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 12[1̄01],12[1̄10],16[2̄11] and 16[112̄] 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 16[112̄](111) 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.