Atomistic simulation of deformation twinning in nanocrystalline body-centered cubic U–Mo alloys

Abstract

Deformation mechanisms of the nanocrystalline body-centered cubic U–Mo alloy were investigated through molecular dynamics simulations, focusing on the influences of the grain size and Mo content. Strain-hardening behavior was only observed under small grain sizes. GB-mediated processes and grain rotation played an important part in the deformation of small-sized grains. Grain rotation competed against twinning in the superplastic area while the combination of twinning and grain rotation could promote merging between the grains. The heterogeneous and homogeneous mechanisms of microtwin nucleation were observed. In the small grain-sized system, twins were formed by the continuous emission of Shockley partials from the grain boundaries onto adjacent slip planes. Moreover, two additional twin nucleation mechanisms (including grain boundary splitting and dynamical overlap of the stacking faults) were also identified in larger-sized grains. Twin–GB interactions may lead to the generation of new twins. The increased Mo content in the U–Mo alloy could not only stabilize the bcc phase of γ-U but also promote grain rotation and coalescence. At ambient temperature, stress-induced phase transitions from body-centered cubic (bcc) to face-centered cubic (fcc) and hexagonal close-packed (hcp) structures appeared locally, and the fcc and hcp phase transitions were reversible and irreversible for the applied stress, respectively.