Optimization of textured Ni-Mn-Ga magnetic shape memory alloys by hot rolling and postprocessing

In contrast to conventional shape memory materials, single crystals of off-stoichometric Ni₂MnGa are reported to show magnetic field induced strain (MFIS) up to 10% due to twin boundary motion (TBM). The goal of the present paper is to show the development status of Ni-Mn-Ga polycrystalline materials with deformation texture. A crystallographic texture could greatly improve the material response. The hot rolling experiments of polycrystalline Ni-Mn-Ga samples encased by steel cans were successful between the ordering temperature of 750 °C and 1000 °C. Samples    show a weak recrystallization texture with {111} and <112> aligned parallel to the rolling plane and rolling direction, respectively. The texture results are discussed with respect to material, processing and phase transformations. The deformation parameters (deformation degree, temperature, time etc.) were varied to get optimal microstructural conditions and to improve the rolling texture. As result a strong grain aspect ratio in rolling-direction was proved. Modulated martensitic phases (7M, 5M) and twins within the recrystallized grains were found. Only the modulated martensitic phases have a uniaxial magnetic anisotropy and are capable to show MFIS. A new set-up for the MFIS measurements was realized and used for a fast sample characterization. It could be proved during the past few months, that coarse grained polycrystals with solidification texture show MFIS by TBM as large as 1.5% after proper treatment including two step annealing and mechanical training. First results after transfer to fine grained samples textured by deformation and recrystallization are presented.
This work is supported by DFG within SPP 1239.