No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
The effect of temperature and strain rate on the interaction between an edge dislocation and an interstitial dislocation loop in α-iron
Journal of Physics: Condensed Matter
Abstract
The influence of temperature, T, and strain rate, , on the reaction between the edge dislocation line and a periodic row of 4 nm interstitial dislocation loops with Burgers vector in α-Fe has been investigated by means of molecular dynamics, using a potential developed recently for body centred cubic Fe (Ackland et al 2004 J. Phys.: Condens. Matter 16 1). A dislocation segment with b = [010] is formed by favourable reaction in all cases: it is sessile in the glide plane and leads to the formation of a screw dipole on the line under increasing stress. The mechanism controlling line breakaway and the corresponding critical stress depend mainly on T rather than . At high T (300 and 600 K here) the length of the screw dipole is short (<10b) and the controlling mechanism is the glide of the [010] segment over the loop surface coupled with cross-slip of the short screws. The loop is totally absorbed on the line by transformation of b to . At low T, where thermal effects are negligible, a long (~100b) screw dipole is drawn out and the controlling mechanism is annihilation of the dipole by screw cross-slip. This results in only partial absorption of the loop. By comparing the results with earlier ones obtained using an older interatomic potential, conclusions are drawn on the effects of interaction between edge dislocations and interstitial loops in iron.
... Moreover, the long-range elastic interaction of dislocations demands atomic modeling on the scale of 1 000 000 atoms to observe the dynamic interaction between dislocations and obstacles. Therefore, based on the techniques of molecular mechanics (MM) and molecular dynamics (MD), empirical interatomic potentials such as the embedded atom method (EAM) are used for investigating the dynamic interactions between dislocations and obstacles [1][2][3][4][5][6][7][8][9][10][11][12]. However, the structure and energetics of the dislocation core predicted using empirical interatomic potentials are often inconsistent with the results * morih@cit.sangitan.ac.jp of first-principles calculations obtained using density functional theory (DFT) [13]. ...
... where A (110) denotes the area of the (110) free surface, and f [111] denotes the atomic forces applied to displace the atoms on each surface in the [111] direction [1][2][3]. We set the time step as 1 fs in the MD simulation. ...
... In contrast, the dislocation half-loop for bowing out predicted by the EAM potential manifested an asymmetric shape, which can also be seen in Fig. 3(b) [35]. This type of asymmetric shape of the dislocation line during the bowing process has been commonly observed using the EAM potential and has been troubling researchers for many years [1][2][3][4][5][6][7][8][9][10][11][12]. Thus, the cause of these shape variations during bowing out from the interaction between the dislocation and the void at the atomic level must be determined for both potentials. ...
The introduction of obstacles (e.g., precipitates) for controlling dislocation motion in molecular structures is a prevalent method for designing the mechanical strength of metals. Owing to the nanoscale size of the dislocation core (≤1 nm), atomic modeling is required to investigate the interactions between the dislocation and obstacles. However, conventional empirical potentials are not adequately accurate in contrast to calculations based on density functional theory (DFT). Therefore, the atomic-level details of the interactions between the dislocations and obstacles remain unclarified. To this end, in this paper, we applied an artificial neural network (ANN) framework to construct an atomic potential by leveraging the high accuracy of DFT. Using the constructed ANN potential, we investigated the dynamic interaction between the (a0/2)〈111〉{110} edge dislocation and obstacles in body-centered cubic (bcc) iron. When the dislocation crossed the void, an ultrasmooth and symmetric half-loop was observed for the bowing-out dislocation. Except for the screw dislocation, the Peierls stress of all the dislocations predicted using the ANN was <100 MPa. More importantly, the results confirmed the formation of an Orowan loop in the interaction between a rigid sphere and dislocation. Furthermore, we discovered a phenomenon in which the Orowan loop disintegrated into two small loops during its interaction with the rigid sphere and dislocation.
... However, up to date few work concerns the interaction between pre-existing dislocations and irradiation-induced loops in Mo or Mo alloy for either in-situ ion irradiation or atomistic simulation. In fact, dislocation line-loop interaction is a complicated issue since the interaction process and result rely on several influencing factors at the same time [57][58][59][60][61] . Previous MD simulation [57] in pure iron system showed that dislocation line-loop interaction was temperature-dependent, where dislocation loop pins dislocation line in an Orowan manner at low temperature and complete line-loop reactions occur at medium and high temperatures ( > 300 K). ...
... In fact, dislocation line-loop interaction is a complicated issue since the interaction process and result rely on several influencing factors at the same time [57][58][59][60][61] . Previous MD simulation [57] in pure iron system showed that dislocation line-loop interaction was temperature-dependent, where dislocation loop pins dislocation line in an Orowan manner at low temperature and complete line-loop reactions occur at medium and high temperatures ( > 300 K). Dislocation loop may or may not be absorbed by the dislocation line during the interaction process [58] . ...
... Dislocation loop may or may not be absorbed by the dislocation line during the interaction process [58] . Besides, superjog was often observed on the dislocation line after the line-loop reaction [57][58][59] , eventually thus enhancing the material strength. These observations in Fe are clarified in some atomistic pictures of dislocation line-loop interaction. ...
In-situ TEM observation and molecular dynamics simulation were used to investigate the evolution of irradiation-induced dislocation loops and their interactions with the pre-existing dislocation lines in molybdenum irradiated by 30 keV He⁺ at 673 K. The nucleation, annihilation, merging, size change, and types of dislocation loops were investigated in detail, and the hardening effect from dislocation loops was analyzed. With the increase of helium fluence, the volume number density of dislocation loops increased rapidly and then decreased slowly to a saturation value, while the average size of dislocation loops increased continuously until it reached an upper limit. Both 1/2<111> and <100> loops were formed, with a proportion of 60.2% to 39.8% at 3.95 × 10¹⁵ He⁺.cm⁻² (0.07 dpa). The pre-existing dislocation lines would suppress the nucleation and growth of the dislocation loops, while the irradiation-induced dislocation loops had a strong pinning effect on the dislocation lines and impeded their motion. Molecular dynamics simulations were used to clarify the atomistic mechanisms of some major features of the line-loop interactions observed in the in-situ experiment, which showed in a good agreement. The different interaction processes depended on the Burgers vector and plane of the dislocation loop, the motion and reaction kinetics of dislocation segments, and the external loading.
... The duality originates from the temperature dependent mobility of a junction segment (JS), formed as a result of dislocation e DL interaction, which depends on both temperature and applied stress. At high enough temperature, an absorption mechanism realizes via thermally activated movement of the JS that propagates across the loop surface that reforms the pre-existing loop [19,20,22]. At low temperature a zipeunzip interaction takes place [19]. ...
... However, the character of the impinging dislocation plays an important role in the absorption reaction. Loops reform as glissile superjogs on edge dislocations [20,21], and accommodate as sessile helical turns on screw dislocations [23,42]. In this section we review the data available in the literature with respect to the direct interaction between dislocation and DL in bcc Fe. ...
... As a final remark we note that some low temperature data (~1 K) from Refs. [20,22] were not included in Fig. 1because the resulting OS was higher than one, which is unphysical. The latter is the result of self-interaction of the dislocation line. ...
Upon irradiation, iron based steels used for nuclear applications contain dislocation loops of both 100 and ½111 type. Both types of loops are known to contribute to the radiation hardening and embrittlement of steels. In the literature many molecular dynamics works studying the interaction of dislocations with dislocation loops are available. Recently, based on such studies, a thermo-mechanical model to threat the dislocation loop (DL) - dislocation interaction within a discrete dislocation dynamics framework was developed for ½111 loops. In this work, we make a literature review of the DL – dislocation interaction in bcc iron. We also perform molecular dynamics simulations to derive the stress-energy function for 100 loops. As a result we deliver the function of the activation energy versus activation stress for 100 loops that can be applied in a discrete dislocation dynamics framework.
... Molecular dynamics (MD) simulations is a natural way to study and evaluate the interaction of dislocations with nanometric defects and reveal the physical nature of the pinning as well as estimate the resulting pinning strength. Up to now, the interaction of edge and screw dislocations with interstitial DLs, vacancy clusters and Cu precipitates has been simulated in bcc iron [17][18][19][20][21][22][23][24][25][26][27][28]. It appears that the relative pinning strength of DLs and precipitates is strongly dependent on ambient temperature and defect size. ...
... Voids act as strong obstacles already at small sizes i.e. above 2 nm (see e.g. [20,21]), while DLs exhibit a dual nature and can be absorbed or bypassed depending on temperature and loop size (see e.g. [19,[21][22][23][24][25][26][27][28]). ...
... [20,21]), while DLs exhibit a dual nature and can be absorbed or bypassed depending on temperature and loop size (see e.g. [19,[21][22][23][24][25][26][27][28]). It should be noted that for computational reasons (Section 3), MD simulations are performed for dislocation velocities of (2-200) m s À1 , which correspond to strain rates of (10 6 -10 8 ) s À1 that is about 10-12 orders of magnitude higher than the strain rate under typical mechanical testing ($10 À4 s À1 ). ...
ESTIMATION OF THE CONTRIBUTION OF RADIATION-INDUCED DISLOCATION LOOPS AND PRECIPITATES TO RADIATION HARDENING OF RPV METAL OF REACTORS WWER-440 AND WWER-1000 S.A. Kotrechko, N.N. Stetsenko, M.V. Qzerskiy, V.I. Dubinko A.E. Volkov, V.A. Borodin For RPV metal (base metal and weld metal) of reactors WWER-440 and WWER-1000, contribution to general radiation-induced hardening of two it components is differentiated, namely, hardening related to dislocation loops and hardening due to precipitates. Radiation dose ranges, where each of these components dominates, are ascertained.
... SIA loops up to 5 nm in diameter have been simulated. [77][78][79][80] When small loops (a few tens of SIAs) reach the core, they are fully absorbed athermally creating a double superjog of the size equivalent to the number of SIAs in the loop. This process does not pin the dislocation and these loops are very weak obstacles to dislocation glide. ...
... Large loops are strong obstacles in this reaction, stronger than voids with the same number of vacancies. 4,80 It should be noted, however, that the interaction just described depends rather sensitively on temperature because of the low mobility of screw segments in the bcc metals. Cross-slip of the screw segments is required to allow the [010] segment to glide down. ...
... 78 More details and examples can be found. 4,[77][78][79][80][81][82] Competition between reactions R1, R2, and R3 for a 1/2h111i{110} edge dislocation and 1/2h111i and h100i SIA loops has been considered in detail. 83,84 We cannot describe all the reactions here but some pertinent features are underlined; note that the favorable Burgers vector reaction between a 1/2h111i dislocation and a h100i loop results in a 1/2h111i segment, for example, ...
Primary damage and microstructure evolution in structural nuclear materials operating under conditions of a high flux of energetic atomic particles and high temperature and stress lead to the formation of a high concentration, nonhomogeneous distribution of defect clusters in the form of dislocation loops, voids, gas-filled bubbles, and radiation-induced precipitates of nanometer scale. They cause changes in many material properties. Being obstacles to dislocation glide, they strongly affect mechanical properties in particular. This gives rise to an increase in yield and flow stress and a reduction in ductility. Atomic-scale computer simulation can provide details of how these effects are influenced by obstacle structure, applied stress, strain rate, and temperature. Processes such as obstacle cutting, transformation, absorption, and drag are observed. Some recent results for body-centered and face-centered cubic metals are described in this chapter and, where appropriate, comparisons are drawn with predictions based on the elasticity theory of crystal defects.
... Several MD studies addressing the interaction of mobile dislocations with ½h1 1 1i loops were published in the past years [7,9,19,20], but not so many for the more complex interaction involving h1 0 0i loops [7]. However, at the PWR relevant temperature range mainly h1 0 0i loops are formed under neutron-irradiation [21]. ...
... The results of such identification are reported in Table 2 and will be discussed in Section 4. For the moment, it can be noted that the dislocation velocity form adopted in the DD simulations does not account for the non-Schmidt effects existing in BCC metals. These effects are indeed not expected to influence irradiation-hardening processes at elevated temperature [20]. ...
... The fourth system solution was the value from [36] when the last system solution was adjusted by trial and error tests made with the DD simulations. For friction stress s 0 s , we used the values found in[20,31] for the first three systems, and set the last two values by trial and error tests made with the DD simulations. ...
... (They are equivalent for the ½[1 1 1]ð1 1 0Þ dislocation.) Hexagonal loops with /1 1 2S sides and centres placed initially below the glide plane have been simulated for a variety of numbers of SIAs: 37 and 331 [184], 99 [185] and 169 [186]. Sensitivity of results to the interatomic potential was examined by using the potential of Ref. [69] in Ref. [184] and that of Ref. [70] in Ref. [186]. ...
... Hexagonal loops with /1 1 2S sides and centres placed initially below the glide plane have been simulated for a variety of numbers of SIAs: 37 and 331 [184], 99 [185] and 169 [186]. Sensitivity of results to the interatomic potential was examined by using the potential of Ref. [69] in Ref. [184] and that of Ref. [70] in Ref. [186]. As mentioned in Section 5.1, ½/1 1 1S loops move easily by one-dimensional glide, and if within a few nanometres of a gliding edge dislocation, slip on their glide prism under attractive elastic interaction and react on contact with the dislocation. ...
... As a result, voids are weaker obstacles than many loops when large, but stronger when small. Furthermore, the strength of voids has weaker T-dependence than that of ½/1 1 1S loops [118,186]. All the t c data obtained by MD simulation with T ¼ 300 K fall below the values predicted by eq. ...
... Molecular dynamics (MD) simulations is a natural way to study and evaluate the interaction of dislocations with nanometric defects and reveal the physical nature of the pinning as well as estimate the resulting pinning strength. Up to now, the interaction of edge and screw dislocations with interstitial DLs, vacancy clusters and Cu precipitates has been simulated in bcc iron [17][18][19][20][21][22][23][24][25][26][27][28]. It appears that the relative pinning strength of DLs and precipitates is strongly dependent on ambient temperature and defect size. ...
... Voids act as strong obstacles already at small sizes i.e. above 2 nm (see e.g. [20,21]), while DLs exhibit a dual nature and can be absorbed or bypassed depending on temperature and loop size (see e.g. [19,[21][22][23]). ...
... [20,21]), while DLs exhibit a dual nature and can be absorbed or bypassed depending on temperature and loop size (see e.g. [19,[21][22][23]). It should be noted, however, that for computational reasons, MD simulations are performed for dislocation velocities of (2 ÷ 200) m·s -1 , which correspond to strain rates of (10 6 ÷10 8 ) s -1 that is about 10÷12 orders of magnitude higher the strain rate under typical mechanical testing (~10 -4 s -1 ). ...
A relative contribution to irradiation hardening caused by dislocation loops
and solute-rich precipitates is established for RPV steels of WWER-440 and
WWER-1000 reactors, based on TEM measurements and mechanical testing at reactor
operating temperature of 563 K. The pinning strength factors evaluated for
loops and precipitates are shown to be much lower than those obtained for model
alloys based on the room temperature testing as well as those evaluated by
means of atomistic simulations in the temperature range of 300 to 600 K. This
discrepancy is explained in the framework of a model of thermally activated
dislocation motion, which takes into account the difference in temperature and
strain rate employed in atomistic simulations and in mechanical testing.
... In the dislocation-assisted plastic deformation of crystalline materials, strain hardening derives from the multiplication of mobile dislocations and their interaction with other dislocations [1][2][3][4] or microstructure defects such as nanosized voids [5][6][7][8][9], secondary phase precipitates [8][9][10][11][12][13][14][15], dislocation loops [16][17][18][19][20][21] and grain boundaries [22][23][24][25][26][27]. Among these mechanisms dislocation-dislocation interactions play a substantial role in the strain hardening of metallic materials. ...
... As described, the bowing of an edge dislocation around any nanosized defect leads to the formation of a more or less perfect screw dislocation dipole segment [5,11,16,18,19]. This dipole can then be stabilized, depending on the local stress state and temperature. ...
... It has been shown that temperature softens the interaction between a ½ a 0 h1 1 1i edge dislocation and a nanosized void in terms of stress response and reduces the screw dipole size [5]. A similar effect has also been observed in the MD simulation of the interaction of an edge dislocation with a dislocation loop at temperatures up to 600 K [18]. In the latter case, the length of the dipole increases with decreasing temperature, reaching about 30 nm at 1 K. ...
Molecular dynamics simulation is used to study the formation of the a 0 h1 0 0i binary dislocation junction in body-centered cubic Fe. Results show that under an applied strain two intersecting ½ a 0 h1 1 1i dislocations, one mobile edge and one immobile screw, form an a 0 h1 0 0i binary junction of mixed character in the glide plane of the mobile edge dislocation. It appears, however, that the binary junction does not necessarily lay in one of the three possible {1 1 0} glide planes of the screw dislocation. The binary junction starts to unzip as the impinging edge dislocation bows around and moves away, which results in the formation of a screw dipole along its Burgers vector. The dipole eventually annihilates, completing the unzipping process of the junction, which liberates the edge dislocation. The effects of tem-perature and strain rate on the unzipping of the junction are quantified by the critical release stress needed to detach the edge dislocation from the screw one. The critical stress decreases when the temperature increases from 10 to 300 K, whereas it increases with increasing applied strain rate, or dislocation speed. The interaction mechanism and strength of the a 0 h1 0 0i binary junction as an obstacle to the edge dislocation are compared to that of other types of defect, namely nanosized voids, Cu and Cr precipitates, and dislocation loops in Fe. It appears that the binary junction strength is in the lowest range, comparable to that of a coherent Cr precipitate.
... In the dislocation-assisted plastic deformation of crystalline materials, strain hardening derives from the multiplication of mobile dislocations and their interaction with other dislocations1234 or microstructure defects such as nanosized voids56789, secondary phase precipitates89101112131415 , dislocation loops161718192021 and grain boundaries222324252627. Among these mechanisms dislocation–dislocation interactions play a substantial role in the strain hardening of metallic materials . ...
... The attractive force between the edge dislocation and the nanosized void or Cu precipitate results from the image forces of the void surface or Fe–Cu interface, whereas in the case of the dislocation–dislocation interaction it results from the elastic fields of both the ½ a 0 [À1 1 1] edge and ½ a 0 [1 1 1] screw dislocations. As described, the bowing of an edge dislocation around any nanosized defect leads to the formation of a more or less perfect screw dislocation dipole segment [5,11,16,18,19] . This dipole can then be stabilized, depending on the local stress state and temperature. ...
... It has been shown that temperature softens the interaction between a ½ a 0 h1 1 1i edge dislocation and a nanosized void in terms of stress response and reduces the screw dipole size [5]. A similar effect has also been observed in the MD simulation of the interaction of an edge dislocation with a dislocation loop at temperatures up to 600 K [18]. In the latter case, the length of the dipole increases with decreasing temperature, reaching about 30 nm at 1 K. ...
Molecular dynamics simulation is used to study the formation of the a <100> binary dislocation junction in body-centered cubic Fe. Results show that under an applied strain two intersecting ½ a 0 <1 1 1> dislocations, one mobile edge and one immobile screw, form an a <100> binary junction of mixed character in the glide plane of the mobile edge dislocation. It appears, however, that the binary junction does not necessarily lay in one of the three possible {1 1 0} glide planes of the screw dislocation. The binary junction starts to unzip as the impinging edge dislocation bows around and moves away, which results in the formation of a screw dipole along its Burgers vector. The dipole eventually annihilates, completing the unzipping process of the junction, which liberates the edge dislocation. The effects of temperature and strain rate on the unzipping of the junction are quantified by the critical release stress needed to detach the edge dislocation from the screw one. The critical stress decreases when the temperature increases from 10 to 300 K, whereas it increases with increasing applied strain rate, or dislocation speed. The interaction mechanism and strength of the a <100>binary junction as an obstacle to the edge dislocation are compared to that of other types of defect, namely nanosized voids, Cu and Cr precipitates, and dislocation loops in Fe. It appears that the binary junction strength is in the lowest range, comparable to that of a coherent Cr precipitate.
... More difficult, though not a priori impossible, is the absorption of other types of defects. Thanks to MD simulations [136][137][138][139][140][141][142][143][144][145][146][147], much is known about the mechanisms of defect absorption by gliding dislocations, at least in pure Fe [136][137][138][139][140][141][142] and, to some extent, in Fe-Cr [144][145][146] and also in Fe-C [143,147]. However, little is known about these same mechanisms in complex alloys, for example in the presence of, simultaneously, substitutional solutes such as Cr, and interstitial ones such as C, which are both found to decorate loops. ...
... More difficult, though not a priori impossible, is the absorption of other types of defects. Thanks to MD simulations [136][137][138][139][140][141][142][143][144][145][146][147], much is known about the mechanisms of defect absorption by gliding dislocations, at least in pure Fe [136][137][138][139][140][141][142] and, to some extent, in Fe-Cr [144][145][146] and also in Fe-C [143,147]. However, little is known about these same mechanisms in complex alloys, for example in the presence of, simultaneously, substitutional solutes such as Cr, and interstitial ones such as C, which are both found to decorate loops. ...
The M4F project brings together the fusion and fission materials communities working on the prediction of radiation damage production and evolution and its effects on the mechanical behaviour of irradiated ferritic/martensitic (F/M) steels. It is a multidisciplinary project in which several different experimental and computational materials science tools are integrated to understand and model the complex phenomena associated with the formation and evolution of irradiation induced defects and their effects on the macroscopic behaviour of the target materials. In particular the project focuses on two specific aspects: (1) To develop physical understanding and predictive models of the origin and consequences of localised deformation under irradiation in F/M steels; (2) To develop good practices and possibly advance towards the definition of protocols for the use of ion irradiation as a tool to evaluate radiation effects on materials. Nineteen modelling codes across different scales are being used and developed and an experimental validation programme based on the examination of materials irradiated with neutrons and ions is being carried out. The project enters now its 4th year and is close to delivering high-quality results. This paper overviews the work performed so far within the project, highlighting its impact for fission and fusion materials science.
... The Ackland-2004 empirical potential [18] is used in the present work. Such potential has been approved to well describe the properties of dislocations and interstitial dislocation loops in iron [19]. After comparing with other potentials, e.g. ...
... Thus, with increasing size of a 〈1 0 0〉 loop, its rotation to a 1/2〈1 1 1〉 loop is expected to be difficult even at high temperatures. Except the rotation from 〈1 0 0〉 to 1/2〈1 1 1〉, the transition of a 1/2〈1 1 1〉 to a 〈1 0 0〉 loop has also been explored with SAAMD method [19]. During the simulation, the accelerations are applied along the direction perpendicular to the Burgers vector, while the normal diffusion of the loop along the Burgers vector is remained. ...
Complex states of nanoscale interstitial dislocation loop can be described by its habit plane and Burgers vector. Using atomistic simulations, we provide direct evidences on the change of the habit plane of a ½<111> loop from {111} to {110} and {211}, in agreement with TEM observations. A new {100} habit plane of this loop is also predicted by simulations. The non-conservation of the Burgers vector is approved theoretically for: (1) dislocation reactions between loops with different Burgers vectors and (2) the transition between <100> loops and ½<111> loops. The rotation from a ½<111> to a <100> loop has also been explored, which occurs at 570 K for time on the order of 10 seconds. The dislocation-precipitate phase duality and change of habit plane are then proposed as new features for nano-scale dislocation loops.
... The size of the inner region of the MD box was 101 Â 3, 30 Â 6 and 25 Â 2 non-equivalent atomic planes along x, y and z (or 25 Â 21 Â 10 nm 3 ), respectively, and it contains around 450 thousands atoms. Such crystallite is large enough to consider the interaction of the dislocation with a loop of diameter up to 5 nm (350 self-interstitial atoms, SIAs), as shown in our previous studies [39][40][41]. During an MD run, the dislocation line and loop were monitored using atomic disregistry analysis, coupled with structural nearest-neighbour analysis (see [42]), as well as by selecting atoms with high potential energy. ...
... (i) Complete removal of the loop, which occurs by its incorporation on the dislocation line or by dynamic drag. (ii) Partial absorption i.e. reduction of the loop size without modification of its orientation, which occurs when the absorption reaction started but did not get to completion (see examples of the interaction with inclined loops at low temperature [41]). Another reaction pathway was identified in the reactions involving loops with BV = a 0 /2[1 1 1] immobilized by the solute decoration, the examples will follow. ...
The exact nature of the radiation defects causing hardening in reactor vessel pressure steels at high doses is not yet clearly determined. While generally it is attributed to solute-rich clusters (precipitates) and point defects clusters (matrix damage), recent fine-scale experiments and atomistic simulations suggest that solute rich clusters, mainly containing Mn, Ni and Cu, might be the result of the segregation of these elements to small dislocation loops (heterogeneous nucleation), so that the distinction between precipitates and matrix damage becomes blurred. Here, we perform an atomistic study to investigate the interaction of a0/2〈1 1 1〉 dislocation loops with moving dislocations and specifically address the effect of solute segregation on the loop’s strength and interaction mechanism, focusing in particular on Mn, alone or with other crucial solute elements such as Cu and Ni. It is found that the enrichment of Mn in the core of dislocation loops causes significant increase of the unpinning stress, especially for small, invisible ones. At the same time, the solute segregation at the dislocation loops enhances their resistance against absorption by moving dislocations.
... The size of the inner region of the MD box was about 25 Â 21 Â 10 nm and contained about 500,000 mobile atoms. This crystallite is large enough to consider interaction of the dislocation with a 3.5 nm loop (169 SIAs), as shown in previous studies (see, for example, Terentyev et al. [27]). MD simulations were performed at temperatures from 150 to 800 K with identical time steps (2 fs). ...
... Dislocation–loop reactions investigated here pass through several stages that depend on the loop state, i.e. pure Fe or Fe enriched with Cr atoms. In the case of pure Fe simulated here the following four stages are fully consistent with earlier studies [20,27]. i. ...
In this work, we have performed an atomic-level study by combining Molecular Dynamics (MD) and Monte Carlo (MC) techniques, employing recently developed Fe-Cr interatomic potentials fitted to ab initio data. The MC method was used to investigate the equilibrium arrangement of Cr atoms in an Fe-Cr matrix containing a dislocation loop. The results reveal that Cr atoms segregate to the tensile strain region and the dissolution temperature is about 50-100 degrees above the solubility limit. The atomic configurations obtained by MC were then used to study the interaction of dislocations with the 'enriched' loops. It was revealed that depinning stress is higher for the 'enriched' loops and the complete absorption of loops on a dislocation line is suppressed. The reason for this has been rationalized in terms of the mechanisms involved in the non-elastic interaction between a dislocation line and dislocation loop. These results clearly show that local micro-chemical changes in the core of dislocation loops have important consequences for the mobility of loops and their interaction with dislocations.
... The size of the inner region of the MD box was about 25 Â 21 Â 10 nm and contained about 500,000 mobile atoms. This crystallite is large enough to consider interaction of the dislocation with a 3.5 nm loop (169 SIAs), as shown in previous studies (see, for example, Terentyev et al. [27]). MD simulations were performed at temperatures from 150 to 800 K with identical time steps (2 fs). ...
... Animated visualizations of the reactions described in the following are attached to this paper as supplementary material on the journal website.Dislocation-loop reactions investigated here pass through several stages that depend on the loop state, i.e. pure Fe or Fe enriched with Cr atoms. In the case of pure Fe simulated here the following four stages are fully consistent with earlier studies[20,27]. ...
The effect of chromium on iron hardening via segregation on dislocation loops was studied by atomic scale computer modeling. A combination of Monte Carlo and molecular dynamics techniques together with the recently determined Fe Cr interatomic potentials fitted to ab initio data was used to investigate Cr segregation on h111i interstitial dislocation loops and its impact on the interaction with moving dislocations. The Monte Carlo results reveal that Cr atoms segregate to the loop tensile strain region and dissolve well above the temperature corresponding to the solubility limit. The molecular dynamics results demonstrated that local micro-chemical changes near the loop reduce its mobility and increase the strength. The stress to move a dislocation through the array of Cr decorated loops increases due to modification of the dislocation loop interaction mechanism. A possible explanation for a number of experimental observations being dependent on the radiation dose and for Cr concentration effects on the yield stress is given on the basis of the modeling results.
... As shown in Fig. 9, changing the strain rate has little effect on the CRSS, and the error in the CRSS calculated using the three strain rates is 16.37 MPa, proving that our simulation is strain rate independent. The effect of strain rate is consistent with the work of Ref. [38]. For the effect of potentials on simulation results, the potential used for interaction simulation in pure Fe in the manuscript was developed by Mendelev [18] (Potential 1), and now we selected other 2 potentials developed by Byggmästar [39] (Potential 2) and Malerba [40] (Potential 3) for supplementary simulations. ...
In this work we study the effects of different factors of dislocation loop on its obstacle strength when interacting with an edge dislocation. At first, the interaction model for dislocation and dislocation loop is established and the full and partial absorption mechanism is obtained. Then, the effect of temperature, size and burgers vector of dislocation loop are investigated. The relation between the obstacle strength and irradiation dose has been established, which bridges the irradiation source and microscale properties. Except that, the obstacle strength of C, Cr, Ni, Mn, Mo and P decorated dislocation loop is studied. Results show that the obstacle strength for dislocation loop decorated by alloy element decreases in the sequence of Cr, Ni, Mn, C, P and Mo, which could be used to help parameterize and validate crystal plasticity finite element model and therein integrated constitutive laws to enable accounting for irradiation-induced chemical segregation effects.
... The interaction of a loop with a gliding dislocation has been well-documented and involves various dislocation reactions. In many cases, loops are incorporated by adding to the halfplane of the gliding dislocation, i.e., climb, thus effectively changing the glide plane for that section of the dislocation [33][34][35][36][37][38]. Various reactions are possible involving the complete or partial absorption of the loop. ...
The production of prismatic dislocation loops in nuclear reactor core materials results in hardening because the loops impede dislocation motion. Yielding often occurs by a localised clearing of the loops through interactions with gliding dislocations called channeling. The cleared channels represent a softer material within which most of the subsequent deformation is localized. Channeling is often associated with hypothetical dislocation pileup and intergranular cracking in reactor components although the channels themselves do not amplify stress as one would expect from a pileup. The channels are often similar in appearance to twins leading to the possibility that twins are sometimes mistakenly identified as channels. Neither twins nor dislocation channels, which are bulk shears, produce the same stress conditions as a pileup on a single plane. At high doses, when cavities are produced (either He-stabilised bubbles at low temperatures or voids at high temperatures), there can be reduced ductility because the material is already in an equivalent advanced stage of microscopic necking. He-stabilised cavities form preferentially on grain boundaries and at precipitate or incoherent twin/ε-martensite interfaces. The higher planar density of the cavities, coupled with the incompatibility at the interface, results in a preferential failure known as He embrittlement. Strain localisation and inter-or intragranular failure are dependent on many factors that are ultimately microstructural in nature. The mechanisms are described and discussed in relation to reactor core materials.
... Важно учесть, что данные зависимости (рис. 2 и 3) не учитывают напряжение трения, так как в [27] показано, что при моделировании с температурой выше 100 K оно почти исчезает. ...
Tungsten is widely used as a material capable of withstanding working conditions in nuclear reactors and other extreme conditions. Under the influence of irradiation, such defects as Frenkel pairs, pores, and dislocation loops are formed in the metal. Therefore, the research aimed at studying the interactions of these defects with each other and their influence on the mechanical properties of the metal are relevant. The paper presents the theoretical study based on the molecular dynamics method, the purpose of which is to investigate the mechanism of strain hardening of tungsten associated with the interaction of dislocations and pores. The authors solved this problem using the LAMMPS package, carried out the integration of atoms motion equations by the fourth order Verlet method. The model under the study is a single crystal of a certain [111], [–1–12], [1–10] orientation along the basic X, Y, and Z coordinate axis relatively, in which the slip of edge dislocations in the main slip system of BCC metals and their interaction with pores is considered. The authors studied the influence of a pore size on the shear stress magnitude: the growth of pore diameter is proportional to the stress growth. The dependences of shear stress on the shear strain in the temperature range of 600–1400 K are calculated, whereby the temperature change does not significantly influence the stress value. The study shows that dislocations cut the pores and, upon the repeated interaction with a pore, a lower value of peak shear stress is observed than during the first one. The presence of pores leads to the flow stress increase, and such an effect becomes more evident with the increasing pore diameter. The flow stress increases thrice for pores with a diameter of 6 nm compared to the material without pores. The authors described the mechanism of interaction between the edge dislocations and pores under the influence of shear stress.
... It can be seen from ABC-T results that, for strain rates higher than 10 5 s -1 , the CRSS increases as a function of strain rate, which shows a normal relation and is consistent with earlier studies[67,68]. It is noticed that under the same conditions, there are quantitative mismatches of CRSS between ABC-T and MD. ...
The energetics and kinetics of interactions between microstructural point, line, and planar defects govern the most significant mechanical properties of structural materials, which is crucial in assessing, predicting, and controlling the material behavior in technological applications such as nuclear reactors and additive manufacturing (AM). Modeling microstructural evolutions under extreme conditions with high atomistic fidelity and at experimental time scales has been known as a longstanding challenge in material science. Under some cases, traditional atomistic modeling techniques like molecular dynamics (MD) enable the elucidation of molecular mechanisms of microstructural evolutions. Nevertheless, conventional MD could hardly go beyond nanoseconds and therefore simple extrapolation of MD results from short timescales could undermine the accuracy or even give a misleading prediction of materials performance at realistic environments. To overcome such challenges, advanced atomistic modeling techniques that can directly tackle the long timescale problems are greatly desired. In this thesis, we present a novel computational framework based on the concept of potential energy landscape (PEL), which enables the investigation of microstructural evolutions at long-time scales while fully retaining the atomistic details. To demonstrate its capabilities, we show the application of this framework to some key problems in material science. The following problems are addressed: (i) We investigate a local interaction between dislocation and a vacancy-type obstacle in BCC Fe at room temperature over a wide range of strain rates, from 108s-1 down to 103s-1. Very surprisingly, a non-monotonic correlation is found between the critical resolved shear stress (CRSS) of the system and applied strain rates. We demonstrate that the nSRS is due to the complex interplays between thermal activation and applied strain rate. (ii) We map the kinetic evolution of metastable <100> symmetric tilt grain boundaries (GBs) in copper under non-equilibrium processing, which is significant in determining the strength and ductility of nanocrystalline materials. Our combined study, providing both atomistic simulations and novel experiments employing femtosecond laser-material interactions, demonstrates striking features of the energetics and kinetic pathways to achieve a multiplicity of grain boundary states. Specifically, it can be divided into an ageing regime and a rejuvenating regime over a broad energy—temperature parameter space. We further ascribe the origin of this ageing/rejuvenating behavior to the inherent energy imbalance along with elementary hopping processes in the system’s underlying PEL. (iii) We also investigate the non-equilibrium relaxations of metastable tilted GBs in copper under ultrafast thermal cycles, which is critical in assessing the structural materials’ properties during AM. We demonstrate that the structural evolution of metastable GBs is mainly driven by disorder and rough energy landscape rather than free volume. Most importantly, a universal scaling is observed between the GBs’ inherent structure energies and their structural transition temperatures during the rapid cooling stage. To further assess the applicability of the obtained scaling correlation, we also investigate a group of GBs in Ni. A similar correlation persists, which gives reason to expect the present study may apply to other materials as well. In summary, the PEL-based modeling framework is employed to investigate the microstructural evolution at long time scales, without invoking empirical assumptions or fitting parameters. This framework might shed light on the predictive design of advanced structural materials to be used in complex environments.
... The obstacle strength parameter is very different for different obstacles and numerous estimations always demonstrated that the dislocation loop strengthening parameter is about 3 times weaker than that of voids [14][15][16][17]. Moreover, atomistic estimation of dislocation loop and void strengthening effects [18][19][20][21] indicates that nanometric voids are much stronger obstacles than dislocation loops of comparable size. Thus, loops smaller than 5 nm were found to be weaker obstacles than even 2 nm voids [18]. ...
Neutron and heavy-ion irradiation of tungsten produces nanometer-size vacancy voids, gas-filled bubbles and dislocation loops. These defect features can affect mechanical properties and the impact can be quite significant because of their high density. Understanding the basic mechanisms of mechanical properties degradation is necessary for predicting radiation effects. Predictions can be made using discrete dislocation dynamics or/and finite element approaches which, however, need local interaction mechanisms as inputs. Such knowledge can be provided only by atomic-scale modeling. This paper reports the results of an extensive atomic-scale modeling study of the interactions between moving edge dislocations and voids in tungsten. The main focus is on the effects of the void size and ambient temperature. Critical resolved shear stress was calculated for voids up to 9 nm in diameter. Atomistic results are compared with the theoretical approach and with those obtained earlier for voids in body centered cubic (bcc) iron. An important role of the void surface has been revealed.
... The mechanical properties of metals at ambient temperatures (from about 0 to 200 • C [1][2][3]) are mainly affected by their microstructures, except for the intrinsic properties determined by their chemical compositions [4,5]. As plastic deformation of metals proceeds via the dislocation motion, including nucleation, multiplication and slip of full dislocations with an edge/screw/mixed characteristics as well as nucleation and propagation of partial dislocations/deformation twins [6][7][8][9]. ...
The interactions between dislocations (dislocations and deformation twins) and boundaries (grain boundaries, twin boundaries and phase interfaces) during deformation at ambient temperatures are reviewed with focuses on interaction behaviors, boundary resistances and energies during the interactions, transmission mechanisms, grain size effects and other primary influencing factors. The structure of boundaries, interactions between dislocations and boundaries in coarse-grained, ultrafine-grained and nano-grained metals during deformation at ambient temperatures are summarized, and the advantages and drawbacks of different in-situ techniques are briefly discussed based on experimental and simulation results. The latest studies as well as fundamental concepts are presented with the aim that this paper can serve as a reference in the interactions between dislocations and boundaries during deformation.
... In the top regions (Figures 6(a) and (d)), the dislocations are intertwined, and some dislocation walls formed by the motion of dislocations are observed. [30] However, in the middle (Figures 6(b) and (c)) and root regions (Figures 6(e) and (f)), the dislocations distribute uniformly in the matrix, and the dislocation walls disappear because of the interaction of dislocations in opposite vector directions during the thermal cycles. In addition, the dislocation walls are translated into sub-grain boundaries under a higher temperature. ...
In the present study, correlation of the microstructure and stress corrosion cracking (SCC) susceptibility in the top, middle and root regions of a multi-pass austenitic stainless steel weld joint fabricated by narrow-gap tungsten inert gas welding (NG-TIG) was
investigated by slow strain rate tests (SSRT) following a microstructure characterization. The results show that from the top to root regions in both the heat-affected zone (HAZ) and weld metal (WM), the dislocation density and length fraction of the Σ3 boundaries present a decreased trend, while the grain size, length fraction of LAB and residual strain all increase. By comparing the microstructure of WM with HAZ, δ-ferrite in WM is excessive and continuous while that in HAZ is island-shaped and discontinuous. In addition, larger grain size, lower dislocation density, higher residual strain, lower length fraction of Σ3 boundaries and higher length fraction of RGB are observed in WM. As a result, the SCC susceptibility in various regions of the weld joint follows the order of root regions > middle regions > top regions and WM > HAZ.
... They found that decorated dislocation loops are of 1 2 ⟨111 ⟩ Burgers vector type and specifically the same 1 2 ⟨111 ⟩ Burgers vector variant ( 1 2 [ 1 1 1 ]). The g • b analysis in our experiment manifests that dislocations and the majority of decorated loops are also of the same 1 2 ⟨111 ⟩ type after annealing at 673 K. D. Terentyev et al. [49] investigated the influence of temperature and strain rate on the reaction between a 1 2 [111] edge dislocation and a 1 2 [ 1 1 1 ] interstitial dislocation loop in -iron by means of MD simulations. In all cases, a dislocation segment with b = [010] is formed by favorable reaction. ...
To better understand the thermal stability of dislocation loops formed by long-term neutron irradiation in reactor pressure vessel (RPV) steels, in-situ scanning transmission electron microscopy (STEM) observation was performed for a surveillance test specimen of a European pressurized water reactor (PWR). The surveillance test specimen was neutron irradiated to a fluence of 8.2 × 10²³ neutrons•m⁻². A membrane sample from the surveillance test specimen was annealed at 673 K and 723 K for 30 minutes using a heating holder, and the same area was in-situ observed under STEM. After annealing, dislocation loops with Burgers vectors of ½ <111> and <100> were quantitatively examined. When annealing temperature increased from 673 K to 723 K, the number of dislocation loops decreases, whereas the size of them becomes larger. Correspondingly, the proportion of <100> dislocation loops changes from 27% to 45%. The ratio of <100> to ½ <111> loops increases with annealing temperature rising. The evolution process of dislocation loops during annealing at 723 K was in-situ observed and analyzed to shed light on the transformation mechanism of dislocation loops going from ½ <111> to <100>. It is the first time to directly observe that two ½ <111> dislocation loops collide with each other and coalesce to form a <100> dislocation loop. Moreover, small ½ <111> dislocation loops could be absorbed by large <100> dislocation loops, whereas the Burgers vector of <100> loops remained unchanged. Dislocation decoration occurs during annealing due to the interaction between dislocations and loops. The dislocations decorated by loops are pretty stable during the continuous annealing process, which is well explained by molecular dynamics simulation.
... Many atomistic researches were dedicated to the description of motion of dislocation in pure metals Yanilkin, 2013, 2015;Cho et al., 2017;Burbery et al., 2017) or in intermetallic systems (Chen and Ma, 2013;Zhou et al., 2016) or, even, in metal oxides (Lunev et al., 2017(Lunev et al., , 2018, where a Peierls barrier and phonon drag produce main contribution to the resistance of metal to deformation. During last decades, atomistic investigation of the interaction of dislocations and obstacles such as precipitates of impurity atoms in the case of alloys or voids typical for irradiated metals were extensively developed (Terentyev et al., 2007;Wu et al., 2007;Bacon and Osetsky, 2009;Fan et al., 2012Fan et al., , 2018aFan et al., , 2018bLeyson et al., 2012;Yanilkin et al., 2014;Xiong et al., 2015;Saroukhani et al., 2016;Sobie et al., 2017;Zhu et al., 2017;Varvenne et al., 2017;Saroukhani and Warner, 2017;Esteban-Manzanares et al., 2019). ...
We investigate the interaction of edge dislocation with θ′ phase in aluminum matrix using atomistic simulations. The thickness of θ′ phase is chosen to be constant of 2.2 nm and the diameter is varied in range from 3 to 10 nm. It is shown that first interactions of dislocation with θ′ phase occur according to the mechanism of Orowan loop formation around the obstacle. In this case, the cross-slip processes, the formation of dislocation jogs in adjacent slip plane and the emission of vacancies upon the return of a segment to the initial slip plane are possible. During these interactions, the material of θ′ phase is subjected to high shear stresses up to 3 GPa in a layer of 2 nm thickness. Such a high stress leads to a θ′ phase cutting on the third or fourth overcoming of the obstacle by dislocation. A study is carried out of the dependence of the average stress in the system on the size of inclusion and the distance between inclusions. It is shown that an increase in the diameter of inclusion causes an increase in the average stresses in the system in proportion close to the square root of the diameter of the inclusion. Increasing the distance between inclusions causes the inversely proportional reduction of the average stress. The conducted investigation of the strain rate sensitivity showed that in the case of high shear rates, the average stresses in the system continuously increase that does not allow applying the time averaging procedure to them. The described effect is also registered in the case of pure aluminum. The existence of two regions on the temperature dependence of the average stresses in the system on the strain rate in the case of θ′ phase, previously described by Yanilkin et al. (2014) for the Guinier-Prestone zones, is confirmed. In the case of high strain rates, heating of the system leads to a decrease in the dislocation velocity, while at low strain rates the dislocation velocity increases with increasing temperature at a fixed shear rate. An interesting result obtained with long-term molecular-dynamics simulations when the tracing time is up to 2 ns, there is a tendency to reduce the average stress in the system through time. This result can be explained by destruction of the structure and form of θ’ phase.
The law of motion of a dislocation in the approximation of the constancy of θ′ phase properties is proposed to describe the response of the system to shear deformation. The model contains a dislocation mass, phonon friction, and takes into account the effect of the inclusion of θ’ phase through an increase in the elastic energy of a dislocation during the formation of the Orowan loop around an obstacle.
... This will enhance the obstacle strength of Cu-Mn precipitates. It is shown that the high temperature [30]. Figure 3 presents the stress-strain curves for the interaction of different diameter precipitates with an edge dislocation in a BCC Fe matrix at 600 K. ...
In order to study the contribution of manganese (Mn) atoms in copper (Cu) precipitates to hardening in body centered cubic (BCC) structure iron (Fe) matrix, the interactions of a 1/2 〈111〉 {110} edge dislocations with nanosized Cu and Cu–Mn precipitates in BCC Fe have been investigated by using molecular dynamics method (MD). The results indicate that the critical resolved shear stresses (τc) of the Cu–Mn precipitates are larger than that of Cu precipitates. Meanwhile, τc of the Cu–Mn precipitates show a much more significant dependence on temperature and size compared to Cu precipitates. Mn atoms exhibit strong attraction to dislocation segment in Cu precipitate and improve the fraction of transformed atoms from BCC phase to nine rhombohedron (R) phase for big size precipitates. Those all lead to the higher resistance to the dislocation glide. Eventually, these features confirmed that the appearance of Mn atoms in Cu precipitates greatly facilitates the hardening in BCC Fe matrix.
... The effect of these different features on the dynamics of individual dislocation lines has been studied in detail with molecular dynamics (MD) simulations for some time. Recent examples include refs [16][17][18][19][20][21][22][23][24] . MD simulations give valuable insight into the physical mechanisms of dislocation-defect interactions, but the computational limitations of simulating materials at atomic resolution constrain the system size usually to a singular dislocation and a few defects. ...
Plastic deformation of crystalline materials is governed by the features of stress-driven motion of dislocations. In the case of irradiated steels subject to applied stresses, small dislocation loops as well as precipitates are known to interfere with the dislocation motion, leading to an increased yield stress as compared to pure crystals. We study the combined effect of precipitates and interstitial glissile [Formula: see text] dislocation loops on the yield stress of iron, using large-scale three-dimensional discrete dislocation dynamics simulations. Precipitates are included in the simulations using our recent multi-scale implementation [A. Lehtinen et al., Phys. Rev. E 93 (2016) 013309], where the strengths and pinning mechanisms of the precipitates are determined from molecular dynamics simulations. In the simulations we observe dislocations overcoming precipitates with an atypical Orowan mechanism which results from pencil-glide of screw segments in iron. Even if the interaction mechanisms with dislocations are quite different, our results suggest that in relative terms, precipitates and loops of similar sizes contribute equally to the yield stress in multi-slip conditions.
... The CRSS under different strain rates are read off from Figs. 3(a)-3(b) and plotted in Fig. 4. According to the ABC-T results, for strain rates higher than 10 5 s −1 the CRSS increases as a function of strain rate, which shows a normal relation and is consistent with earlier studies [24,25]. It is noticed that under the same conditions, there are quantitative mismatches of CRSS between ABC-T and MD. ...
The interaction between an edge dislocation and a sessile vacancy cluster in bcc Fe is investigated over a wide range of strain rates from 108 down to 103 s−1, which is enabled by employing an energy landscape-based atomistic modeling algorithm. It is observed that, at low strain rates regime less than 105 s−1, such interaction leads to a surprising negative strain rate sensitivity behavior because of the different intermediate microstructures emerged under the complex interplays between thermal activation and applied strain rate. Implications of our findings regarding the previously established global diffusion model are also discussed.
... It appears from the MD simulations that the binary junction induces a comparable hardening to that of nanometric Cr precipitate [16]. Actually, the ½ a 0 <111> screw dipole formation was also observed in the interaction of an edge dislocation with a nanometric dislocation loop [13,17,18], a spherical cavity as void or He bubble [15,19], performed under constant strain rate at finite temperatures. It appears that for the annihilation of this dipole, one of the screw arms must first cross slip, and then glide towards the other arm, resulting in the release of the dislocation [12]. ...
We report the experimental observation of the <111> edge dislocation dipole formation and annihilation in ultra-high purity Fe using transmission electron microscopy (TEM) in-situ straining. The observation is confirmed by TEM image simulations. The edge dipole is formed by the interaction of a moving screw dislocation with an obstacle of dislocation character. It results from the glide of the two arms of the dislocation on two different glide planes, which stabilizes the dipole that is closed by a jog. The dipole is later released from the obstacle and disappears, presumably by gliding of the dipole's edge segments along their Burgers vector and freeing the mobile screw dislocation from the jog. This mechanism leads to enhanced obstacle strength of the immobile dislocation well above Orowan critical stress, promoting forest strength.
... Here, we make a step further and develop CPM to account for the DL-induced hardening and thermally-stress activated absorption of DLs in the course of plastic deformation, following the activation mechanisms extracted directly from atomistic simulations (see e.g. [10][11][12]). We apply this model to compute the stress-strain response of neutron irradiated polycrystalline Fe using the elastic-viscoplastic self-consistent (EVPSC) theory [13,14]. ...
Crystal plasticity model (CPM) for BCC iron to account for radiation-induced strain softening is proposed. CPM is based on the plastically-driven and thermally-activated removal of dislocation loops. Atomistic simulations are applied to parameterize dislocation-defect interactions. Combining experimental microstructures, defect-hardening/absorption rules from atomistic simulations, and CPM fitted to properties of non-irradiated iron, the model achieves a good agreement with experimental data regarding radiation-induced strain softening and flow stress increase under neutron irradiation.
... The loops are initially introduced randomly, but then the loops located at a distance ± D L (the loop diameter) from the glide plane are positioned exactly on the glide plane. By doing this, we accounted for the limited loop glide near the approaching dislocation, which occurs in MD simulations[15]. To compare the DD resolved flow stress with the experimental data, we scaled the simulation results by a factor of 3, the Schmidt factor. ...
We propose a computationally fast and physically justifiable method to treat dislocation loops as stochastic thermally activated finite-size obstacles in discrete dislocation dynamics simulations. The method was parameterized using molecular dynamics data for the interaction of dislocations with a(0)/2 < 111 > dislocation loops. As demonstration, the method is applied to rationalize experimental hardening of neutron-irradiated iron. The obtained results show good agreement with experimental data.
... The details of the interaction mechanism can be determined by molecular dynamics (MD) simulation or experimentally by post-mortem or in situ transmission electron microscopy (TEM) deformation experiments. MD simulations of the interaction between a ½ a 0 h1 1 1i dislocation and a nanometric void [3][4][5][6][7][8][9], coherent Cr precipitate [10][11][12] and Cu precipitate [4] in body centred crystalline (bcc) Fe show that they are sheared by the dislocation, whereas in the case of obstacles of complex dislocation character, e.g. a nanometric dislocation loop, the moving dislocation reacts with it, which may result in a sessile segment and/or form a secondary loop around it, which consequently changes its character and morphology [13][14][15][16]. ...
In this study, we report our observation of the interaction between a moving dislocation and an obstacle of dislocation character in pure bcc-Fe using a TEM In situ straining experiment at room temperature. Our observations reveal that the interaction of a ½1 1 1 dislocation with a dislocation debris leads to a dislocation loop at the obstacle by Orowan mechanism. Hence, the strength of these obstacles is estimated to be in the range of Gb/L, which is higher than the strengthening induced by different nanometric defects of similar size.
... To rationalize the formation of clear bands in bcc Fe, molecular dynamics (MD) modelling was applied to address the interaction of moving dislocations with dislocation loops [13][14][15][16][17][18]. The absorption of dislocation loops was explained on the basis of reactions between different dislocation segments observed using dislocation-core analysis in atomistic simulations. ...
This work closes a series of molecular dynamics studies addressing how solute/interstitial segregation at dislocation loops affects their interaction with moving dislocations in body-centred cubic Fe-based alloys. We consider the interaction of 〈100〉 interstitial dislocation loops decorated by different numbers of carbon atoms in a wide temperature range. The results reveal clearly that the decoration affects the reaction mechanism and increases the unpinning stress, in general. The most pronounced and reproducible increase of the unpinning stress is found in the intermediate temperature range from 300 up to 600K. The carbon-decoration effect is related to the modification of the loop–dislocation reaction and its importance at the technologically relevant neutron irradiation conditions is discussed.
... Besides studies in pure Fe of the interaction of an edge dislocation with dislocation loops [70][71][72][73] and voids [60,74], performed using early interatomic potentials, also the Fe-Fe potential used in OLS (very similar to the one used in BON) has been extensively used to investigate dislocation/defect interaction, in the absence of Cr [56,[75][76][77][78]. The latter studies, which represent the unavoidable reference for studies in Fe-Cr, cover the interaction of both edge and screw dislocations with loops and voids, although not all combinations of type of dislocation and type of defect have been investigated in detail. ...
... It has been demonstrated that deformation twinning at low temperature governs the plastic deformation, particularly in the case of low and medium stacking fault energy materials like copper [10]. In contrast to FCC materials, experimental studies on low-temperature deformation behavior of BCC materials are limited111213. However, the possibilities and concerned mechanisms have been investigated theoretically [14, 15]. ...
Taylor & Francis makes every effort to ensure the accuracy of all the information (the "Content") contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content.
A classical crystal plasticity formulation based on dislocation slip was extended to include the mechanisms of dislocation channelling, with associated strain softening which is observed in many alloys post irradiation. The performance of the model was evaluated against experimental data on Zircaloy-4, which included engineering stress-strain response and high-resolution digital image correlation strain mapping. Variants of the model were developed to evaluate the influence on the strain hardening law used, comparing hardening based on a linear relationship for effective plastic strain with that based on the evolution of geometrically necessary dislocations. In addition, governing equations for simulating the interaction between gliding dislocations with various types of irradiation defect were investigated; this included the comparison of isotropic and anisotropic interactions based on the resultant reaction segments for each interaction. It was shown that the engineering stress-strain response measured by experiment could be captured by many of the model variants, but the simulation of characteristic strain heterogeneity was more sensitive to the model used. Direct modelling of the HRDIC experiments indicated that the model successfully predicted the activation of slip systems in many cases and exhibited localised strain distribution as observed in the experiment. In all models localised kink band formation was predicted, which is not observed experimentally, which highlights limitations in modelling of softening materials with a local crystal plasticity approach and a required area of development going forward.
The crystalline/amorphous dual-phase structure is a new design strategy proposed in recent years to achieve high strength and excellent toughness of high-entropy alloys (HEA). Here, molecular dynamics simulation is used to investigate the effect of amorphous nanopillar size, amorphous nanopillar spacing and temperature on the behavior of extended dislocation overcoming amorphous obstacle in the HEAs. The results indicate that the introduction of amorphous nanopillar can improve the strength the HEA, and the larger the amorphous nanopillar size, the more obvious the strengthening effect. It is worth noting that two stress peaks of the stress-strain curve of the HEA containing amorphous nanopillar correspond to the maximum shear stress required for the leading and trailing dislocations to break away from amorphous nanopillar pinning, while the two peaks of the HEA without amorphous nanopillar represent the shear stress required to drive the leading and trailing dislocations to reach the maximum velocity.
The interactions of a 1/2<110>{111} edge dislocation with a void in Fe10Ni20Cr and Fe33Ni33Cr concentrated solid-solution alloys are investigated by using molecular dynamics simulation method. The edge dislocation dissociates into two shockley partial dislocations with stacking fault in both alloys. Compared to the Fe10Ni20Cr alloy, the dislocation motion becomes difficult in Fe33Ni33Cr alloy due to the big fluctuation of stacking fault energy. The obstacle strength for a void with diameter of 2 nm in Fe33Ni33Cr alloy is slightly weaker than that in Fe10Ni20Cr alloy at temperature range from 300K to 800K. It is attributed to easy vacancies migration in Fe33Ni33Cr alloy. Interestingly, a significant increase of obstacle strength for void at 900K is noted only in Fe10Ni20Cr alloy because of the transformation from void to stacking fault tetrahedra (SFT). It is found that the compress strain upon the edge dislocation glide plane promotes the transformation from void to SFT at 900K in Fe10Ni20Cr alloy. While in Fe33Ni33Cr alloy, sluggish diffusion induced by atomic-level heterogeneity suppresses the transition from void to SFT. Consequently, Fe33Ni33Cr concentrated solid-solution alloy exhibits better irradiation hardening resistance than Fe10Ni20Cr alloy, especially at high temperature.
This work aims to reproduce the individual interactions between screw dislocations and radiation-induced loops using dislocation dynamics in good agreement with molecular dynamics simulations. Such agreement is characterized by reproducing the dynamics of the reaction and obtaining the critical resolved stress to overcome the obstacles. This approach provides the mean to calibrate our dislocation dynamics code with parameters from the molecular dynamics simulations. Consequently, it permits to perform massive simulations at the mesoscopic scale. In this scope, this work consists of two parts, an identification of the energetic model and identification of elementary mechanisms. In the first part we propose a procedure to calibrate the line tension based on Orowan's mechanism using a sensibility study. In the second part, we have identified the cross-slip and twining/anti-twinning mechanisms to be essential to reproduce the individual dislocation-loop interactions. The dislocation dynamics simulations are done using a 3D nodal code called NUMODIS, where the recent developments in this code are presented. The uniqueness of this code is its ability to manage and control collisions and core reactions between dislocation segments. This is done through a set of generic algorithms with the minimum amount of local rules.
The interactions of a 1/2 <111> {110} edge dislocation with nano-sized Cu, Cu-Ni and Cu-Mn precipitates have been investigated by using of molecular dynamics method. It is found that the increase of precipitates size enhances their obstacle strength, while, the rise of temperature causes the reducing of obstacle strength. The results prove that Cu-Mn precipitates will have maximum resistance for dislocation gliding, followed by Cu-Ni and Cu precipitates. It is originated from Mn atoms in Cu-rich precipitate that exhibit attractive to dislocation segment. And Mn atoms can improve the fraction of transformed atoms from bcc structure to 9R structure for Cu-Mn precipitates with a diameter of 4 nm. These will lead to the increase of obstacle strength of Cu-Mn precipitates. For 4 nm Cu-Ni precipitate, the critical resolved shear stress is much bigger than that of 4 nm Cu precipitate, due to Ni atoms promoting the phase transition from bcc to 9R structure. Moreover, the fraction of transformed atoms is inversely proportional to temperature. Eventually, these features are confirmed that the appearance of Ni or Mn atoms enhances the obstacle strength of Cu precipitates for dislocation gliding in bcc Fe matrix, especially for Mn atoms.
The dependence of the interactions of intermediate-size ½<111> self-interstitial atom (SIA) loops with an edge dislocation on strain rate and temperature was investigated by molecular dynamics (MD) simulations for the interatomic potential derived by Ackland et al. (A97). For low temperatures (T = 1 K), the mechanisms of the interactions were in agreement with recent literature. It was shown that a second passing of the dislocation through the loop led to a different mechanism than the one that occurred upon first passing. Since these mechanisms are associated with different SIA loop sizes, and since the loop lost a number of SIAs upon first interaction, it was deduced that the dividing threshold between large and small loops (rendering them strong or weak obstacles, respectively) is at the vicinity of the loop size studied (169 SIAs). For higher temperatures (T = 300 K), the strain rate dependence proved strong: for low strain rates, the dislocation absorbed the loop as a double super-jog almost immediately and continued its glide unimpeded. For a high strain rate, the dislocation was initially pinned due to the formation of an almost sessile segment leading to high critical stress.
The role of edge dislocations as sinks for small radiation induced defects in bcc-Fe is investigated by means of atomistic computer simulation. In this work we investigate by Molecular Statics (T = 0K) the interaction between an immobile dislocation line and defect clusters of small sizes invisible experimentally. The study highlights in particular the anisotropy of the interaction and distinguishes between absorbed and trapped defects. When the considered defect intersects the dislocation glide plane and the distance from the dislocation line to the defect is on the range between 2 nm and 4 nm, either total or partial absorption of the cluster takes place leading to the formation of jogs. Residual defects produced during partial absorption pin the dislocation. By the calculation of stress-strain curves we have assessed the strength of those residues as obstacles for the motion of the dislocation, which is reflected on the unpinning stresses and the binding energies obtained. When the defect is outside this range, but on planes close to the dislocation glide plane, instead of absorption we have observed a capture process. Finally, with a view to introducing explicitly in kinetic Monte Carlo models a sink with the shape of a dislocation line, we have summarized our findings on a table presenting the most relevant parameters, which define the interaction of the dislocation with the defects considered.
Description
Seventeen peer-reviewed papers focus on the latest research on the effects of radiation on nuclear materials. These papers focus on:
Light Water Reactor Sustainability Issues and Programs that address the multiple challenges of extended reactor life, including detailed coverage of irradiation embrittlement of reactor pressure vessel steels.
Synergistic Effects of Gas Atoms (i.e. helium and hydrogen) and Displacement Damage, with an emphasis on the unprecedented challenge to structural and plasma-facing materials in nuclear fusion reactors.
Sections cover
Dislocation interactions with distributed condensed vacancy clusters in fcc metals were simulated via a concurrent atomistic-continuum method. Due to void strengthening, the dislocation lines are found to bow as a result of pinning on the original glide plane and undergo depinning through drawing out screw dipoles and forming prismatic loops on the secondary slip plane. We discovered an inertia-induced transition between Hirsch looping and void shearing mechanisms as the void spacing ranges from the scale of nm to hundreds of nm. Contrary to prior understanding, simulations suggest that large voids (∼5 nm in diameter) can behave as weak barriers to dislocation motions under high strain-rate dynamic conditions.
In this study, we present atomistic mechanisms of 1/2 [111](1 1¯0) edge dislocation interactions with point defects (hydrogen and vacancies) and hydrogen solute atmospheres in body centered cubic (bcc) iron. In metals such as iron, increases in hydrogen concentration can increase dislocation mobility and/or cleavage-type decohesion. Here, we first investigate the dislocation mobility in the presence of various point defects, i.e., change in the frictional stress as the edge dislocation interacts with (a) vacancy, (b) substitutional hydrogen, (c) one substitutional and one interstitial hydrogen, (d) interstitial hydrogen, (e) vacancy and interstitial hydrogen, and (f) two interstitial hydrogen. Second, we examine the role of a hydrogen-solute atmosphere on the rate of local dislocation velocity. The edge dislocation simulation with a vacancy in the compression side of the dislocation and an interstitial hydrogen atom at the tension side exhibit the strongest mechanical response, suggesting a higher potential barrier and hence, the higher frictional stress (i.e., ∼83% higher than the pure iron Peierls stress). In the case of a dislocation interacting with a vacancy on the compressive side, the vacancy binds with the edge dislocation, resulting in an increase in the friction stress of about 28% when compared with the Peierls stress of an edge dislocation in pure iron. Furthermore, as the applied strain increases, the vacancy migrates through a dislocation transportation mechanism by attaining a velocity of the same order as the dislocation velocity. For the case of the edge dislocation interacting with interstitial hydrogen on the tension side, the hydrogen atom jumps through one layer perpendicular to the glide plane during the pinning-unpinning process. Finally, our simulation of dislocation interactions with hydrogen show first an increase in the local dislocation velocity followed by a pinning of the dislocation core in the atmosphe- e, resulting in resistance to dislocation motion as the dislocation moves though the hydrogen-solute atmospheres. With this systematic, atomistic study of the edge dislocation with various point defects, we show significant increase in obstacle strengths in addition to an increase in the local dislocation velocity during interaction with solute atmospheres. The results have implications for constitutive development and modeling of the hydrogen effect on dislocation mobility and deformation in metals.
Zirconium is an important metal for internal components of nuclear reactors, yet there have been few computer simulation studies of the interaction between dislocations and interstitial dislocation loops that are created in this metal by radiation damage. Reaction mechanisms have been simulated in this work by using an interatomic potential developed by Mendelev and Ackland (2007 Phil. Mag. Lett. 87 349) that has been shown to provide a good description of the core structure and glide resistance of dislocations on the principal slip plane
. The interaction of both edge and screw dislocations of the
slip system with small prismatic loops containing up to 156 interstitial atoms has been considered. If a loop intersects the dislocation glide plane it becomes absorbed on the dislocation line in most situations. If it does not intersect the glide plane but has a Burgers vector inclined to that of the dislocation, it glides to and is absorbed by the line in most cases. The obstacle resistance of loops is relatively strong for screw dislocations in comparison with edges, but loop absorption by screws is only temporary.
We investigated the interaction of a brittle crack with low and high angle grain boundaries in BCC Iron by means of atomistic simulations at finite temperatures. It is demonstrated that both low and high angle grain boundaries exhibit resistance to brittle crack propagation. The resistance is controlled by the ability of a grain boundary interface to structurally transform, which can involve grain boundary sliding or the emission of misfit dislocations. Here, we observed deformation mechanisms which generally correspond to those reported in experiments for Fe–Si polycrystals. We show that low and high angle grain boundaries exhibit different intensities of plastic deformation upon interaction with a brittle crack, thereby offering different resistance to its propagation.
In situ straining in a transmission electron microscope was performed in order to investigate dislocation interactions with a prismatic loop, which as a mobile obstacle is expected to be displaced by the strain-field of dislocation prior to physical contact. It was found that when a gliding dislocation approached a critical distance, the prismatic loop was certainly attracted to the dislocation. The captured loop disrupted the dislocation motion and was not dragged along with the mobile dislocation. Instead, the dislocation bypassed the loop via cross-slip to another slip plane with a resolved shear stress estimated to be 40% lower than that of the original plane.
The performance of four recent semi-empirical interatomic potentials for iron, developed or used within the FP6 Perfect Project, is evaluated by comparing them between themselves and with available experimental or, more often, density functional theory data. The quantities chosen for the comparison are of specific interest for radiation damage studies, i.e. they concern mainly properties of point-defects and their clusters, as well as dislocations. For completeness, an earlier, widely used (also within the Project) iron potential is included in the comparison exercise as well. This exercise allows conclusions to be drawn about the reliability of the available potentials, while providing a snapshot of the state-of-the-art concerning fundamental properties of iron, thereby being also useful as a kind of handbook and as a framework for the validation of future semi-empirical interatomic potentials for iron. It is found that Mendelev-type potentials are currently the best choice in order to “extend density functional theory” to larger scales and this justifies their widespread use, also for the development of iron alloy potentials. However, a fully reliable description of self-interstitial atom clusters and dislocations with interatomic potentials remains largely an elusive objective, that calls for further effort within the concerned scientific community.
Self-interstitial atom (SIA) type dislocation loop is one of the possible candidates of the so-called matrix damage that causes hardening and embrittlement of blanket structural materials of fusion reactors and/or pressure vessel materials of light water reactors. We present in this paper molecular dynamics computer simulation results on the interactions between an edge dislocation and a SIA loop with Burgers vectors of b = a(o)/2 [1 (1) over bar1] and b = a(o)/2 [(1) over bar 11], respectively, which are introduced in bec-Fe crystal. Then shear stresses of several different magnitudes are applied so that the dislocation moves to meet the SIA loop. General observation is that the SIA loops with diameter of similar to 2 nm can be obstacles to dislocation motion, and the strength as obstacles to dislocation motion depends on applied stress. The origin of the stress dependent strength can be explained athermally using the elastic theory of dislocation interaction. In most cases, the SIA loops are absorbed by the edge dislocations to form a large super-jog after the interactions. This suggests a possibility of localized deformation of irradiated bcc-Fe due to the formation of dislocation channeling.
We present the derivation of an interatomic potential for the iron-phosphorus system based primarily on ab initio data. Transferability in this system is extremely problematic, and the potential is intended specifically to address the problem of radiation damage and point defects in iron containing low concentrations of phosphorus atoms. Some preliminary molecular dynamics calculations show that P strongly affects point defect migration.
Clusters of self-interstitial atoms (SIAs) are formed in metals by high-energy displacement cascades, often in the form of small dislocation loops with a perfect Burgers vector, b. Atomic-scale computer simulation is used here to investigate their reaction with an edge dislocation gliding in α-iron under stress for the situation where b is inclined to the dislocation slip plane. The b of small loops (37 SIAs here) changes spontaneously and the interstitials are absorbed as a pair of superjogs. The line glides forward at critical stress τc when one or more vacancies are created and the jogs adopt a glissile form. A large loop (331 SIAs here) reacts spontaneously with the dislocation to form a segment with b = ⟨100 ⟩, which is sessile on the dislocation slip plane, and as applied stress increases the dislocation side arms are pulled into screw orientation. At low temperature (100 K), the ⟨100⟩ segment remains sessile and the dislocation eventually breaks free when the screw dipole arms cross-slip and annihilate. At 300 K and above, the segment can glide across the loop and transform it into a pair of superjogs, which become glissile at τc. Small loops are weaker obstacles than voids with a similar number of vacancies, large loops are stronger. Irrespective of size, the interaction processes leading to superjogs are efficient for absorption of SIA clusters from slip bands, an effect observed in flow localization.
Clusters of self-interstitial atoms are formed in metals by high-energy displacement cascades, often in the form of small dislocation loops with a perfect Burgers vector. In isolation, they are able to undergo fast, thermally activated glide in the direction of their Burgers vector, but do not move in response to a uniform stress field. The present work considers their ability to glide under the influence of the stress of a gliding dislocation. If loops can be dragged by a dislocation, it would have consequences for the effective cross-section for dislocation interaction with other defects near its glide plane. The lattice resistance to loop drag cannot be simulated accurately by the elasticity theory of dislocations, so here it is investigated in iron and copper by atomic-scale computer simulation. It is shown that a row of loops lying within a few nanometres of the dislocation slip plane can be dragged at very high speed. The drag coefficient associated with this process has been determined as a function of metal, temperature and loop size and spacing. A model for loop drag, based on the diffusivity of interstitial loops, is presented. It is tested against data obtained for the effects of drag on the stress to move a dislocation and the conditions under which a dislocation breaks away from a row of loops.
Irradiation of metals with high-energy atomic particles creates obstacles to glide, such as voids, dislocation loops, stacking-fault tetrahedra and irradiation-induced precipitates through which dislocations have to move during plastic flow. Approximations based on the elasticity theory of defects offer the simplest treatment of strengthening, but are deficient in many respects. It is now widely recognised that a multiscale modelling approach should be used, wherein the mechanisms and strength parameters of interaction are derived by simulation of the atomic level to feed higher-level treatments based on continuum mechanics. Atomic-scale simulation has been developed to provide quantitative information on the influence of stress, strain rate and temperature. Recent results of modelling dislocations gliding under stress against obstacles in a variety of metals across a range of temperature are considered. The effects observed include cutting, absorbing and dragging obstacles. Simulations of 0K provide for direct comparison with results from continuum mechanics, and although some processes can be represented within the continuum treatment of dislocations, others cannot.
The stability and mobility of self-interstitials and small interstitial clusters, In, in α-Fe is investigated by means of calculations performed in the framework of the density functional theory using the SIESTA code. The mono-, di- and tri-interstitials are shown to be made of (parallel) 〈110〉 dumbbells and to migrate by nearest-neighbor translation–rotation jumps, according to Johnson’s mechanism. The 〈111〉 orientation of the dumbbells becomes energetically more favourable for I5 and larger clusters. The performance of a semi-empirical potential recently developed for Fe, including ab initio self-interstitial data in the fitted properties, is evaluated over the present results. The superiority over previous semi-empirical potentials is confirmed. Finally the impact of the present results on the formation mechanism of 〈100〉 loops, observed experimentally in α-Fe is discussed.
According to the production bias model, glissile defect clusters and small dislocation loops play an important role in the microstructural evolution during irradiation under cascade damage conditions. The atomic scale computer simulations carried out in recent years have clarified many questions about the structure and properties of glissile clusters of self-interstitial atoms that are formed directly in the cascade volume. It has been found that such clusters consist of sets of crowdions and are highly mobile in the crowdion direction. Very recently, one-dimensional glide of similar character has been observed in the computer simulation of small vacancy loops in α-Fe. In the present paper we summarise results obtained by molecular dynamics simulations of defect clusters and small dislocation loops in α-Fe(bcc) and Cu(fcc). The structure and stability of vacancy and interstitial loops are reviewed, and the dynamics of glissile clusters assessed. The relevance and importance of these results in establishing a better understanding of the observed differences in the damage accumulation behaviour between bcc and fcc metals irradiated under cascade damage conditions are pointed out.
Structural materials for first–wall and breeding-blanket components will be exposed to 14\3MeV neutrons, plasma particle and electromagnetic radiation. In magnetically confined systems, the operation mode will be quasi–continuous. Typical operatio conditions for next–step devices and demonstration plants will be described. The selection of suitable structural material is based on conventional properties, their resistance to radiation–induced damage phenomena and the additional requiremen of low neutron–induced radioactivity. Presently, low–activating ferritic–martensitic steels, vanadium alloys and ceramic composite are investigated as promising candidates. In the paper the present status of knowledge is reviewed and the critical issue for the different alternatives are elaborated. The necessity of an appropriate neutron source for the testing and qualificatio of the materials under fusion–specific conditions is also stressed.
The behaviour of copper atoms in dilute solution in α-iron is important for the microstructural changes that occur in ferritic pressure vessel steels under fastneutron irradiation. To investigate the properties of atomic defects that control this behaviour, a set of many-body interatomic potentials has been developed for the Fe-Cu system. The procedures employed, including modifications to ensure suitability for simulating atomic collisions at high energy, are described. The effect of copper on the lattice parameter of iron in the new model is in good agreement with experiment. The phonon properties of the pure crystals and, in particular, the influence of the instability of the metastable, bcc phase of copper that precipitates during irradiation are discussed. The properties of point defects have been investigated. It is found that the vacancy has lower formation and migration energy in bcc copper than in α-iron, and the self-interstitial atom has very low formation energy in this phase of copper. The threshold displacement energy in iron has been computed as a function of recoil orientation for both iron-and copper-atom recoils. The differences between the energy for the two species are small.
We study self-interstitial cluster migration properties, such as dimensionality of the motion and activation energy barrier, as functions of the cluster size, by means of molecular-dynamics simulations in bcc-Fe. The atomic interactions are described using a recently proposed potential, fitted to reproduce self-interstitial atom (SIA) configuration energies in close agreement with the results of ab initio calculations. We show that this potential provides a dynamic migration energy for the single SIA in agreement with the experimental value. We also show that, in the case of clusters formed by up to five SIAs, the migration energy decreases with increasing cluster size, but remains higher than previously believed. This is the consequence of the change of the migration mechanism of these small clusters from purely three dimensional (3D) to preferentially one dimensional (1D) and of the fact that these clusters take different configurations during migration, including anomalous ones. While the concept of fast 1D diffusion of large SIA clusters remains valid, the obtained results suggest a revision of both the rapidity and the dimensionality of the motion of small interstitial clusters.
Recently a model has been developed by Osetsky and Bacon to study edge dislocations moving over large distances on the atomic scale. It permits investigation of motion of a dislocation under different conditions of applied shear stress with constant or variable strain rate and temperature, and in the presence of obstacles. In this paper we apply the model to study the motion of an infinite straight but flexible edge dislocation through a row of either voids or coherent copper precipitates in bcc iron. Stress–strain curves, energy barrier profile and strength characteristics of obstacles and other dislocation configuration information have been obtained from the modelling and compared with continuum treatments. Some specific atomic-scale mechanisms associated with strengthening due to voids and precipitates over a range of size have been observed and discussed.
A model for simulating the dynamic behaviour of edge dislocations in metals at the atomic level is presented. The model extends an earlier approach based on an array of edge dislocations periodic in the Burgers vector direction and allows the external action (either shear strain or resolved shear stress), crystal energy, plastic displacement and dislocation position and velocity to be determined unambiguously. Two versions of the model for either static or dynamic conditions, i.e. zero or non-zero temperature, are described. The model is tested for elastic response of a perfect crystal and the atomic properties of a ½111 edge dislocation in a model of bcc Fe. Several examples of dislocation glide behaviour and dislocation–obstacle interactions at zero and non-zero temperature are presented and discussed.
The REVE project (REactor for Virtual Experiments) is an international effort aimed at developing tools to simulate irradiation effects in light water reactors materials. In the framework of this project, a European team developed a first tool, called RPV-1 designed for reactor pressure vessel steels. This article is the third of a series dedicated to the presentation of the codes and models used to build RPV-1. It describes the simplified approach adopted to simulate the irradiation-induced hardening. This approach relies on a characterization of the interactions between a screw dislocation and irradiation-induced defects from molecular dynamics simulations. The pinning forces exerted by the defects on the dislocation were estimated from the obtained results and some hypotheses. In RPV-1, these forces are used as input parameters of a Foreman and Makin-type code, called DUPAIR, to simulate the irradiation-induced hardening at 20 °C. The relevance of the proposed approach was validated by the comparison with experimental results. However, this work has to be considered as an initial step to facilitate the development of a first tool to simulate irradiation effects. It can be improved by many ways (e.g. by use of dislocation dynamics code).
High-energy particle irradiation of low stacking fault energy, face centered cubic (fcc) metals produces significant degradation of mechanical properties, as evidenced in tensile tests performed at or near room temperature. Post-irradiation microstructural examination reveals that approximately 90% of the radiation-induced defects in copper are stacking fault tetrahedra (SFT). Radiation damage is an inherently multiscale phenomenon involving processes spanning a wide range of length and time scales. Here we present a multiscale modeling methodology to study the formation and evolution of defect microstructure and the corresponding mechanical property changes under irradiation. At the atomic scale, molecular dynamics (MD) simulations have been used to study the evolution of high energy displacement cascades, SFT formation from vacancy rich regions of displacement cascades, and the interaction of SFTs with moving dislocations. Defect accumulation under irradiation is modeled over diffusional length and time scales by kinetic Monte Carlo (KMC), utilizing a database of displacement cascades generated by MD. The mechanical property changes of the irradiated material are modeled using three-dimensional dislocation dynamics (DD). Key input into the DD includes the spatial distribution of defects produced under irradiation, obtained from KMC, and the fate of dislocation interactions with SFTs, obtained from MD.
Molecular dynamics simulations of screw dislocations interacting with interstitial Frank loops are performed using specific boundary conditions in a model face-centered-cubic nickel crystal, in a configuration favorable to the formation of a helical turn on the dislocation. Both the interaction mechanism and the pinning stress caused by the defects are studied. In particular, we show (1) that the interactions involve athermal cross-slip events, (2) that the shape of the loop has a strong influence: loops with edges along 〈1 1 0〉 directions are unfaulted while loops with 〈1 2 1〉 edges are just sheared and (3) that the Frank loops are strong obstacles with unpinning reactions involving Orowan processes. The consequences of these observations on clear band formation are discussed.
Strengthening due to voids can be a significant radiation effect in metals. Treatment of this by elasticity theory of dislocations is difficult when atomic structure of the obstacle and dislocation is influential. In this paper, we report results of large-scale atomic-level modelling of edge dislocation–void interaction in fcc (copper) and bcc (iron) metals. Voids of up to 5 nm diameter were studied over the temperature range from 0 to 600 K. We demonstrate that atomistic modelling is able to reveal important effects, which are beyond the continuum approach. Some arise from features of the dislocation core and crystal structure, others involve dislocation climb and temperature effects.
The small, coherent BCC precipitates of copper that form during fast-neutron irradiation of ferritic steels are an important component in the irradiation hardening that occurs during service. The conventional explanation of hardening due to copper precipitates is given in terms of the Russell–Brown (M. Acta Metall. 20 (1972) 969) modulus hardening model, in which precipitates are treated as soft spots in iron. In the present paper, the core structure and energy of a <111> screw dislocation are computed as it approaches a row of precipitates. The results indicate that the hardening observed in experiments is due to the effect of the screw dislocation core on the BCC copper structure rather than elastic interaction. This is a new precipitate strengthening effect. The increase in the flow stress is estimated from the interaction energy between the dislocation and the precipitate row, and the estimated value for precipitates of a size and spacing found in irradiated reactor pressure vessel steels is encouragingly close to that found experimentally.
Ferritic/martensitic steels considered as candidate first-wall materials for fusion reactors experience significant radiation hardening at temperatures below similar to400 degreesC. A number of experimental studies in ferritic alloys, performed at 1/2(111) and (100) in the higher temperatures, have shown the existence of large interstitial loops with Burgers vector 2 bulk, which may provide a significant contribution to the hardening caused during irradiation at lower temperatures. Hardening arises from a high number density of loops, voids and small precipitates, which pin system dislocations, impeding their free glide. In this work, we review the nature of the different interstitial dislocation loops observed in alpha-Fe and ferritic materials, assess the effect of substitutional impurities on migrating 1/2 (111) clusters, and apply atomistic modeling to investigate the mechanisms of formation and growth of (100) loops from smaller cascade-produced 1/2 (111) clusters. The proposed mechanism reconciles experimental observations with continuum elasticity theory and recent MD modeling of defect production in displacement cascades. In addition, the interaction of screw dislocations, known to control the low-temperature plastic response of b.c.c. materials to external stress, with (10 0) dislocation loops is investigated with MD, where the main physical mechanisms are identified, cutting angles estimated and a first-order estimation of the induced hardening is provided. (C) 2003 Elsevier B.V. All rights reserved.
Molecular dynamics (MD) simulations are used to investigate the response of a/2<111> screw dislocation in iron submitted to pure shear strain. The dislocation glides and remains in a (110) plane; the motion occurs exclusively through the nucleation and propagation of double kinks. The critical stress is calculated as a function of the temperature. A new method is developed and used to determine the activation energy of the double kink mechanism from MD simulations. It is shown that the differences between experimental and simulation conditions lead to a significant difference in activation energy. These differences are explained, and the method developed provides the link between MD and mesoscopic simulations.
- Osetsky Yu
- N Bacon
Osetsky Yu N and Bacon D J 2005 Mater. Sci. Eng. A 400/401 374
- Osetsky Yu
- N Bacon
- D J Serra
- A Singh
- S Golubov
Osetsky Yu N, Bacon D J, Serra A, Singh B N and Golubov S I 2000 J. Nucl. Mater. 276 65