It was shown that different surface treatments in combination with repeated mechanical deformation experiments significantly reduce the twinning stress, regardless of whether or not electropolishing was used. The reduction of the twinning stress was 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.

Effects of Surface Roughness and Training on the Twinning Stress of Ni–Mn–Ga Single Crystals. M.Chmielus, K.Rolfs, R.Wimpory, W.Reimers, P.Müllner, R.Schneider: Acta Materialia, 2010, 58[11], 3952-62