Martensitic transformation and mechanical properties of Ni_49+xMn_36–xIn₁₅ (x=0, 0.5, 1.0, 1.5 and 2.0) alloys

Five polycrystalline Ni_49+xMn_36–xIn₁₅ (x=0, 0.5, 1.0, 1.5 and 2) alloys were prepared by triple arc-melting and examined to understand their martensitic transformation and mechanical properties. Martensitic transformation temperatures were determined by differential scanning calorimetry (DSC) and observed to increase with increasing Ni content. Powder X-ray diffraction (XRD) and transmission electron microscopy (TEM) showed that Ni₄₉Mn₃₆In₁₅ is austenitic at room temperature while modulated 7M martensitic structure was observed in other alloys. Different twinning relationships between martensitic variants were revealed by TEM. Reduced elastic modulus and hardness were measured by nanoindentation. For the martensites, the reduced elastic modulus increased as the e/a increases, while hardness did not vary. The austenitic phase exhibited a lower reduced elastic modulus and hardness. A larger scatter in the reduced elastic modulus and hardness was observed for the martensitic phase in conjunction with variants of different orientation. The martensitic transformation behavior and nanoindentation results were also compared with Ni_53+xMn_22–xGa₂₅ (x=0.5, 1.0, 1.8 and 2.5) alloys. For both Ni–Mn–In and Ni–Mn–Ga alloys, the martensitic transformation temperature and reduced elastic modulus increased as the e/a ratio increased.