Single crystals of Ni-Mn-Ga are reported to show magnetic-field-induced strain (MFIS) due to twin boundary motion (TBM). This mechanism is substantially different from the phase transformations responsible for the large strains in the conventional shape-memory alloys. A large uniaxial magnetic anisotropy, a low barrier against twin boundary motion, and an appropriate orientation of the twins are prerequisites to enable field-induced twin boundary motion. Both features depend not only on composition, but also on crystal structure and microstructure. In the last time different routes (hot forming process, bar extrusion) were studied to prepare textured polycrystals. The goal is to establish the conditions under which MFIS can be achieved in Ni-Mn-Ga polycrystals. The plastic deformation experiments of Ni-Mn-Ga samples encased by steel plates were successful above the ordering temperature up to T = 1000 °C. The hot deformed samples were characterized by means of DSC, XRD, electron microscopy using EDX and EBSD. The final thickness reduction of Ni-Mn-Ga samples was about 60 %. The influence of Fe-diffusion processes on the Ni-Mn-Ga samples was very small. The presence of martensitic twins can be clearly seen within the recrystallized grains. Modulated martensitic phases (crystal structure 7M or 5M) were found. The optimization of microstructure using different hot-rolling steps was performed. As result a strong grain aspect ratio in rolling-direction was found. Further, large grain growth can be obtained by heat treatments. The resulted grain size was approximately 500 µm. The measurements of orientational dependence of the magnetization have shown, that the perpendicular direction is the easy axis of magnetization. Further magneto-mechanical treatments (“training”) were performed and are essential to obtain MFIS. The results will be used to prepare improved magnetically driven actuators by efficient preparation routes.

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1. Preparation of textured Ni₄₈Mn₃₀Ga₂₂
2. Structure and mechanical training of 5M and 7M Ni-Mn-Ga magnetic shape memory alloys