Flow Behaviors and Processing Maps of NiTi Shape Memory Alloy with Microstructural Observations on Austenite Phase during Hot Compression

The flow stress–strain curves, strain rate sensitivity (m) and processing maps of Ni_50.8Ti_49.2 shape memory alloy were analyzed in the present study. The microstructure of deformed samples at some processing parameters was observed via optical micrograph (OM) and electron backscatter diffraction (EBSD) techniques. The results showed that an early yield reduction phenomenon were observed at the deformation temperatures 800 and 900 °C and strain rates of 1 and 0.1 s⁻¹, and it extended at 600 °C as the strain rate was 0.01 s⁻¹ due to the noticeable softening effect after escaping the pinning site. The maximum m-value of 0.29 occurred at a deformation temperature of 800 °C due to the occurrence of complete dynamic recrystallization. The instability region of NiTi shape memory alloy increased with increasing strain, in which the austenite grains were obviously elongated and showed an oriented characteristics.