Dresden 2011 – scientific programme
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MM: Fachverband Metall- und Materialphysik
MM 41: Topical Session Diffussionless Transformations II
MM 41.3: Talk
Thursday, March 17, 2011, 16:45–17:00, IFW D
Ab initio derivation of chemical trends in the phase temperatures of Ni2MnGa-based shape memory alloys — •Ali Al-Zubi, Tilmann Hickel, and Jörg Neugebauer — Max Planck Institut für Eisenforschung, 40237 Düsseldorf
Since martensitic phase transitions are essential for the shape memory effect, their dependence on the chemical composition is of utmost importance for the application of these alloys. In this study we systematically investigated chemical modifications of the magnetic Heusler alloy Ni2MnGa, with a particular focus on an excess Ni content. We combine density functional theory with thermodynamic concepts in order to derive accurate temperature dependencies of the free energies for the austenitic, pre-martensitic and martensitic phase. Vibrational excitations are treated in the quasiharmonic approximation and magnetic excitations are considered in a fixed-spin moment approach, yielding a delicate interplay of these two degrees of freedom. Particular care has been taken to determine the shuffling structures related to soft phonons. Using this approach we were able to successfully describe the phase transitions for different chemical compositions. In very good agreement with experiment, we find an increase of the martensitic phase transition temperature with x in Ni2+xMn1−xGa and revealed a sensitive dependence of this behaviour on the Curie temperature of the system. Further, a strong dependence of the pre-martensitic phase stability on the Ni content was resolved. The information have been combined in temperature-dependent phase diagrams.