Loading rate dependency of maximum nanoindentation depth in nano-grained NiTi shape memory alloy

We examined the effect of loading rate on maximum nanoindentation depth for nano-grained superelastic NiTi shape memory alloy during stress-induced phase transformation. The depth decreases significantly with increasing loading rate. It is argued that the observed rate dependency is attributed to the release and transfer of latent heat during indentation and the temperature dependence of the material's transition stress. Dimensional analysis further shows that the depth is mainly governed by the normalized average stress in the phase transition zone and the ratio of heat conduction time over loading time. Experimental results support the rationale.