Abstract
The effect of shear deformation on Ni 2 MnGa magnetic shape memory alloy has been investigated using ab initio electronic structure calculations. We used the projector-augmented wave method for the calculations of total energies and stresses as functions of applied affine shear deformation. The studied nonmodulated martensite (NM) phase exhibits a tetragonally distorted L2 1 structure with c / a > 1. A large strain corresponding to simple shears in {001}, {100} and {100} systems was applied to describe a full path between two equivalent NM lattices. We also studied {101} shear which is related to twining of NM phase. Twin reorientation in this system is possible, because applied positive shear results in path with significantly smaller energetic barrier than for negative shear and for shears in other studied systems. When the full relaxation of lattice parameters is allowed, the barriers further strongly decrease and the structures along the twinning path can be considered as orthorhombic.
Highlights
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) of several percent [1]
As a measure of the shear deformation we have chosen the value of applied strain İ100||001, which is calculated from the displacement ǻx100 of the top plane (001) relative to the bottom plane (001) of the computational cell divided by their distance
This inequivalency of three shear systems is result of tetragonal symmetry of nonmodulated martensite (NM), because in cubic structure all six mentioned shears will exhibits same results. In this work another shear system {101} is studied. It corresponds to the twin reorientation, because both twin variants of NM structure can be found on this deformation path
Summary
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) of several percent [1] This effect occurs in martensite phase, i.e. below the martensitic transformation temperature at which a high-temperature cubic phase, austenite, transforms to a phase with lower symmetry, martensite. When the monoclinic variation of the simulated supercell is allowed, the (5-2) nano-twinned structure exhibits almost the same total energy as NM structure and the energy of the (3-2) nano-twinned structure is just slightly larger [21] These energies are considerably lower than that of the modulated pseudo-tetragonal and premartensite structures. Whereas the relative stability of different phases has been clarified, the detailed atomistic description of martensitic or intermartensitic transformations is still missing as well as the mechanism of twin reorientation, i.e. twin boundary motion. It allows us to estimate the transformation path and energetic barrier between two equivalent NM lattices as well as the shear stress along the path and ideal shear strength
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