Non-monotonic grain size dependence of phase transformation behavior in NiTi microscale samples

Abstract Superelastic NiTi shape memory alloy (SMA) cuboidal micropillars (1 μm  × 1 μm  × 3 μm ) with average grain sizes ( GS ) of 26 nm, 127 nm and 421 nm are compressed by flat-tip nanoindentation. Increasing GS from 26 nm to 127 nm enhances the transformation by promoting nucleation and growth of martensite. Nevertheless, by further increasing GS to 421 nm, the overall transformation is significantly suppressed, leading to a non-monotonic variation in transformation stress, recoverable strain and hysteresis loop area with GS . Such strong effects of GS on microscale phase transformation behavior of SMA are quantitatively elucidated by a simple three-length-scale model in this paper.