Fig 11 - uploaded by Lijun Xu
Content may be subject to copyright.
MD simulated the evolution of ''expanded'' fivefold symmetry. (a) The initial cell consists of a threefold and a quasi-fourfold twin connected by GB1. (b) TB3 forms by partial b1 slipping. (c) Kink-like steps cause the motion of node1 and node2. (d) A complete fivefold twin forms at 573 K.
Source publication
The formation mechanisms and grain size dependence of annealing coherent multiple-fold twins, such as twofold and fivefold twins, were investigated in nanocrystalline Cu with zero applied stress by a combination of transmission electron microscopy and molecular dynamics (MD) simulation. It was found that the formation frequency of twofold and fivef...
Similar publications
Conoidal cracks with elliptical bases within cubic crystals are investigated. Under general loading (tension and shears), the crack is represented by a continuous distribution of three dislocation families m (m= 1, 2 and 3). Expressions for the elastic fields (displacement u (m) and stress (σ) (m)) of the crack dislocations with elliptic forms are...
The evolution of microstructure and texture in three non-equiatomic CrMnFeCoNi high-entropy alloys (HEAs) with varying stacking fault energy (SFE) has been studied in up to 90% rolling reductions at both room and cryogenic temperature. All the HEAs deform by dislocation slip and additional mechanical twinning at intermediate and shear banding at hi...
The present work deals with revealing the fatigue and dwell-fatigue behavior and correlated damage mechanisms of Fe-Mn-Al-C lightweight steel. Surprisingly, alteration in loading mode from monotonic to cyclic induces reversible dislocation movement and facilitates the occurrence of dynamic strain aging. Additionally, applying dwell time by an accel...
It has been found that the DO22 Ni3(Cr0.2W0.4Ti0.4) particles play a dominant role in improving the strength of the aged Ni-Cr-W-Ti superalloy. The precipitation of DO22 phase also changed the preferred deformation mode from slip in the as-solutionized alloy into twinning in the aged alloy. Strengthening of the aged superalloy has been determined t...
Citations
... For instance, Liao et al., 12,21 An et al. 22 and Zhu et al. 11 reported that high external stress, an orientation change in applied stress (ball milling and high-pressure torsion), and low experimental temperature were required to activate partial dislocation activities from grain boundaries (GBs) and TBs on different twinning systems, causing FFT formation in nanocrystalline metal materials. Furthermore, Huang et al., 23 Cao et al., 24 and Bringa et al. 25 demonstrated that FFT formed with zero external stress in nanocrystalline metals at high temperature during annealing treatment, where the splitting and migration of a GB segment, 23,26 grain rotation, 24 stacking fault (SF) motion and overlapping, 24 and successive partial emission driven by high internal stresses at TBs and GBs 25 were the dominant mechanisms. However, the formation mechanisms of FFT proposed in these previous studies were all deduced from the post-mortem observation, which fell short in obtaining direct time-resolved atomic-scale observation. ...
... For instance, Liao et al., 12,21 An et al. 22 and Zhu et al. 11 reported that high external stress, an orientation change in applied stress (ball milling and high-pressure torsion), and low experimental temperature were required to activate partial dislocation activities from grain boundaries (GBs) and TBs on different twinning systems, causing FFT formation in nanocrystalline metal materials. Furthermore, Huang et al., 23 Cao et al., 24 and Bringa et al. 25 demonstrated that FFT formed with zero external stress in nanocrystalline metals at high temperature during annealing treatment, where the splitting and migration of a GB segment, 23,26 grain rotation, 24 stacking fault (SF) motion and overlapping, 24 and successive partial emission driven by high internal stresses at TBs and GBs 25 were the dominant mechanisms. However, the formation mechanisms of FFT proposed in these previous studies were all deduced from the post-mortem observation, which fell short in obtaining direct time-resolved atomic-scale observation. ...
... For instance, Liao et al., 12,21 An et al. 22 and Zhu et al. 11 reported that high external stress, an orientation change in applied stress (ball milling and high-pressure torsion), and low experimental temperature were required to activate partial dislocation activities from grain boundaries (GBs) and TBs on different twinning systems, causing FFT formation in nanocrystalline metal materials. Furthermore, Huang et al., 23 Cao et al., 24 and Bringa et al. 25 demonstrated that FFT formed with zero external stress in nanocrystalline metals at high temperature during annealing treatment, where the splitting and migration of a GB segment, 23,26 grain rotation, 24 stacking fault (SF) motion and overlapping, 24 and successive partial emission driven by high internal stresses at TBs and GBs 25 were the dominant mechanisms. However, the formation mechanisms of FFT proposed in these previous studies were all deduced from the post-mortem observation, which fell short in obtaining direct time-resolved atomic-scale observation. ...
A 5-fold twin is usually observed in nanostructured metals after mechanical tests and/or annealing treatment. However, the formation mechanism of a 5-fold twin has not been fully elaborated, due to the lack of direct time-resolved atomic-scale observation. Here, by using in situ nanomechanical testing combined with atomistic simulations, we show that sequential twinning slip in varying slip systems and decomposition of high-energy grain boundaries account for the 5-fold twin formation in a nanoscale gold single crystal under bending as well as the reversible formation and dissolution of a 5-fold twin in a nanocrystal with a preexisting twin under tension and shearing. Moreover, we find that the complex stress state in the neck area results in the breakdown of Schmid's law, causing 5-fold twin formation in a gold nanocrystal with a twin boundary parallel to the loading direction. These findings enrich our understanding of the formation process of high-order twin structures in nanostructured metals.
... In this work, the twins were all growth twins for both the pure Cu and the GNS/Cu composite. For the formation of this kind of twin, the growth accident theory suggests that a coherent twin boundary forms at a migrating grain boundary due to a stacking error [28]. The migration distance and migration velocity of the grain boundary are two key positive factors to annealing twin generation [29]. ...
... In this work, the twins were all growth twins for both t Cu and the GNS/Cu composite. For the formation of this kind of twin, the growth a theory suggests that a coherent twin boundary forms at a migrating grain bound to a stacking error [28]. The migration distance and migration velocity of the grain ary are two key positive factors to annealing twin generation [29]. ...
The strength–ductility trade-off has been a long-standing challenge when designing and fabricating a novel metal matrix composite. In this study, graphene-nanosheets (GNSs)-reinforced copper (Cu)-matrix-laminated composites were fabricated through two methods, i.e., the alternating electrodeposition technique followed by spark plasma sintering (SPS) and direct electrodeposition followed by hot-press sintering. As a result, a Cu-GNS-Cu layered structure formed in the composites with various Cu layer thicknesses. Compared with the pure Cu, the yield strength of the GNS/Cu composites increased. However, the mechanical performance of the GNS/Cu composites was strongly Cu-layer-thickness-dependent, and the GNS/Cu composite possessed a brittle fracture mode when the Cu layer was thin (≤10 μm). The fracture mechanism of the GNS/Cu composites was thoroughly investigated and the results showed that the premature failure of the GNS/Cu composites with a thin Cu layer may be due to the lack of Cu matrix, which can relax the excessive stress intensity triggered by GNSs and delay the crack connection between neighboring GNS layers. This study highlights the soft Cu matrix in balancing the strength and ductility of the GNS/Cu-laminated composites and provides new technical and theoretical support for the preparation and optimization of other laminated metal matrix composites.
... Yet, the formation mechanism of multifold twins remains controversial in the literature. For instance, multifold annealing twins can form as a part of recrystallization process in the absence of applied stress, wherein formation mechanisms, such as successive partial emission (Bringa et al., 2008), grain rotation and SFs overlapping (Cao et al., 2015), as well as GB migration and grain growth (Huang et al., 2009;Thomas et al., 2016), have been proposed. In severe plastic deformations, e.g., ball-milling (Liao et al., 2003a) and high-pressure torsion Zhu et al., 2005), the formation of multifold deformation twins has been attributed to partial dislocation emission from GBs or TBs Cao and Wei, 2006;Zhang et al., 2017;Zhu et al., 2005). ...
... In real deformation, other secondary factors can influence the development of a multifold twin, such that some structures with TB(+), e.g., regions 4 and 5 in Fig. 6g-h, can only developed to a twofold or threefold twin instead of a fivefold one. These factors include: (i) twin network with small twin thickness provides insufficient spaces for further twinning (see region 5 in Fig. 6h); (ii) geometrical constraints or disclination dipoles between two junctions (see region 4 in Fig. 6g) can stabilize the structure to some extent (Roesner et al., 2011); (iii) sessile Lomer-Cottrell dislocation at the intersection can stabilize twofold twins connecting a Ʃ9 GB (see region 5 in Fig. 6h) from further development (Cao et al., 2015); (iv) unfavorable deformation conditions, including loading orientation and local stress condition. On the other hand, GB-TB intersections with TB(-) preferred to evolve into a maximum of threefold twin with a Ʃ27 GB emanating from its core rather than a fourfold twin, no matter how much the GB-TB intersection deviates from the ideal geometry ( Fig. 6g-h, this will be discussed in Section 3.3.3 ...
... Once the first partial was emitted under a high stress concentration (Fig. 7b), it required much low stress to activate another partial on an adjacent plane such that a twin (i.e., T4 in Fig. 7c) formed with the straining. Simultaneously, T5 with a low-angle GB (GB3, ~7 • ) was generated by continuous GB migration (Fig. 7c), which was similar to the multifold twinning mechanism of high-angle GB transformation via a minor grain rotation (Cao et al., 2015). It is also noted that the migration of the left GB segment (GB1) proceeded in a disconnection mode throughout the process (see the yellow arrows). ...
The formation of multifold twins in nanocrystalline face-centered-cubic metals and alloys was frequently ascribed to a grain boundary (GB) mediated process. However, the geometrical requirement for the formation of multifold twins remains largely unclear. In this work, using combined in situ nanomechanical testing and atomistic simulation, we show that multifold twins can be generated through the migration of GBs associated with intersecting twin boundaries (TBs). We systematically evaluate a number of factors, including deformation geometry, loading condition, and coordinated deformation between GB and TB, that influence the underlying formation mechanisms. Such GB migration mediated formation of multifold twins exhibits a strong dependence on TB orientation (polarity) at GB-TB intersections and matrix orientation. Based on experimental observations, a general geometrical model is proposed to further reveal the intrinsic and extrinsic factors controlling the formation of multifold twins, which helps to establish a comprehensive map for the origin of multifold twins in nanocrystalline metals and also provides an accessible guidance for designing multifold twin structures in materials with high strength and good ductility.
... In metallic nanowires with a longitudinal penta-twin, surface-nucleated partial dislocations commonly slip inclined to the axial direction of nanowires, which then propagate and interact with the constituent TBs and the penta-twin core, inducing the formation of SF decahedrons [22,23]. In other loading conditions, penta-twins in synthesized nanoparticles [24,25], as well as the ones in thin films processed through extreme mechanical [26][27][28] or annealing conditions [29,30] in other nanomaterials, are expected to interplay with edge-on dislocations slipping parallel to the common {110} axis. Within the bulk of these penta-twins, dislocation motions and their interactions with the constituent TBs and the penta-twin core are expected to be significantly affected by the intrinsic disclination stress field of penta-twins, inducing some unique interaction features other than the ones in conventional single or coplanar nanotwins. ...
Dislocation interactions with twin boundary (TB) have been well-established in nanotwinned metals. Penta-twins, as an extreme of crystal twinning, are tacitly assumed to be more effective at blocking dislocation motions than conventional single or coplanar nanotwins. However, the mechanism underlying the interactions between dislocations and penta-twins remains largely unclear. Here, by combining in situ transmission electron microscope (TEM) nanomechanical testing and atomistic simulations, we rationalize the fundamental interactions between dislocations and penta-twins in Au nanocrystals. Our results reveal that the interactions between dislocations and penta-twins show some similar behaviors to the ones in the cases of coplanar nanotwins, including dislocation impedance at TBs, cross-slip into the twinning plane and transmission across the TB. In addition, penta-twins also exhibit some unique behaviors during dislocation interactions, including multiple cross-slip, dislocation-induced core dissociation and climb-induced annihilation/absorption at the penta-twin core. These findings enhance our mechanistic understanding of dislocation behaviors in penta-twins, shedding light on the accessible design of high-performance nanomaterials with multi-twinned nanostructures.
... In bulk metals, penta-twins often form under severe plastic deformation 1,2 or annealing conditions associated with grain growth. 3,4 In natural minerals and synthetic nanomaterials, penta-twins become more common, especially in chemically synthesized face-centered cubic (fcc) metallic nanoparticles. 5−7 These unique pentatwins can significantly modify the atomic/electronic structures and thereby stress distributions in nanomaterials, 8,9 providing plenty of room to tune the physical, chemical, and mechanical properties of nanomaterials. ...
... Furthermore, it can be seen from Figure 18c that the lode parameter of the sample during FCE deformation is closer to 0, which means that the shear strain plays the dominant role in FCE. As compared to tensile strain, the shear strain can promote grain refinement and the formation of twin boundaries effectively [46]. In other words, FCE is able to provide much stronger grain refinement ability and improve the generation of twin boundary, hence the strength of FCEed sample can be improved significantly. ...
Bimodal grain structure leads to high strength and strain hardening effect of metallic materials. In this study, an effective approach called flow control extrusion (FCE) is proposed to achieve heterostructures of pure copper. Compared with conventional extrusion (CE), FCE shows much stronger grain refine ability and much weaker grain orientation concentration. The significant grain refinement and heterostructures depend on the severe shear strain from FCE. The heterostructures of sample subject to FCE transfer from bimodal structure to gradient structure with the decrease of temperature, as the grains in the surface of sample are all refined to ultrafine scale. Both these two heterostructures can realize the improvement of strength and strain hardening effect simultaneously.
... In addition, the TB1 is broadened by tens of atomic-layer migration. Considering the higher stability of twofold NT among the multifold NTs, it can be inferred that the occurrence of this twofold is caused by the detwinning process of multifold NTs [42]. In addition, the migration of the twin pole from grain interior to GBs as well as TBs broadening indicate that the detwinning process is overwhelming in NGs with reduced grain sizes. ...
Low-excess energy twin boundary can effectively stabilize the conventional grain boundary. It has been reported that deformation-activated nanotwins in nanograined metals produced by severe plastic deformation techniques can significantly enhance mechanical-thermal stability. However, fabrication, structural evolution and the effect of grain size and twin thickness on the mechanical stability of nanograined-nanotwinned metals, where both the grain size and twin thickness reach the nanometer scale (especially grain size is lower than 40 nm), remain unclear. In this study, a gradient nanostructured layer containing a nanograined-nanotwinned sub-layer region and an extremely refined twin-free nanograined top surface layer with grain size as small as ∼10 nm is achieved on copper by using an ultrahigh-strain rate single point diamond turning technique. High-resolution transmission electron microscope observations, atomistic molecular dynamic simulations, and nanoindetation tests were performed to reveal the size-dependent mechanisms of grain refinement and hardness along the gradient direction. The propensity of deformation multifold twinning is increased firstly in large-size nanograins and then decreased once grain size is below ∼48 nm, finally replaced by detwinning to form extremely fine twin-free nanograins at the topmost surface layer. In other words, both the zero-macrostrain-induced deformation multifold twinning and symmetry-breaking-based detwinning processes can continuously refine nanograins along the gradient direction. Critical grain sizes for deformation multifold twinning and detwinning are discussed. Interestingly, a Hall-Petch strengthening-softening transition is discovered at a critical grain size of ∼30 nm in the gradient nanostructured layer. The softening mechanisms are elucidated to be attributed to the twin thickness effect on deformation mode in nanograined-nanotwinned structures and the pure grain boundary-mediated plasticity in extremely fine twin-free nanograins. A series of critical twin thicknesses for softening in nanograins with different grain sizes are discussed; that is, the smaller the grain size is, the smaller the critical twin thickness will be. This study offers the potential for understanding and developing stable nanostructured metals.
... However, a relatively high stress or high strain rate is needed to activate these mechanisms. Multifold annealing twins have been also reported in nanostructured Cu with zero applied stress [60] . Dislocation mediated grain rotation and SFs overlapping were claimed as the predominant formation mechanisms of the multiple-fold twins. ...
Manipulating the twin morphology to achieve the reinforcement of strength is a great challenge in Al with high stacking fault energy. In this work, the influence of Zr addition on the twin morphology and strengthening response of nanostructured Al films was symmetrically studied. The results showed that, for low Zr addition (≤ 4.0 at.%), the Zr atoms were homogeneously distributed within the matrix, while Zr segregation at grain boundaries was evident at higher Zr addition (> 4.0 at.%). Twins were substantially observed in all the films, and the twin morphology was highly dependent on the Zr addition. In the pure Al film, only twins with a single coherent twin boundary were detected. In comparison, nanotwins with coplanar twin boundaries (C-nanotwins) and 9R phase were predominant in the Al-Zr films with Zr addition ≤ 4.0 at.%. Further raising the Zr content, multiple nanotwins (M-nanotwins) coexisted with the C-nanotwins and 9R phase. In particular, a zero-strain twinning mechanism was applied to account for the C-nanotwins and 9R phase formation, and a zig-zag grain boundary feature induced by Zr segregation was responsible for the M-nanotwin formation. The hardness also exhibited a strong Zr dependence that increased monotonically with the Zr addition. The Al-13.4 at.% Zr film displayed a hardness of ∼4.3 GPa, about 11 times greater than the pure Al film. Strengthening mechanisms were quantitatively evaluated, and the highly-promoted hardness was mainly ascribed to the 9R phase and solid solution strengthening.
... It shows that the Σ3 n type grain boundaries are geometrically related to Σ3 boundaries. Twin-twin interactions/junctions have also been characterized in nanocrystalline materials [12,16,20]. Different twin junctions containing five, four, three and two TBs along with other grain boundaries have been observed. ...
... These studies on nanopillars have shown only the formation of fivefold twins under the bending and torsional loading conditions [21,22]. Further, most of the studies pertaining to the twin-twin interactions in bulk/ nanocrystalline materials were experimental [16][17][18]20], where it is difficult to obtain the atomistic details. In view of this, the aim of the present investigation is to characterize and provide the atomistic mechanisms of how twin-twin interactions can lead to different twin-twin junctions in Cu nanopillars. ...
... However, it is of significant interest to understand the size dependence of twin-twin junctions in nanopillars and nanocrystalline materials over a wide range of sizes. To this, Cao et al. [20] have analysed the formation frequency of different twin-twin junctions with respect to the grain size in nanocrystalline Cu. It has been found that formation frequency increases with decreasing grain size, then reaches a peak around 35-45 nm, and again decreases at lower grain sizes [20]. ...
Twinning is an important mode of plastic deformation in metallic nanopillars. When twinning occurs on multiple systems, it is possible that twins belonging to different twin systems interact and forms a complex twin–twin junctions. Revealing the atomistic mechanisms of how twin–twin interactions lead to different twin junctions is crucial for our understanding of mechanical behaviour of materials. In this paper, we report the atomistic mechanisms responsible for the formation of two different twin–twin interactions/junctions in Cu nanopillars using atomistic simulations. One junction contains two twin boundaries along with one Σ9 boundary, while the other contains five twin boundaries (fivefold twin). These junctions were observed during the tensile deformation of [1 0 0] and [11¯0] Cu nanopillars, respectively.
... It shows that the Σ3 n type grain boundaries are geometrically related to Σ3 boundaries. Twin-twin interactions/junctions have also been characterized in nanocrystalline materials [13,18,22]. Different twin junctions containing five, four, three and two TBs along with other grain boundaries have been observed. ...
... Further, most of the studies pertaining to the twin-twin interactions in bulk/nanocrystalline materials were experimental [18,19,20,22], where it is difficult to obtain the atomistic details. In view of this, the aim of the present investigation is to characterize and provide the atomistic mechanisms of how twin-twin interactions can lead to different twin-twin junctions in Cu nanopillars. ...
... However, it is of significant interest to understand the size dependence of twin-twin junctions in nanopillars and nanocrystalline materials over a wide range of sizes. To this, Cao et al. [22] have analysed the formation frequency of different twin-twin junctions with respect to the grain size in nanocrystalline Cu. It has been found that formation frequency increases with decreasing grain size, then reaches a peak around 35-45 nm, and again decreases at lower grain sizes [22]. ...
Twinning is an important mode of plastic deformation in metallic nanopillars. When twinning occurs on multiple systems, it is possible that twins belonging to different twin systems interact and forms a complex twin-twin junctions. Revealing the atomistic mechanisms of how twin-twin interactions lead to different twin junctions is crucial for our understanding of mechanical behaviour of materials. In this paper, we report the atomistic mechanisms responsible for the formation of two different twin-twin interactions/junctions in Cu nanopillars using atomistic simulations. One junction contains two twin boundaries along with one $\Sigma$9 boundary, while the other contains five twin boundaries (five-fold twin). These junctions were observed during the tensile deformation of [100] and $[1\bar1 0]$ Cu nanopillars, respectively.
