Multiple lengths scale can affect materials’ behavior. Internal and intrinsic lengths such as grain size, grain boundary thickness and phase transformation interface thickness along with external lengths such as sample size are key lengths in shape memory alloys. This study aims to capture the effects of altering such lengths on shape memory alloys phase transformation behavior. Grain sizes ranging between 10 and 421 nm have been fabricated by severe cold rolling and subsequent heat treatment of commercial grade near equiatomic superelastic NiTi sheets. Macroscale pillars of 1 × 1 × 3 mm and microscale pillars with an aspect ratio of 1 to 3 and diameters of 0.5, 1.0 and 2.5 μm were fabricated on each grain size and monotonically compressed at room temperature by a UTM machine an...[ Read more ]

Multiple lengths scale can affect materials’ behavior. Internal and intrinsic lengths such as grain size, grain boundary thickness and phase transformation interface thickness along with external lengths such as sample size are key lengths in shape memory alloys. This study aims to capture the effects of altering such lengths on shape memory alloys phase transformation behavior. Grain sizes ranging between 10 and 421 nm have been fabricated by severe cold rolling and subsequent heat treatment of commercial grade near equiatomic superelastic NiTi sheets. Macroscale pillars of 1 × 1 × 3 mm and microscale pillars with an aspect ratio of 1 to 3 and diameters of 0.5, 1.0 and 2.5 μm were fabricated on each grain size and monotonically compressed at room temperature by a UTM machine and a flat tip diamond Nano indenter respectively. The results show that hysteresis loop area and transformation strain of the microscale pillars of 0.5 and 1.0 μm diameters vary non-monotonically with grain size, first increasing and then decreasing. In contrast, hysteresis loop area and transformation strain monotonically increase with grain size in macroscale pillars and microscale pillars of 2.5 μm diameter. Analysis of the results reveals that the monotonic increasing trend is observed when grain size is increased relative to grain boundary and phase transformation interface thicknesses. This enhances the transformation inside the grains by promoting nucleation and growth of martensite domains. The initial increasing trend observed in the non-monotonic behavior of the pillars is also caused by the aforementioned mechanism, nevertheless, the subsequent decreasing trend is observed once grain size becomes comparable with the sample size. This would decrease the number of grains across the pillars’ diameter which owing to the transformation behavior discrepancy between surface and interior grains, increases the inhomogeneity of the pillars and suppresses the transformation.