Large superplastic strain in non-modulated epitaxial Ni–Mn–Ga films

Abstract

The phase transformation and superplastic characteristics of free-standing epitaxial Ni–Mn–Ga stripes are reported. The stripes are prepared by micromachining a 1μm thick Ni–Mn–Ga film sputter-deposited on a single crystalline MgO (100) substrate using optical lithography and a Chromium-based sacrificial layer technology. The stripes are oriented at angles of 0 and 45 degrees with respect to the Ni–Mn–Ga unit cell. Electrical resistance versus temperature characteristics reveal a reversible thermally induced phase transformation between 169∘C and 191∘C. Stress-strain measurements are performed with the stress applied along the [100]Ni−Mn−Ga as well as [110]Ni−Mn−Ga direction. Depending on the orientation, the twinning stress ranges between 25 and 30MPa, respectively. For the [100]Ni−Mn−Ga and [110]Ni−Mn−Ga directions, superplastic behaviour with a strain plateau of 12% and 4% are observed, respectively, indicating stress-induced reorientation of non-modulated martensite variants.