Microscopic shape memory and superelastic effects under complex loading conditions

The microscopic shape memory (SM) and superelastic (SE) effects of martensitic and austenitic NiTi alloys were probed by instrumented indentation techniques. Both spherical and pyramidal indenters (i.e. Berkovich and Vickers) were used to determine the mechanical response of the NiTi alloys over a wide range of indentation loads and depths. The magnitude of any SM effects was quantitatively characterized by the thermally activated depth recovery ratio of the residual indentation depth. The magnitude of SE was quantitatively characterized by the depth and work recovery ratios obtained from the load-displacement curves. We show that (1) microscopic SM and SE effects exist under complex loading conditions, (2) the magnitude of the SM and SE effects can be rationalized using the concept of the representative strain and maximum strain and (3) instrumented indentation techniques are useful in quantifying SM and SE effects in the micro- and nano-meter length scales.