Shear Deformation of Nonmodulated Ni$_2$MnGa Martensite: An Ab Initio Study

TL;DR

This study uses ab initio simulations to analyze shear deformation in non-modulated Ni2MnGa martensite, revealing the role of nanotwins and their impact on structural stability and transformation mechanisms.

Contribution

It introduces a new advanced procedure to calculate the generalized planar fault energy and elucidates the role of nanotwins in martensitic transformations.

Findings

Nanotwins have lower energy than defect-free structures.

Shear barriers decrease with lattice modulation.

Stability of NM lattice increases with Mn content.

Abstract

The impact of shear deformation in $(1\,0\,1)[1\,0\,\bar{1}]$ system of non-modulated (NM) martensite in Ni$_2$MnGa ferromagnetic shape memory alloy is investigated by means of ab initio atomistic simulations. The shear system is associated with twinning of NM lattice and intermatensitic transformation to modulated structures. The stability of the NM lattice increases with increasing content of Mn. The most realistic shear mechanism for twin reorientation can be approximated by the simple shear mechanism, although the lowest barriers were calculated for pure shear mechanism. The energy barrier between twin variants further reduces due to spontaneous appearance of lattice modulation or, in other words, the nanotwins with thickness of two atomic planes. Such nanotwins appear also on the generalized planar fault energy (GPFE) curve calculated using a newly developed advanced procedure and exhibits even lower energy than the defect free NM structure. These nanotwin doublelayers are also basic building blocks of modulated structures and play an important role in intermartensitic transformation.