The role of strain glass state in the shape memory alloy Ni_{50 +x}Ti_{50 −x}: Insight from an atomistic study

Molecular dynamics simulations are performed to study the influence of Ni concentration on the phase transformations in Ni_{50 +x}Ti_{50 −x}(0 ≤ x ≤ 2) alloys during thermal cycling between 100 K and 500 K. We successfully reproduce the experimental observations that the martensitic transformation temperature and the thermal hysteresis decrease with increasing Ni concentration when x ≤ 1.5, and the thermally induced martensitic transformation is prohibited when x > 1.5. Three types of martensitic twins are observed; increasing disorder from local distortion in martensitic variants is revealed with increasing Ni concentration at 100 K. Furthermore, a strain glass state with disordered local lattice distortion exists when x > 1.5. In addition, it is found that the atomic distributions of the Ni-Ni and Ti-Ni pairs in strain glass can be significantly altered by temperature. Shape memory effect and pseudoelasticity in strain glass are also analyzed. It is found that the shape memory effect can be characterized by the transformation from frozen strain glass to the martensitic B19₁^′twin and the following detwinning, while the pseudoelasticity can be characterized by the transformation from unfrozen strain glass to the B19₁^′phase.