The critical role of grain orientation and applied stress in nanoscale twinning

Abstract

Numerous recent studies have focused on the effects of grain size on deformation twinning in nanocrystalline fcc metals. However, grain size alone cannot explain many observed twinning characteristics. Here we show that the propensity for twinning is dependent on the applied stress, grain orientation and stacking fault energy. The lone factor for twinning dependent on grain size is the stress necessary to nucleate partial dislocations from a boundary. We use bulk processing of controlled nanostructures coupled with unique orientation mapping at the nanoscale to show the profound effect of crystal orientation on deformation twinning. Our theoretical model reveals an orientation-dependent critical threshold stress for twinning, which is presented in the form of a generalized twinnability map. Our findings provide a newfound orientation-based explanation for the grain size effect: as grain size decreases the applied stress needed for further deformation increases, thereby allowing more orientations to reach the threshold stress for twinning.

Segregation-Assisted Plasticity in Ni-Based Superalloys

“…[1][2][3] This is because dislocations, even when dissociated, cannot pass through the ordered lattice without their creation in one form or another. A corollary is a range of interesting but practically important plastic phenomena: anomalous yielding, a substantial strain hardening effect and an anisotropy of tensile/compressive behavior which is non-Schmidian.…”

Relationship between monolayer stacking faults and twins in nanocrystals

“…Taken together, observations and atomic-scale simulations clearly indicate that crystal orientation, nanocrystalline grain size, fault energies or the entire c-surface can influence which of these defects is emitted from grain boundaries [2,[10][11][12][13][14][15][16][17]. In general, when the intrinsic stacking fault energy, c I , is low enough, or the nanocrystal small enough, and/or the crystal orientation is situated to promote nucleation of the second leading partial nucleation over nucleation of the trailing partial, then twinning becomes likely.…”

Understanding dislocation mechanics at the mesoscale using phase field dislocation dynamics

“…Observations from experiment suggest that the propensity for twinning in fcc nanocrystalline materials depends, at least, on grain size and fault energies [31,[44][45][46][47]. For twinning, the grain size must be sufficiently small for partial-mediated slip.…”