Strain field in Ni₂MnGa magnetic shape memory alloys with different twin densities under combined tensile and magnetic loading

This study investigates the strain fields developed in two Ni₂MnGa samples, with fine and coarse twin structures, respectively when loaded in tension and/or with a magnetic field. The strain fields have been recorded using the digital image correlation technique, which allowed for the observation of the strain field over the entire sample as it evolves with load. This allows for visual observation of the evolution of the sample’s twin microstructure. This investigation provides a more comprehensive insight into the localized and bulk material behavior than the traditional strain measurement techniques used in previous studies. The results show that the twin density, the uniformity of the magneto-mechanical loading along the sample, and the presence of pinning sites are all contributing to the profile of the tensile strain field. Particularly, the presence of pinning sites and the emergence of perpendicular twin boundaries along the sample inhibit full variant reorientation and recovery. Both samples showed no visible signs of damage or crack formation during tensile testing, and their magneto-mechanical response in tension and compression was found to be similar, but there is a clear tension/compression asymmetry.