A geometrical model for grain boundary migration mediated formation of multifold twins

Hierarchical twinning governed by defective twin boundary in metallic materials

“…Eventually, a crystallographic extreme of fivefold twin was generated, consistent with pentatwin structures reported in previous literatures (36,37). However, our in situ experiments provide a direct atomistic understanding of TB defect-induced formation of fivefold twin, which differs fundamentally from previously proposed mechanisms involving either complex GB activities or coincidental collision of TBs under consecutively hanging orientations (26,27,38). MD simulations further confirm this hierarchical twinning mechanism facilitated by TB kinks under tensile loading (fig.…”

“…Unfortunately, the construction processes of these hierarchical twin structures remain largely unknown. Several theories have been proposed, such as the coincidental collision of multiple twinning systems ( 26 ) or the multifold twinning mediated by grain boundary (GB) migration ( 27 ), all of which require highly specific conditions and may not sufficiently account for the frequent occurrence of hierarchical twins. On the other hand, recent experimental and simulation results implied a hierarchical twinning mechanism from the primary TBs ( 28 ), given the ample space for the hierarchical plastic deformation inside the primary twin lamella ( 15 , 29 ).…”

Interactions between Dislocations and Penta-Twins in Metallic Nanocrystals

“…Such stress distribution renders dislocation cross-slip preferred at constituent TBs and the penta-twin core. Nevertheless, the local stress state within a deformed penta-twin is far more complicated than expected [28,39]; therefore, the detailed routes for dislocation cross-slip within a penta-twin are difficult to precisely predict based merely on the rule of Schmid factor.…”

“…Such stress distribution renders dislocation cross-slip preferred at constituent TBs and the penta-twin core. Nevertheless, the local stress state within a deformed penta-twin is far more complicated than expected [28,39]; therefore, the detailed routes for dislocation cross-slip within a pentatwin are difficult to precisely predict based merely on the rule of Schmid factor. The penta-twin core may also act as a preferential site for dislocation annihilation (Figure 4), as experimentally verified in Figure 4a-d The penta-twin core may also act as a preferential site for dislocation annihilation (Figure 4), as experimentally verified in Figure 4a-d and schematically illustrated in Figure 4e-h.…”

“…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.…”