Effect of Severe Plastic Deformation by Torsion on Structure and Properties of Ni_(54)Mn_(21)Ga_(25) and Ni_(54)Mn_(20)Fe_1Ga_(25)

摘要

We report on the effect of severe plastic deformation by torsion under the Bridgman anvil pressure (SPDT) on structure and physical properties of the Ni_(54)Mn_(21)Ga_(25) and Ni_(54)Mn_(20)Fe_1Ga_(25) alloys. Two types of samples were studied: ordered (cast) samples and nanocrystalline samples disordered by SPDT. The thermal expansion, thermo EMF, electro- and magnetoresistivity, Hall effect, and magnetization were measured from 4.2 to 800 K and in magnetic fields of up to 15T. We show that the deviation from the stoichiometric Ni_(50)Mn_(25)Ga_(25) composition shifts the temperatures of the martensitic transition T_M and the magnetic (Curie) transition T_C to room temperature. Large changes in the physical properties near T_m and T_C were discovered. The alloys become amorphous and nanocrystalline after the SPDT treatment. This process is accompanied by the disappearance of the martensitic transformations and of the nanocrystalline modulated substructures and, consequently, by the suppression of the magnetic shape-memory effect. The physical properties of the disordered alloys are also strongly changed. Subsequent annealing at T ≥ 700 K leads to a practically full restoration of the structure and all properties characteristic of the initial cast alloys.