Indentation stress dependence of the temperature range of microscopic superelastic behavior of nickel-titanium thin films

The microscopic superelastic behavior of thin-film NiTi is investigated by instrumented indentation experiments conducted at different temperatures. The indentation-induced superelastic effect is found to be persistent to about 100 K above the austenite transformation finish temperature (Af). In contrast, the upper temperature where superelastic effect exists is only around Af plus 40 K in uniaxial tension and compression tests, beyond which the plasticity of the austenite phase overwhelms the transformation-induced superelasticity. By combining the Clausius-Clapeyron equation and spherical cavity model for indentation, we show that the high hydrostatic pressure under the indenter is capable of elevating the transformation temperatures and increase the upper temperature limit of indentation-induced superelastic behavior.