Thermal phonons affecting the long-time evolution of Ni–Mn–Ga martensite under magnetic field

The experimental data illustrating a slow evolution of the microstructure, deformation and magnetization of Ni–Mn–Ga martensite under the stationary magnetic field have been analyzed. The thermal phonons that effectively transform the twin structure of martensite have been considered. A strong interdependence between the twin width and the effectiveness of thermal phonons transforming the twin structure has been predicted theoretically. The temperature dependencies of the square average values of the thermally induced random stress and strain have been computed for the different values of twin width. The twin width providing the availability of time-dependent effects in the Ni–Mn–Ga martensite has been estimated by comparison of the effective random stress with the stress, which is sufficient for the transformation of twin structure. The estimated twin width proved to be of the order of 0.1 μm. It was concluded, therefore, that the experimentally observed time evolution of the elastic twins of 10 μm width is a secondary effect accompanying a thermally induced transformation of the submicron elements of martensite microstructure.