Effects of surface pinning, locking, and adaption of twins on the performance of magnetic shape-memory alloys

We study the effect of surface modifications and constraints on the mechanical properties of Ni-Mn-Ga single crystals, which are imposed by (i) structural modifications near the surface, (ii) mounting to a solid surface, and (iii) guiding the stroke. Spark eroded samples were electropolished and characterized before and after each polishing treatment. Surface damage was then produced with spark erosion and abrasive wearing. Surface damage stabilizes and pins a dense twin-microstructure and prevents twins from coarsening. The density of twins increases with increasing degree of surface deformation. Twinning stress and hardening rate during mechanical loading increase with increasing surface damage and twin density. In contrast, when a damaged surface layer is removed, twinning stresses, hardening rate, and twin density decrease. Constraining the sample by mounting and guiding reduces the magnetic-field- induced strain by locking twins at the constrained surfaces. For single-domain crystals and for hard magnetic shape-memory alloys, external constraints strongly reduce the magnetic-field-induced strain and the fatigue lifetime is short. In contrast, for self-accommodated martensite and for soft magnetic shape-memory alloys, the twin-microstructure adapts well to external constraints and the fatigue lifetime is long. The performance of devices with MSMA transducers requires managing stress distributions through design and control of surface properties, microstructure, and constraints.