Influence of stress aging process on variants of nano-Ni4Ti3 precipitates and martensitic transformation temperatures in NiTi shape memory alloy

Highlights: • Application of stress during age hardening process reduces precipitates variants. • No more than two variants can be found at 5 h-150 MPa stress aged sample. • The precipitates have a coffee-bean morphology in the stress aged NiTi. • The martensite start temperature increases by increasing the tensile stress level of aging. In this study, the effect of a stress aging process on the microstructure and martensitic phase transformation of NiTi shape memory alloy has been investigated. NiTi samples were aged at 450 °C for 1 h and 5 h under different levels of external tensile stress of 15, 60 and 150 MPa. Transmission electron microscopy (TEM) was used to characterize different variants and morphology of precipitates. The results show that application of all stress levels restricts the formation of precipitates variants in the microstructure after 1 h stress aging process. However, all variants can be detected by prolonging aging time to 5 h at 15 MPa stress level and the variants formation is again restricted by increasing the stress level. Moreover, the stress aging process resulted in changing the shape of precipitates in comparison with that of the stress-free aged samples. Coffee-bean shaped morphologies were detected for precipitates in all stress levels. According to the Differential Scanning Calorimetry (DSC) results, the martensite start temperature (M_s) on cooling shifts to higher temperatures with increasing the tensile stress during the aging process. This can be related to the change of austenite to martensite interface energy due to the different volume fractions and variants of precipitates.