Structural Analysis of Ni-Mn-Sn Ferromagnetic Shape Memory Alloy

Ni-Mn-Sn-based Heusler alloys are considered as one of the new Ga-free FSMAs that can offer an alternative to the more established Ni-Mn-Ga alloy system. These alloys are also known for their inverse magnetocaloric effect. The parent austenite phase is ordered cubic L2₁ but, there are a number of conflicting reports about the structure of martensite in the following range of Sn concentrations (0.125 ≤ x ≤ 0.14). A four-layer modulation of orthorhombic martensitic 4O structure was initially reported in Ni₅₀Mn_37.5Sn_12.5 and Ni₅₀Mn₃₇Sn₁₃ alloys but, later 10M martensite was reportedly observed in the same composition range. In view of this ambiguity, a detailed structural analysis of Ni-Mn-Sn alloy in this composition range is carried out using a combination of techniques namely TEM, XRD and Neutron Diffraction. This article will present results of the structural analyses and provide conclusive evidence of the presence of a 4-layer modulated structure of the internally-faulted orthorhombic martensite. These results will be supported by the computation of the crystallographic parameters of the martensitic transformation by the phenomenological theory.