Stabilizing a Fine Twin Structure in Ni–Mn–Ga Samples by Coatings and Ion Implantation

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The surfaces of Ni–Mn–Ga samples were coated with hard TiN and soft elastomer and implanted Nb+ ions. All of these treatments stabilized a dense twin structure. After removing the treated surface layers, the original coarse twin structure reappeared. Compression tests of the ion-implanted sample with fine twins exhibited a continuously increasing, smooth stress–strain curve. Without the surface layer, the stress–strain curve was more rugged and flat, the twinning stress decreased, and one twin boundary moved throughout the entire sample.