Cailiao gongcheng (Jun 2023)
Martensitic transformation and superelasticity of Ni-Mn-Ga-Fe alloy microwires
Abstract
Ni-Mn-Ga and Ni-Mn-Ga-Fe microwires were prepared by high vacuum arc melting furnace and melt drawing liquid forming equipment, and the microwires were subjected to step-ordered heat treatment. The microstructure and phase structure of the microwire were characterized by field emission scanning electron microscopy, transmission electron microscopy and X-ray diffractometer, the martensitic transformation process of the microwires was analyzed by differential scanning calorimeter, and the superelasticity of the microwires was tested by dynamic mechanical analyzer. The results show that the orderly arrangement of atoms inside the microwires is improved after the ordering heat treatment, and the adjacent twin crystals grow in the direction of the grain boundaries at a large angle of nearly 90°, with the grain boundaries being straight. Fe doping can refine the cell structure and improve the structure density. Fe doping increases the number of free electrons in the lattice and increases the concentration of electrons, resulting in a significant increase in the phase transformation temperature (Ms) of martensitic transformation (MT); through the ordering heat treatment, the free electrons at high temperatures are freely arranged, forming new Brillouin zones with differences, constituting new twin boundaries and further increasing the density of the interior of the lattice. At the same time, Fe has the characteristics of high temperature resistance and high tensile strength, and Fe-doped Ni-Mn-Ga microwires reduce their intrinsic brittleness. The superelasticity curves show two basic characteristics of thermoelastic martensitic transformation: complete superelasticity (SE), and low temperature recovery property. In the superelasticity experiments, Ni50Mn25Ga20Fe5 microwires reach complete SE at 355 K; at a test temperature of Ttest >Ms+8 K, 100% strain recovery rate is achieved, which is an improvement over Ni-Mn-Ga microwires (≈90%). Ni-Mn-Ga-Fe fibres show larger critical stress values and a wider SE temperature space than other alloys such as Ti-Ni and Cu-Al-Ni alloys.
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