Intermartensitic Transformations in Ni-Mn-Ga Alloys: A General View

Abstract

Off-stoichiometric Ni2MnGa ferromagnetic shape memory alloys undergo a martensitic transformation (MT) from the L21 cubic phase to a martensitic crystal lattice consisting, in the majority of cases, of a periodic stacking sequence of nearly closed-packed planes with periodicity of 5, 7 or 2 planes, denoted as 10M (five layered tetragonal), 14M (seven layered orthorhombic) and 2M (non-modulated tetragonal). In addition to the parent to martensite transformation, Ni-Mn-Ga alloys tend to show stress or temperature induced intermartensitic transformations (IMTs) towards the most stable 2M phase, through the sequences 10M→14M→2M or 14M→2M depending on the first formed martensite. The IMTs reported in the literature show a variety of characteristics such as reversibility, completeness, hysteresis and temperature of occurrence, but, as a general trend, the role of internal stresses in favouring the occurrence of IMTs is recognised. Recently it has been shown that the L21 order degree favours the occurrence of the intermartensitic transformation from 14M to 2M martensite, stabilising the non modulated martensite through a decrease of its free energy with respect to the layered martensite. From this point of view, the occurrence of intermartensitic transformations in Ni-Mn-Ga alloys appears as a “chemical“ free energy effect. Aiming to go deeply into this aspect, in this work the occurrence of IMTs and their properties have been examined for an extensive set of off-stoichiometric Ni2MnGa. The results show the existence of a relationship between the IMTs temperatures and the alloys composition, as well as the dependence of all observed IMTs (i.e., 10M→14M, 14M→2M and their corresponding reverse transformations) on the L21 order degree.