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![4. Cross-slip mechanisms in FCC crystals. (a) Sequence of events (i), (ii), (iii) of an expanding SF ribbon where the screw component undergoes non-dissociative cross-slip at "S". Shaded area denotes SF plane. Double cross-slip is shown in (iv). Adapted from Ref. [3]. (b) Illustration of the Friedel-Escaig model for dissociative cross-slip where the screw dislocation gliding on the (•••) plane dissociates onto the (•• • •) cross-slip plane. Double cross slip attains when the dislocation cross-slips back to the initial (•••) plane. Figure adapted from Ref. [82].](profile/Javier-Varillas/publication/336876966/figure/fig6/AS:819318958866436@1572352270924/Cross-slip-mechanisms-in-FCC-crystals-a-Sequence-of-events-i-ii-iii-of-an.png)
4. Cross-slip mechanisms in FCC crystals. (a) Sequence of events (i), (ii), (iii) of an expanding SF ribbon where the screw component undergoes non-dissociative cross-slip at "S". Shaded area denotes SF plane. Double cross-slip is shown in (iv). Adapted from Ref. [3]. (b) Illustration of the Friedel-Escaig model for dissociative cross-slip where the screw dislocation gliding on the (•••) plane dissociates onto the (•• • •) cross-slip plane. Double cross slip attains when the dislocation cross-slips back to the initial (•••) plane. Figure adapted from Ref. [82].
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This study aims to investigate the underlying mechanisms which govern the development of dense defect networks in nanoscale crystal plasticity, either under contact and uniaxial loading conditions, with emphasis on the onset of intermittent avalanche phenomena. The investigation is based on a comprehensive set of massive molecular dynamics (MD) sim...
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Citations
... Atomistic simulations based on molecular dynamics (MD) can provide information about plastic deformation mechanisms, although there are limitations in some cases. These simulations are now capable of saving financial and technological resources [1][2][3][16][17][18][19][20][21][22]. ...
... The effect of the crystal orientation is presented in the Supplemental Material (SM) [43], where a sequence is found by every approach used following the characteristic maximum pop-in load for [001] orientation and the minimum one for [111] orientation for BCC metals [2,21,44]. contact mechanics formulation [5,20]: ...
... The results seem to follow the universal linear relationship as [20] 0.844/(1 − ν 2 )a/R i , with R i as the indenter radius for all the approaches with tabGAP results reaching the best agreement, which indicates that the atomic ensembles fulfill coarse-grained, linear elastic contact mechanics and boundary conditions do not affect the simulation dynamics. As observed in Fig. 3, the recorded load curves can be affected by both the spatial arrangement of the surface atoms and the thermally induced atomic vibrations [2]. ...
In this study, we employed molecular dynamics simulations, both traditional and machine learned, to emulate spherical nanoindentation experiments of crystalline W matrices at different temperatures and loading rates using different approaches, such as EAM, EAM with Ziegler, Biersack, and Littmark corrections, modified EAM, analytic bond-order approach, and a recently developed machine-learned tabulated Gaussian approximation potential (tabGAP) framework for describing the W-W interaction and plastic deformation mechanisms. Results showed similarities between the recorded load-displacement curves and dislocation densities, for different interatomic potentials and crystal orientations at low and room temperature. However, we observe concrete differences in the early stages of elastic-to-plastic deformation transition, revealing different mechanisms for dislocation nucleation and dynamics during loading, especially at higher temperatures. This is attributed to the particular features of orientation dependence in crystal plasticity mechanisms and, characteristically, the stacking fault and dislocation glide energies information in the interatomic potentials, with tabGAP being the one with the most well-trained results compared to density functional theory calculations and experimental data.
... Figure 5 vividly shows that this fact is quite true for our investigations for both interatomic potentials. Also, Varilla et al. 31 investigated nanoindentation pile up patterns for Ta oriented in (111) directions. Although the temperature of their simulation (77 K) and their indenter radius (24 nm) is different to our calculations, the pile up patterns they reported is similar to ours due to the BCC geometry of W samples. ...
The design of the next generation of nuclear fusion machines needs efficient Plasma Facing materials (PFMs) that can withstand extreme operating conditions due to the direct interaction with the fusion plasma, and BCC metals can fulfill these requirements, in particular tungsten. However, the understanding of the behavior of these materials at extreme operating conditions, such as irradiation and high temperatures, also depends on the capacity for efficient molecular simulations using appropriate interatomic potentials. In this work, we perform Molecular dynamics (MD) simulations to emulate experimental nanoindentation tests of crystalline pure [111] W by two different Embedded Atom Method(EAM)-based interatomic potentials for describing the interaction of W-W and W-H/W-W, respectively. The characterization of W mechanical properties is done by a detailed analysis of the dislocation nucleation and evolution during the early stages of the elastic to plastic deformation transition. Results leads to similarities between load displacements curves and pileup formation for both MD potentials. However, a discrepancy is observed in the dislocation dynamics and stacking fault formation mechanism during nanoindentation, that is attributed to the difference of lattice constants, thus the Burgers vector magnitude, being directly related to stacking fault and dislocation energies.
... It thus seems that the use of the JE should be validated on a case by case basis. Other counterexamples, such as those of "absolute irreversibility" 20 or of indentation21,22 also suggest that this is the case. That violations of the JE are generically possible in systems that drastically violate the assumptions of the theory has already been pointed out. ...
Derivations of the Jarzynski equality (JE) appear to be quite general, and applicable to any particle system, whether deterministic or stochastic, under equally general perturbations of an initial equilibrium state at given temperature T. At the same time, the definitions of the quantities appearing in the JE, in particular the work, have been questioned. Answers have been given, but a deeper understanding of the range of phenomena to which the JE applies is necessary, both conceptually and in order to interpret the experiments in which it is used. In fact, domains in which the JE is not applicable have been identified. To clarify the issue, we scrutinize the applicability of the JE to a Hamiltonian particle system in a variable volume. We find that, in this case, the standard interpretation of the terms appearing in the JE is not adequate.
... Dislocation interactions in FCC metals: (a) lomer-cottrell lock; (b) Glissile junction (c,d) Collinear junction; (e,g) Illustration of multi-junction formation[163]. ...
Fatigue is one of the main failure mechanism for metallic components. The early stages of fatigue refer to the period before the initiation and propagation of fatigue cracks, and accounts for up to 90% of fatigue life. Therefore, the understanding of fatigue damage mechanisms at the early stages is a key issue to improve the operational lifetime of components. Experimental studies have shed light on the importance of the Persistent Slip Band (PSB) as the sites of plasticity localization and subsequent initiation of cracks. The purpose of this thesis is to contribute to the understanding of the formation of fatigue microstructure at the scale of dislocations using 3D Discrete Dislocation Dynamics (DDD). First, simulations of single slip on Cu single crystal are realized. The formation process of the dislocation related microstructures inside PSBs and the surface roughness evolution are elucidated. Under a large strain (> 10−3), a transformation process of the homogeneously distributed dislocations into the organized PSBs is observed, which can be well explained from the diminution of the shear stress on the cross-slip plane. The stability of the numerical PSB is verified with a decreasing loading and is found to be comparable to experimental results. Besides, the comparison between Cu and AISI 316L austenitic stainless steel confirms the importance of cross-slip probability to the distribution and number of PSBs. Simulations of different double slip combinations are also realized to identify the effect of dislocation interactions on cyclic behavior. In the end, the preliminary cyclic simulations for bi-crystals and aggregates are launched with a newly developed DDD code for poly-crystals.
