Multiphase Microstructure and Extended Martensitic Phase Transformation in Directionally Solidified and Heat Treated Ni₄₄Co₆Mn₃₉Sn₁₁ Metamagnetic Shape Memory Alloy

Abstract

Directionally solidified Ni–Co–Mn–Sn alloy shows a multiphase solidification microstructure relatable primarily to the varying Co–Mn/Sn ratio. Thermal treatment at 1220 K lasting for 72 h encourages chemical homogeneity with average stoichiometry of Ni_45.1Co_6.2Mn_37.2Sn_11.5. At room temperature, despite the chemical uniformity, the as-homogenized alloy shows a multiphase microstructure with coexisting L2₁ austenite and 6M and 4O martensite phases. The martensite phase preferentially locates at grain boundaries. The onset of the martensitic transition temperature is estimated at 402 K, which overlaps with the Curie transition of austenite. The martensitic transition appears to initially take place at the grain boundaries and then it extends to low temperature as the volume of the grains transforms to martensite.