Control property and behavior of nano-structured NiTi SMAs by grain size engineering

We report recent advances in the experimental and theoretical study of effects of grain size (from 1500nm to 10nm) and sample size (from 1 mm to 1 micron) on the thermal, mechanical, cyclic and fatigue properties of NiTi polycrystalline SMAs which have nano-grain and grain-size gradient structures. Particular emphasis is paid to the significant effects of internal and external length scales on the properties and behaviors of the material. It is proposed that the emergence of the observed significant changes in the behavior of the material with the length scale reduction originates from the large increase in the area-to-volume ratios of the interfaces (grain boundary and phase boundary) in the polycrystal and that there is huge potential to improve and control the performance of the existing NiTi SMA by grain size engineering.