Ultrafine Grained Magnetic Shape Memory Ni₂MnGa - Based Alloys

In this work we report the results of investigations of ferromagnetically ordered Heusler Ni₂MnGa-based alloys. Our recent studies have shown that super-rapid solidification (SRS) from melt and high-pressure torsion could be effectively used to form homogeneous ultrafine grained structures (nano- and submicrocrystalline) and even amorphous state in these alloys. For the first time nanostructured alloys with magnetically controlled thermoelastic martensitic transformation and shape memory effects were produced using combined methods (SRS+HPT+thermal treatment). Various structural states and their thermal stability, and also martensitic transformations have been analyzed by X-ray and electron diffraction and in situ transmission electron microscopy. We identify in submicro-SRS and nano-HPT alloys a premartensitic state, structures of the low-temperature 5M and 7M martensites, their morphology and temperature evolution. The physical properties were determined by measuring electrical resistance, magnetic susceptibility and other characteristics in a wide temperature interval. We also demonstrate that mechanical and shape memory properties can be enhanced by forming ultrafine-grained structures in these alloys.