Plastic deformation of NiMnGa polycrystals

Ferromagnetic materials based on the Heusler alloy Ni 2 MnGa have been recognized as interesting actuating materials. Large strains can be obtained by applying an external magnetic field acting on the reorientation of the mesoscopic structure of martensite. Even if a great part of the investigation has been performed on single crystals, an interest exists for the development of polycrystalline technology. It has been recognized that, in order to balance the strong uncoupling effect due to the large number of fixed interfaces (e.g. the grain boundaries), the polycrystalline sample should have some texture. This should result in some pre-orientation of the twinned martensitic structure and, if possible, in a modification of the random distribution of the easy axis orientation within the material. Strong material texturing can be obtained basically by using directional solidification, rapid solidification techniques or plastic deformation. The effects of directional solidification, and of melt spinning have been evaluated in the past. In the present work, we report on successfully plastic deformation of polycrystalline NiMnGa alloys. In literature, there are few (if any) papers devoted to this topic. The great brittleness of the alloy at room temperature is well known. The rare attempts performed to deform the material are reported as failures. The approach used here is based on the hot working of NiMnGa ingots in special metallic cans filled with Argon and sealed. Hot deformation of a 7 × 7 × 40 mm 3 small slab down to 1 . 25 × 11 . 9 × 130 mm 3 was successful. Even after this severe plastic deformation, it was possible to easily remove the deformed material from the special can. The structural and functional characterization of the final product, which maintained the martensitic transformation, will be presented and discussed.