The Effect of Low Temperature Aging on Ni-rich Ti-Ni

The effects of aging at temperatures below £250°C were studied using DSC, tensile testing and TEM, finding distinct suppression of Martensite at temperatures as low as 100°C. The mechanical properties exhibited a complex behavior, from no effect on stiffness at 100°C to an increase and then loss of stiffness at higher temperatures and exposure times. At slightly higher temperatures, a distinct separation between the R-phase and B19’ temperature domains were observed. This stabilization of the R-phase in combination with suppression of Martensite, resulted in an R-M-R superelasticity with A_f temperatures above body temperature. Moreover, ds/dT was found to decrease due to the increased scope of the R-phase and attendant decrease in DS accompanying Martensite formation. TEM characterization revealed the existence of Ni clustering along the <111> directions (precursor of Ni4Ti3 precipitates) and tiny Ni4Ti3 precipitates at 100 °C aged samples. These observations can explain the observed shift in phase transformation temperatures for different aging conditions.