Correlation between Hardness Behavior, Shape Memory, and Superelasticity in Ni-Rich NiTi Shape Memory Alloy

The relationship between hardness behavior, shape memory and superelastic properties of an aged Ni-rich NiTi shape memory alloy (SMA) is investigated at various test temperatures. Rockwell hardness test, with a spheroconical indenter, is considered as mechanical loading/unloading test. The influence of the aging temperature on hardness measurements is also discussed. Based on calorimetric analysis, the initial phases to be examined by hardness test are identified (austenite, R phase and austenite + R phase). For lower aging temperatures, it is reported that test temperature has a great effect on hardness, residual indentation depth and thermomechanical behavior. However, for higher aging temperatures, no significant changes are revealed. Compressive superelastic tests are conducted to uncover the correlation between measured Rockwell hardness, forward transformation stress, transformation strain and martensite elastic modulus as function of aging temperature. The findings of this study can be very helpful for the comprehension of the spheroconical Rockwell indentation in the NiTi SMA.