A phase field study of the grain-size effect on the thermomechanical behavior of polycrystalline NiTi thin films

The grain-size-dependent microstructure evolution and overall mechanical behavior of polycrystalline NiTi thin films have not been widely studied under thermomechanical loading. We propose a non-isothermal phase field model with an additional grain boundary energy term introduced into the local energy density to take into account the contribution from the grain boundary in the polycrystalline system. We investigate the grain-size effects in the process of temperature-induced martensitic transformation, detwinning and reorientation of martensitic variants, and superelastic deformation. This multi-variant two-dimensional phase field study showed that, as the average grain size of the thin film drops from 1.5 μm to 10 nm, the temperature-induced martensitic transformation tends to be inhibited. The polytwinned martensitic microstructure tends to be replaced by single-variant martensite microstructure in the grains. The martensitic transformation will not even take place once the grain size is around 10 nm. Meanwhile, the stress-induced martensitic detwinning and reorientation are less active for all types of martensite variants. The shape memory functionality may be weakened with decreasing grain size. Regarding the superelastic behavior, the stress-induced martensitic transformation is delayed with decreasing average grain size. The stress plateau tends to be higher and less noticeable as the grain size drops. The underlying mechanism of this grain-size effect is associated with the inhibiting effect of the grain boundary, the fraction of which will noticeably increase as the average grain size drops, and thus the phase transformation area will significantly reduce in the polycrystalline thin film.