Dislocation slip and twinning in Ni-based L1₂ type alloys

We report theoretical results on dislocation slip and twinning in Ni ₃ (Al, Ti, Ta, Hf) compositions with L1₂ crystal structures utilizing first-principles simulations. The lattice parameters of Ni₃Al, Ni₃Al_0.75Ta_0.25, Ni ₃Al_0.5Ta_0.5, Ni₃Ta, Ni₃Ti and Ni₃Al_0.75Hf_0.25 are calculated, and the crystal structures with lower structural energies are determined. We established the Generalized Stacking Fault Energy (GSFE) and Generalized Planar Fault Energy (GPFE), and calculated stacking fault energies APB (anti-phase boundary) and CSF (complex stacking fault) matched other calculations and experiments. Based on the extended Peierls-Nabarro model for slip and the proposed twin nucleation model, we predict slip and twinning stress and the results show a general agreement with available experimental data. The results show that in the studied intermetallic alloys, twinning stress is lower than slip stress; Ta and Hf ternary addition are substantial to increase flow stress in Ni ₃Al. The models proposed in the paper provide quantitative understanding and guidelines for selecting optimal precipitate chemistry and composition to obtain higher mechanical strength in Shape Memory Alloys.