Magnetic Domain-Twin Boundary Interactions in Ni–Mn–Ga

The stress required for the propagation of twin boundaries in a sample with fine twins increases monotonically with ongoing deformation. In contrast, for samples with a single twin boundary, the stress exhibits a plateau over the entire twinning deformation range. We evaluate the twin boundary and magnetic domain boundary interactions for increasing twin densities. As the twinned regions get finer, these interaction regions result in additional magnetic domains that form magnetoelastic defects with high magnetostress concentrations. These magnetoelastic defects act as obstacles for twinning disconnections and, thus, harden the material. Whereas in a low twin density microstructure, these high-energy concentrations are absent or dilute and their effectiveness is reduced by the synergistic action of many twinning disconnections. Therefore, with increasing twin density, the interaction of twin boundaries and magnetic domain boundaries reduces the twin boundary mobility. The defect strength has a distribution such that twinning disconnections overcome soft obstacles first and harder obstacles with ongoing deformation. The width of the distribution of obstacle strength and the density of obstacles increase with increasing twin density and, thus, the hardening coefficient increases with increasing twin density.