A Ternary Map of Ni–Mn–Ga Heusler Alloys from Ab Initio Calculations

In the search for new magnetic functional materials, non-stoichiometric compounds remain a relatively unexplored territory. While experimentalists create new compositions looking for improved functional properties, their work is not guided by systematic theoretical predictions. Being designed for perfect periodic crystals, the majority of first-principles approaches struggle with the concept of a non-stoichiometric system. In this work, we attempt a systematic computational study of magnetic and structural properties of Ni–Mn–Ga, mapped onto ternary composition diagrams. Compositional stability was examined using the convex hull analysis. We show that the cubic austenite has its stability region close to the stoichiometric Ni2MnGa, in agreement with experimental data, while the tetragonal martensite spreads its stability over a wider range of Mn and Ni contents. The unstable compositions in both austenite and martensite states are located in the Ga-rich corner of the ternary diagram. We note that simultaneous stability of the austenite and martensite should be considered for potentially stable compounds suitable for synthesis. The majority of compounds are predicted to be ferrimagnetically ordered in both austenitic and martensitic states. The methodology used in this work is computationally tractable, yet it delivers some predictive power. For experimentalists who plan to synthesize stable Ni–Mn–Ga compounds with ferromagnetic order, we narrow the target compositional range substantially.

Analysis of the crystallogeometry of tetragonal martensite in indium-cadmium alloys by the method of electron back scattering diffractions

It was shown in the work using electron back scattering diffractions analysis that when cooling lower than temperature of the martensitic fcc-hct transformation in the alloy In - 4.5 wt% Cd a packet structure is formed consisting of colonies of tetragonal lamella plates. At the next packages there are always three grades of the plates of a tetragonal martensite which are in twinning situation and differing with the direction of the tetragonality axis. Each martensitic plate consists in turn of a set of more shallow submicrotwinning plates alternating in the direction of an axis of tetragonality. On the basis of the obtained data it is possible to conclude that the structure of the package consisting of twinning plates and the structure of the separate martensitic plate consisting, in turn, of submicro-twins submits to the general regularities. At each structural level in group of the neighboring areas the building blocks of three grades differing with the direction of a tetragonality axis are found.