Nucleation and growth mechanisms of type II twinning in uranium

Owing to the irrational twin plane for type II twin, the classic twinning mechanism associated with twinning disconnections (TDs) on twin plane is unlikely. Previous works suggest that type II twins form following kinking mechanism by successive gliding of TDs on conjugate twin plane. Pile-up and relaxation of edge TDs with irrational line senses forms irrational twin plane, which can breakup into low-index terraces with equally distributed interfacial disconnections (IDs). Here, we conduct atomic-level study of type II twins in α-U through topological analysis and atomistic simulations. For both type I {112}“<37¯2>” and type II “{17¯2}”<312> conjugate twins, their elementary TDs are ( b ₂ , 2 h _{112}). Their dominant nucleation mechanisms depend on the ratio between nucleation-to-motion of TDs, changing from kinking mechanism associated with type II twinning to glissile mechanism associated with type I twinning. The irrational “{17¯2}” twin boundary (TB) manifests serrated morphology, consisting rational {17¯2} terraces decorated by array of ( b ₂ , 2 h ₍₁7¯₂₎) IDs. The rational terraces form by pile-up of TDs with rational line senses, while the IDs can be treated as TD kinks. The ultra-low kinetic barriers of IDs suggest the associated double-kink mechanism, and result in easy growth of type II twins. Lastly, a crystallographic criterion is proposed to determine rational terraces and the associated rational line senses for TDs. This work provides atomistic insights into twinning mechanism in low-symmetric crystals.