Micromechanics of magnetic-field-induced twin-boundary motion in Ni-Mn-Ga magnetic shape-memory alloys

Magnetoplasticity is the plastic response of a material to a magnetic field, usually implying the motion of twin boundaries. Magnetoplasticity has been found in ferromagnets like rare-earth metals and a number of intermetallic compounds. Especially the martensitic phases of Ni₂MnGa have attracted particular attention. In cyclic magneto-mechanical experiments, singly twinned crystals display large cyclic strains but short lifetime, whereas the cyclic strain is smaller in polyvariant crystals but the lifetime is extended. Twinning dislocations (disconnections) are the elementary carriers of twin-boundary motion. Depending on martensite structure, twinning disconnections may dissociate into partial twinning disconnections in various ways. Mutual interaction of twinning disconnections and their interaction with interfaces control magnetic-field-induced deformation and fracture initiation. In singly twinned crystals, twinning disconnections run through the entire crystal, and the full twinning shear is released. In polysynthetically twinned crystals, the path of twinning disconnections is limited, and the twinning shear is released only partially. In coarse-twinned crystals (large twin thickness), the magnetic force on disconnections exceeds the disconnection-disconnection interaction force and large magnetic-field-induced strains are possible. However, large stress concentrations finally lead to fracture. In fine-twinned crystals (thin twins), the disconnection-disconnection interaction force exceeds the magnetic force and suppresses to a large extent twin boundary motion and the formation of strain concentrations.