Simultaneous measurement of mechanical and electrical contact resistances during nanoindentation of NiTi shape memory alloys

The nanoindentation technique can be employed in shape memory alloys (SMAs) to discern the transformation temperatures as well as to characterize their mechanical behavior. In this paper, we use it with simultaneous measurements of the mechanical and the electrical contact resistances (ECR) at room temperature to probe two SMAs: austenite (RTA) and martensite (RTM). Two different types of indenter tips – Berkovich and spherical – are employed to examine the SMAs’ indentation responses as a function of the representative strain, εR. In Berkovich indentation, because of the sharp nature of the tip, and in consequence the high levels of strain imposed, discerning the two SMAs on the basis of the indentation response alone is difficult. In the case of the spherical tip, εR is systematically varied and its effect on the depth recovery ratio, ηd, is examined. Results indicate that RTA has higher ηd than RTM, but the difference decreases with increasing εR such that ηd values for both the alloys would be similar in the fully plastic regime. The experimental trends in ηd vs. εR for both the alloys could be described well with a ηd ∝ (εR)−1 type equation, which is developed on the basis of a phenomenological model. This fit, in turn, directs us to the maximum εR, below which plasticity underneath the indenter would not mask the differences in the two SMAs. It was demonstrated that the ECR measurements complement the mechanical measurements in demarcating the reverse transformation from martensite to austenite during unloading of RTA, wherein a marked increase in the voltage was noted. A correlation between recovery due to reverse transformation during unloading and increase in voltage (and hence the electrical resistance) was found.