Large cyclic deformation of a Ni-Mn-Ga shape memory alloy induced by magnetic fields

A single-crystalline ferromagnetic Ni-Mn-Ga shape memory specimen was deformed in the martensitic state by uniaxial compression to a total strain of 2%. The stress-strain curve displays a yield point at 6 MPa followed by jerky flow. After mechanical deformation, magnetomechanical tests in a rotating magnetic field of constant strength as well as experiments in a magnetic field of variable strength and constant direction were performed. Upon field rotation, a cyclic-field-induced strain of 1.8% is obtained repeatedly. Upon repeated field variation with constant field direction, a large field-induced strain occurs only in the first test after a change of the field direction. The force of a magnetic field on a twinning dislocation and the resulting magnetostress are discussed. For small fields, the magnetic force is proportional to the applied field. Above the saturation field, the magnetic stress is constant and equals the ratio of the magnetic anisotropy constant and the twinning shear. A microscopic model relating large field-induced strain to the motion of twinning dislocations is presented and applied to the experiments. It is concluded that cyclic magnetic-field-induced strain can be obtained only by changing the field direction because the magnetic force cannot be reversed otherwise. (C) 2002 American Institute of Physics.