Precipitation and thermal fatigue in Ni–Ti–Zr shape memory alloy thin films by combinatorial nanocalorimetry

Abstract

Thin-film samples of Ni-Ti-Zr shape memory alloys were studied by combinatorial nanocalorimetry to determine the effects of high-temperature (900°C) heat treatments and low- temperature (450°C) thermal cycling on the characteristics of the martensite transformation. The response of the samples to heat treatments depends on composition and is controlled by a precipitation mechanism. Two precipitate types, a Ti2Ni base phase at low Zr concentration and a Ni10Zr7 base phase at high Zr concentration, affect the martensite transformation characteristics by altering the composition and the stress state of the shape memory phase. Thermal fatigue behavior, induced by thermal cycling, is improved compared to previous results. The most stable sample demonstrates a transformation temperature reduction of just 11°C for 100 cycles. The improved stability of the samples is attributed to the very small grain size of approximately 5-20 nm. The high heating and cooling rates characteristic of nanocalorimeters allowed this study to be performed in a high-throughput manner with efficiencies not previously achieved.