Berlin 2008 – wissenschaftliches Programm
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MM: Fachverband Metall- und Materialphysik
MM 36: Phase Transitions II
MM 36.3: Vortrag
Donnerstag, 28. Februar 2008, 12:15–12:30, H 0107
Phase transformations of Ni2MnGa shape memory alloy from first principles. — •Matthe’ Uijttewaal, Tilmann Hickel, and Joerg Neugebauer — MPI für Eisenforschung, Düsseldorf, Germany
Ni2MnGa is a typical example of a Heusler alloy that undergoes a martensitic transformation. The high-temperature austenite has a cubic L21 structure, whereas below 200 K the structure is orthorhombically distorted. The transformation is completely reversible despite lattice deformations of up to 10% and large strains connected to this change. Ni2MnGa is attractive for application in actuators and sensors because magnetism can control its phases. Central to the shape memory properties is a soft phonon mode in the austenite, leading to modulated (pre-) martensitic phases. Despite ample research, the physics of this soft mode is not well understood. We, therefore, systematically studied the soft mode in the various phases (austenite, pre-martensite and modulated martensites) and their transitions using ab initio DFT (GGA, PAW). First, phonon spectra were computed in the quasiharmonic approximation. The eigenvectors of the unstable phonon modes were used next to set up the corresponding modulated harmonics in supercell calculations and to identify the stable shuffling structures. Based on the derived double-well potentials we successfully explained the mechanism of the transitions and determined the corresponding temperatures. We conclude that for the pre-martensitic transition the dynamics of the phonons are important. The resulting temperature dependence of the phonon frequencies compares favorably to available neutron scattering experiments for this material.