Training, Constraints, and High-Cycle Magneto-Mechanical Properties of Ni-Mn-Ga Magnetic Shape-Memory Alloys

Magneto-mechanical experiments with a rotating magnetic field of 0.97-T were performed with a Ni-Mn-Ga single crystal. Periodic strains exceeding 1% were recorded over a hundred million magneto-mechanical cycles. The twin microstructure of the cycled crystal was characterized using atomic force microscopy (AFM) and magnetic force microscopy (MFM). In the center of the sample, no twin boundaries were found. At the sample edges, the microstructure shows a dense twin pattern. The results are compared with previous experiments of differently trained crystals. It is useful to distinguish between "ineffective training", which results in a nearly self-accommodated martensite, and "effective training", which results in a nearly single-variant crystal. The evolution of twin structure is discussed in terms of training, magneto-mechanical cycling, and extrinsic constraints imposed by the experimental setting. It is concluded that the response of a magnetic shape memory alloy to an alternating excitation depends strongly on the initial twin microstructure established through training. In particular, ineffective training results in a twin microstructure which can adapt to extrinsic constraints resulting in continued large periodic magnetic-field-induced deformation. In contrast, the twin microstructure of an effectively trained crystal can not adapt to extrinsic constraints resulting in early failure by fracture.