Effects of grain size on phase transition behavior of nanocrystalline shape memory alloys

We report recent advances in the experimental and theoretical study of grain size (GS) effects on the thermal and mechanical properties of nanostructured NiTi polycrystalline shape memory alloy (SMA). It is shown that when GS < 60 nm, the superelastic stress-strain hysteresis loop area (H) of the polycrystal decreases rapidly with GS and tends to vanish as GS approaches 10 nanometers. At the same time, the temperature dependence of the transition stress also decreases with GS and eventually approaches zero, leading to a wide superelastic temperature window and breakdown of the Clausius-Claperyon relationship. Rate dependence of the stress-strain responses is significantly reduced and the cyclic stability of the material is improved by the nanocrystallization. It is proposed that the emergence of such significant changes in the behavior of the material with GS reduction originate from the large increase in the area-to-volume ratios of the nanometer-thick interfaces (grain boundary and Austenite-Martensite (A-M) interface) in the polycrystal. In particular, with GS reduction, interfacial energy terms will gradually become dominant over the bulk energy of the crystallite, eventually bring fundamental changes in the phase transition responses of the material. Modelling strategy leading to the establishment of quantitative relationships among GS, grain boundary, A-M interfaces and the macroscopic responses of the material are outlined.