Investigation of the structural stability of Co_2NiGa shape memory alloys via ab initio methods

摘要

CoNiGa alloys have received considerable interest due to their high-temperature shape memory properties. Although there have been many investigations on the mechanical and magnetic behavior of these materials, little is known about the microscopic basis for the observed macroscopic behavior. In this work, we discuss the stability of Co_2NiGa-based structures. The austenite phase is modeled as fully ordered L2_1 and as partially ordered B2. The stability of the austenite phase with respect to tetragonal distortions is examined. Lattice dynamics calculations suggest that the mechanism for the transformation is different from that of the much more studied and chemically similar Ni_2MnGa shape memory alloy (SMA). The electronic basis for the observed metastability of the cubic Co_2NiGa austenite is found to be qualitatively different from that observed in other ferromagnetic SMAs, especially Ni_2MnGa.