Transformation Behavior And Properties Of Ni₅₀Mn₂₅Ga_25-xFe_x Shape Memory Alloy Microwires

In this paper, the continuous polycrystalline alloy microwires were efficiently prepared by melt-extraction technique. The microstructure, martensite transformation behavior, superelasticity, shape-memory effects, magnetic properties and magnetocaloric effects of the as-fabricated and heat-treated Ni-Mn-Ga-Fe microwires were systematically studied. The effects of Fe content on the microstructure, martensite transition and properties were also investigated. The influence of Fe on critical stresses,strain recovery ratios and superelastic strains during the superelastic and shape memory process were analyzed between the as-extracted and heat-treated Ni-Mn-Ga-Fe microwires. Furthermore, the effect of Fe on magnetic entropy change, the relationship between the e/a and positive and negative magnetic entropy change and the related mechanism of direct and inverse magnetocaloric effect were also studied.The results show that the chemical composition variation induced by Fe-doping has significant impacts on the microstructure and phase microstructure at room temperature.The transformation from a parent austenite phase to single martensite phase was realized with the increase of Fe in Ni₅₀Mn₂₅Ga_25-xFe_x microwires. The parent phase and martensite microstructures are determined to be tweed and 7M modulation structure,respectively. The martensite exhibited straight and clear twin boundary after heat-treatment. Based on principle of priority size rules in the Heusler alloy, a axis is increased, while the b and c axises are decreased which result in the contraction of the unit cell volume with increasing of Fe substituting Ga.Martensite transformation behaviors of both as-fabricated and heat-treated microwires show compositional dependence, and the temperature increases of two states were 30.6 and 28.1 K/at.%, respectively. The apparent formula of the transformation temperature and electron density before and after heat treatment are Ms（K）=681（e/a）-4993 and Ms（K）=705（e/a）-5113, respectively. Based on the Hume-Rothery mechanism, the increase of martensite transition temperature is related to both of the increase of e/a and contraction of the unit cell volume. The chemical ordering heat treatment also increased the transition temperatures due to the changes of the ordering degree, and the reduction of the crystal defect and internal stress.Microwires of both before and after heat treatment show good superelasticity and shape memory effect. Results show that the stress induced martensite transition had a good reproducibility and Fe-doping widened the superelastic interval and improved the strain recovery ratio. Complete superelastic recovery appeared in the microwires after chemical ordering heat treatment. The critical stresses of the superelasticity in the Fe-doping microwires increased linearly with temperature, which may be described by the Clausius-Clapeyron relation and show more obvious temperature dependence ofsuperelastic stresses compared with Ni-Mn-Ga single crystals and microwires.Compared with the as-fabricated microwires, the heat treatment microwires showed improved elongation due to the higher degree of atomic order, the decrease in the number of twin variants and enhancement of twin boundary mobility. During the two-way shape memory cycles, two characteristics of thermoelastic martensite transition were displayed in both the as-cast and heat-treatment microwires: reversibility and thermal hysteresis. Compared with other shape memory alloys, the melt-extracted microwires exhibit the least dependence of strain on stress, which is favorable for the micro-devices in which the constant stress output is required.The influence of magnetic field on the transition temperature can be ignored for these Fe-doping wires. The Curie point of Ni-Mn-Ga-Fe microwires keeps stable at³90 K which is ²5 K higher than that of Ni₂ Mn Ga. The magnetization fo the martensite phase exhibited higher magnetocrystalline anisotropy due to its poor crystalline symmetry than the parent phase which has higher symmetry. The saturation magnetization of the microwires decreased with increase of Fe content. Magnetocaloric effects of these microwires were calculated by isothermal magnetization curves. The results show that the positive and negative magnetic entropy changes appear in the same temperature when different fields were applied. The critical field for the maximum magnetic entropy change corresponds to the intersection point from the ferromagnetic martensite to ferromagnetic austenite. Critical field is reduced with the increase of Fe content, which means that the effect of the positive magnetic entropy change is diminished. The direct and inverse magnetocaloric effects were caused by the coupling effect between martensite twin variants and the magnetic moment, which is different from the mechanisms in Gd₂ In alloys. Direct and inverse magnetocaloric effects present compositional dependence. These microwires can act as competitive candidate materials for micro-driven magnetic refrigeration at room temperature.In conclusion, with increasing Fe content, martensite transition temperature, Curie temperature and magnetization of the Ni-Mn-Ga-Fe microwires were improved effectivly compared with ternary alloy. The changes of chemical composition, heattreatment process and stress field have significant influence on the martensite transition while the magnetic field is negligible factor. The Fe-doped Ni-Mn-Ga-Fe microwires show application potentials as outstanding superelastic, shape memory and magnetocaloric materials.