Twinning stress in shape memory alloys: Theory and experiments

Utilizing first-principles atomistic simulations we present a twin nucleation model based on the Peierls-Nabarro formulation. We investigated twinning in several important shape memory alloys, starting with Ni ₂FeGa (14M modulated monoclinic and L1₀ crystals) to illustrate the methodology, and predicted the twin stress in Ni₂MnGa, NiTi, Co₂NiGa, and Co₂NiAl martensites, all of which were in excellent agreement with the experimental results. Minimization of the total energy led to determination of the twinning stress accounting for the twinning energy landscape in the presence of interacting multiple twin dislocations and disregistry profiles at the dislocation core. The validity of the model was confirmed by determining the twinning stress from experiments on Ni ₂FeGa (14M and L1₀), NiTi, and Ni₂MnGa and utilizing results from the literature for Co₂NiGa and Co ₂NiAl martensites. This paper demonstrates that the predicted twinning stress can vary from 3.5 MPa in 10M Ni₂MnGa to 129 MPa for the B19′ NiTi case, consistent with the experimental results.