The influence of aging treatment on the microstructural and mechanical behavior by ultra-micro hardness tester in Ni-rich NiTi alloy

The present study aims to assess the superelasticity behavior in Ni-rich NiTi alloy wire produced by rotary forging process. T he thermomechanical process involved four steps of hot working at 800 deg C, two steps of cold working with solution heat treatment at 800 deg C between them, and subsequently a solution heat treatment (950 deg C during 2 hours) followed by aging treatment at 350, 400 and 450 deg C during 30 minutes. X-ray diffraction (XRD) and instrumented ultra-micro hardness testers evaluated the present phases at each aged sample and were compared with their mechanical behavior by ultramicrohardness technique. The results put in evidence the work-hardening effect on a forged condition associated with the final step of cold rotary forging. The solution treatment promotes stress relaxation and precipitate dissolution. The sample heat-treated shows the presence of the precipitated (Ni₄Ti₃) and R phase. The presence of these precipitates is beneficial because precipitation-hardening increases the yield strength of austenite, which in turn contributes to better functional stability. (author)