Ni-Mn-Ga microwire twist caused by stress-magnetic coupling

A new kind of magneto-mechanical effect of Ni-Mn-Ga microwire is explored by combining stress field-induced twist (SFIT) with magnetic field-induced twist (MFIT) under the condition of stress-magnetic coupling. It has been found that Ni-Mn-Ga shape memory microwire exhibits a large recoverable strain up to 12%, superelastic strain of similar to 9% with critical stress for martensitic transformation similar to 66 MPa. On the basis of excellent mechanical properties, a reversible SFIT angle of 5.2 degrees for the total length (18 cm) of the microwire and a recoverable MFIT angle up to 1250 ''/cm have been achieved. By controlling the stress to make the microwire in different stages during martensitic transformation, the total MFIT angle (excluding SFIT angle) decreases slightly at the stage of austenite elastic deformation, then increases again during martensitic variant reorientation. The MFIT angle changes periodically as a function of the magnetic field magnitude and direction. The relationships between the MFIT angle and the static force, magnetic field, magnetic field torsional angle are summarized in this paper. The coupling coefficient k corresponding to the austenite and martensite is obtained. This phenomenon provides a new way to achieve large twist under the condition of stress-magnetic coupling for smart materials. [GRAPHICS] .