Exceptional Resistance to Functional Fatigue of Nanocrystalline NiTi at Microscale

The effect of grain size on the compressive functional fatigue of nanocrystalline NiTi at microscale is investigated. It is found that the micropillars with smaller grain size are more resistant to functional fatigue than large-grain-size counterparts. The sample with an average grain size of 110 nm shows significant functional fatigue, i.e. an increase in residual strain, reductions of hysteresis and transformation stress as the cycle number increases. The functional degradation is mainly attributed to the transformation-induced dislocations and the residual martensite where the former contributes to irreversible traces of shear bands at the surface of the micropillars and leads to formation of the latter by impeding the reverse transformation of martensite. In contrast, the sample with a small grain size of around 11 nm demonstrates exceptional resistance to functional fatigue and shows stable superelastic stress-strain curve without significant plastic deformation even after being compressed under a maximum cyclic compressive stress of 1.8 GPa for more than ten million cycles. The nanocrystalline NiTi micropillars with ultra-high cycles of reversible phase transformation show promising potential for application in microscale devices.