Ab initiostudy of Ni₂MnGa under shear deformation

Abstract

Abstract. The effect of shear deformation on Ni2MnGa 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 L21 structure with c/a > 1. A large strain corresponding to simple shears in <100>{001}, <001>{100} and <010>{100} systems was applied to describe a full path between two equivalent NM lattices. We also studied <10-1>{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.

Ab initio prediction of stable nanotwin double layers and 4O structure in Ni2MnGa

“…We have found a very tiny energy barrier (in order of calculation error) between austenite and 4O. Using the same G-SSNEB procedure to calculate transformation between austenite and NM martensite we obtained the barrier and transformation path corresponding to the tetragonal deformation path already described in literature 11,12,21,35 confirming the validity of this approach. The barrierless transformation between austenite and 4O is not surprising as austenite, unstable at 0 K, transforms easily to low-symmetry ground state due to the Fermi surface nesting [11][12][13][14] and Jahn-Teller effect [15][16][17] .…”

“…1. The transformation can be viewed as application of local shear in (101) plane along the [101] direction 35 . The doublelayer nanotwin reorientation was indicated in recent experimental in-situ study of stress-induced 14M → NM transformation using high resolution TEM 36 .…”

Different <i>Ab Initio</i> Approaches for Doping Descriptions: Tetragonal Deformation of Ni-Mn-Ga Alloys

“…[24]. The SCA and the VCA were performed using the Vienna ab-initio simulation package (VASP) [25] in which the PAW method is implemented [20,26]. The VCA method can be applied only for Co doping in Ni sublattice.…”

“…The employed coherent-potential approximation (CPA) [18,19] is able to provide electronic structure and total energies for alloys with chemical disorder caused by doping elements, but it is not capable to describe short-range interactions around the doping atoms and also force-based relaxations of atomic positions or lattice parameters are problematic. Nonetheless, it can be crucial in complex lattice transformations, e. g. shear deformation, which are necessary for understanding of twin reorientation process [20]. To take these effects into account a supercell approach (SCA) has to be applied within a plane wave method for description of the electron-ion interaction, e. g. the projector-augmented wave (PAW) method [21].…”

Modes and Twinning Stresses of Martensite Variants Rearrangements in Near-Stoichiometric Ni$_{2}$MnGa Single Crystal

“…It is worth to note that the nanotwinned 10M and 14M martensites, which discussed to be transient adaptive phases [16,11,[17][18], were surprisingly stable to mechanical disturbance. The stresses caused intervariant transitions in nanotwinned martensites were insufficient to initiate the intermartensitic transformation of 10M and 14M martensites to perfect NM martensite.…”