Dislocation slip stress prediction in shape memory alloys

We provide an extended Peierls–Nabarro (P–N) formulation with a sinusoidal series representation of generalized stacking fault energy (GSFE) to establish flow stress in a Ni2FeGa shape memory alloy. The resultant martensite structure in Ni2FeGa is L10 tetragonal. The atomistic simulations allowed determination of the GSFE landscapes for the (1 1 1) slip plane and image and image slip vectors. The energy barriers in the (1 1 1) plane were associated with superlattice intrinsic stacking faults, complex stacking faults and anti-phase boundaries. The smallest energy barrier was determined as 168 mJ/m2 corresponding to a Peierls stress of 1.1 GPa for the image slip system. Experiments on single crystals of Ni2FeGa were conducted under tension where the specimen underwent austenite to martensite transformation followed by elasto-plastic martensite deformation. The experimentally determined martensite slip stress (0.75 GPa) was much closer to the P–N stress predictions (1.1 GPa) compared to the theoretical slip stress levels (3.65 GPa). The evidence of dislocation slip in Ni2FeGa martensite was also identified with transformation electron microscopy observations. We also investigated dislocation slip in several important shape memory alloys and predicted Peierls stresses in Ni2FeGa, NiTi, Co2NiGa, Co2NiAl, CuZn and Ni2TiHf austenite in excellent agreement with experiments.