Stabilizing a fine twin structure in Ni-Mn-Ga samples by coatings and ion implantation

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.