First-principles investigation of the composition dependent properties of Ni_(2+x):Mn_(1-x)Ga shape-memory alloys

摘要

The composition dependent lattice parameter, phase stability, elastic moduli, and magnetic transition temperature of the Ni_(2+x)Mn_(1-x)Ga shape-memory alloys are studied by using the first-principles exact muffin-tin orbital method in combination with the coherent potential approximation. The lattice parameter and tetragonal shear modulus of the cubic L2_1 austenite phase decreases linearly with increasing concentration x of excess Ni atoms. The heats of formation of both cubic L2_1 and tetragonal β''' phases and their difference increase with x, indicating decreasing stability of the cubic and tetragonal phases and increasing driving force for the L2_1 to β''' martensitic transition. Investigating the electronic density of states, we find that the Ni-induced decreasing phase stability can mainly be ascribed to the weakening of the covalent bonding between minority spin states of Ni and Ga. Using the computed parameters, the composition dependence of the martensitic transition temperature is discussed. The theoretical Curie temperature, estimated from the Heisenberg model in combination with the mean-field approximation, is larger for the β''' phase than for the L2_1 phase. For both phases, the Curie temperature decreases nearly linearly with increasing x.