Ageing effect and rate dependency of a NiTi shape memory alloy after hydrogen charging

Abstract

In this work, we will be interested in examining the strain rate effect on the mechanical behavior of the Ni–Ti superelastic wires immediately after hydrogen charging and after ageing. Specimens have undergone one cycle of loading–unloading, reaching a stress value of 650MPa. During loading, strain rates from 10−5/s to 10−2/s have been achieved. Immediately after 8h to 16h of hydrogen charging, there has not been any embrittlement detected for the higher tensile strain rate of 10−2/s. In contrast, after 16h of hydrogen charging and loading with the lower strain rate of 10−5/s, the Ni–Ti alloy fractures in a brittle manner during the martensite transformation stage. Moreover, after 8h of hydrogen charging and annealing for 24h at room temperature, the embrittlement occurs in the austenite–martensite transformation for 10−5/s and 10−4/s of the strain rate. These results show that it is difficult to study the strain rate response of the superelastic Ni–Ti wire immediately after hydrogen charging since the diffused hydrogen is more located near the subsurface. However, after ageing, where the whole specimen is affected due to the hydrogen diffusion, the embrittlement of the superelastic Ni–Ti alloy strongly depends on the strain rate. This embrittlement takes place for the low strain rate rather than the higher one.