The motion of twin boundaries in a Ni48Mn27Ga25 (at%) single crystal was investigated by a low-frequency mechanical relaxation spectrum analyser. A distinct internal friction peak was observed when cooling from room temperature to −70C at a cooling rate of about 1K/min in this single crystal with an Ms temperature above 35C. This internal friction was measured as a function of temperature at four frequencies: 0.1, 0.5, 1 and 5Hz. The internal friction peaks were found to shift to high temperature with increasing frequency demonstrating a relaxation process. The activation enthalpy of the process was estimated as 0.6eV according to the Arrhenius equation. Associated with the literature results, the underlying dynamic process was attributed to the movement of partial twin dislocations within the {112} twinning planes. Interaction between twin boundaries and defects during the movement in this magnetically induced shape memory alloy was considered.

Relaxation of Twin Boundaries in a Ni2MnGa Single Crystalline. Z.Peng, X.Jin, T.Y.Hsu, Z.Xu, E.Pagounis: Materials Science and Engineering A, 2008, 481-482, 310-3