Influence of surface condition and training on the twinning stress of Ni-Mn-Ga magnetic shape-memory alloys

While the effects of composition of Ni-Mn-Ga magnetic shape memory alloys (MSMAs) on structure, thermal, and magnetic properties have been well studied, effects related to the processing of MSMA single crystals have attracted less attention. Annealing, cutting, and surface preparation are known to impact the magneto-mechanical properties but no quantitative data is available. In this work, the influence of the surface roughness on the twinning stress is evaluated. During spark erosion cutting - commonly used to prepare single crystalline samples - a rough surface layer is produced. Directly after cutting, the single crystals exhibit a high twinning stress. After removal of a surface layer through electro polishing, the twinning stress reduces significantly. It is shown here, however, that a reduction of the twinning stress after each of several electro polishing steps is induced by mechanical training. This softening also occurs for unpolished Ni-Mn-Ga single crystals when subjected to the same training procedure. The results are discussed in terms of twin-surface and twin-twin interactions.