Who: Martin Zeleny, Brno University of Technology, Czech Republic
Place: DIPC, floor 1
Date: Friday, 30 November 2018, 11:00
Much attention has been paid to the Ni-Mn-Ga magnetic shape memory alloys because they exhibit interesting properties such as a giant magnetic field-induced strain (MFIS) . The MFIS is related to the high mobility of twin boundaries in connection with a large magneto- crystalline anisotropy . It occurs below the martensitic transformation temperature at which a high-temperature cubic phase with L21 structure, austenite, transforms to a phase with lower symmetry, martensite. Several types of martensites have been observed in the Ni Mn-Ga system . The modulated five-layered (10M) or seven-layered (14M) phases exhibit MFIS up to 10%. Giant MFIS has never been reported for the third martensitic phase which has a purely tetragonal lattice without modulation (NM) . However, recent electronic structure calculations predict orthorhombic structure with four-layered modulation (4O) as the ground state at 0 K  but so far there is no experimental report.
The martensitic transformation to low-symmetry structures is associated with characteristic features in the electronic DOS . Using first-principles calculations combined with the generalized solid state nudged elastic band method  we determined the minimum energy path leading to the different modulated phases of martensite (4O, 10M, 14M) and to NM phase. In stoichiometric Ni2MnGa alloy there is no energy barrier on the path to the 10M phase. Geometry of the lattice in the initial part of the path confirms that this transformation is driven by a softening of the TA2 phonon branch  corresponding to the shift of (110) planes. In later part of the path individual NM nanotwins are stabilized by band Jahn-Teller effect. Transformation paths to other structures including NM martensite exhibit more or less significant barriers in the beginning, hindering such a transformation from austenite although these structures exhibits lower total energy. This finding corresponds to experiment and demonstrates that the kinetics of the transformation is decisive for the selection of the particular low-symmetry structure of martensite . On the other hand, in off-stoichiometric alloys the barrier-less transformation was found between austenite and NM martensite corresponding to tetragonal distortion of the lattice due to Jahn-Teller effect.
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