Size effects in the superelastic response of Ni ₅₄Fe ₁₉Ga ₂₇ shape memory alloy pillars with a two stage martensitic transformation

The superelastic behavior of Ni ₅₄Fe ₁₉Ga ₂₇ shape memory alloy (SMA) single crystalline pillars was studied under compression as a function of pillar diameter. Multiple pillars with diameters between 10 μm and 200 nm were cut on a single crystalline bulk sample oriented along the [1 1 0] direction as the compression axis and that had undergone fully reversible two stage martensitic transformation, i.e. L2 ₁ austenite to 10M/14M modulated martensite and then to L1 _o martensite. The results revealed an increase in the critical stress for stress-induced martensitic transformation and the yield strength of martensite with decreasing pillar size. The stress hysteresis also increased with the reduction in pillar size and the superelastic response started to diminish below 500 nm pillar diameter. Two-stage martensitic transformation was suppressed for pillar sizes of 1 μm and below, which were shown to exhibit a direct austenite to L1 _o transformation. Such a change in the transformation pathway, i.e. from a two stage to one stage transformation, was also observed in bulk single crystals with increasing temperature. We demonstrated the absence of two stage transformation in bulk at high temperatures. This finding suggests that decreasing the sample size and increasing the temperature have similar effects on the superelastic response of NiFeGa SMAs that had undergone two-stage transformation and indicates that a reduction in pillar diameter decreases the transformation temperature due to the difficulty of martensite nucleation on small scales. The damping coefficients of the pillars were also calculated and the results highlighted that damping capacities higher than those of bulk metallic alloys can be achieved using submicron sized pillars.