Effects of Surface Roughness and Training on the Twinning Stress of Ni–Mn–Ga Single Crystals

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Spark eroding, which is commonly employed to cut samples out of magnetic shape-memory alloy single crystals, produces a rough surface layer. Directly after cutting, the single crystals exhibit a high twinning stress. After removal of a surface layer by electropolishing, the twinning stress reduces significantly and stress–strain curves become serrated. The reduction of twinning stress has previously been attributed to the removal of the defective surface layer. In this work, it is shown that different surface treatments in combination with repeated mechanical deformation experiments significantly reduce the twinning stress, regardless of whether or not electropolishing is used. The reduction of the twinning stress is due to softening that takes place as a mechanical training effect, which occurs with mechanical testing. In addition, the stress–strain curves of samples subjected to different surface treatments differed in so far as the curves of electropolished samples showed serrated flow, while the curves of unpolished samples and those of mechanically polished samples were smooth. Furthermore, the unpolished samples displayed significant hardening at higher strain. Following subsequent mechanical polishing, this hardening reduced to nearly zero, and the average twinning stress decreased another 30–50% to 1.6 MPa and below. For these samples, the twinning stress stayed at a very low level until twinning was complete.