Influence of Ni4Ti3 precipitate on pseudoelasticity of austenitic NiTi shape memory alloys deformed at high strain rate

The influence of heat treatment on mechanical properties of austenitic NiTi alloys during impact loading is studied in this paper. Hot-rolled rod samples were annealed at 800 °C for 5 h to obtain homogenized texture. After homogenization, samples were aged at 500 °C and 550 °C for 1–4.5 h. Fine and/or coarse Ni₄Ti₃ precipitates disperse homogeneously or heterogeneously in grain interior and at grain boundaries depending on aging temperatures and time. The high strain rate compressive behaviors of austenitic NiTi SMAs after various heat treatments were investigated using a Kolsky (split Hopkinson) compression bar. The phase transformation temperatures were determined by differential scanning calorimeter (DSC). The microstructures of the samples were studied by scanning electron microscopy (SEM) and electron backscattered diffraction (EBSD). It is observed that the initial stress for phase transformation, work hardening during phase transformation, and transformation temperatures vary with heat treatments. The initial stress for martensitic transformation increases with increasing heating temperature and time. Ni₄Ti₃ precipitates are shown to improve the pseudoelasticity of NiTi SMAs during impact loading.