Magnetostriction And Structural Characterization Of Polycrystalline Fe-Ga Alloy

Magnetostrictive materials are ideally required to have high strength, good ductility, large magnetostriction at low saturation magnetic field, high magnetomechanical coupling coefficients and low cost for engineering applications. Intermetallic compound, Terfenol-D single crystal exhibits giant magnetostriction λ=2000ppm at room temperature. However, Terfenol-D single crystal is expensive and brittle to be worked. Fe-Ga alloy, as a kind of new magnetostrictive material, was concentrated by people in recent years because it has high mechanical strength, good ductility, large magnetostriction at low saturation magnetic fields, and have low associated cost relative to the materials of Tb-Dy-Fe. Research results indicate that the magnetostriction and microstructure of Fe-Ga alloy are influenced by composition, preparing method and heat treatment.It has been reported that the magnetostriction of Fe_100-xGa_x(4100) versus Ga concentration at room temperature. The unusual double peak dependence of magnetostriction on Ga concentration is interpreted on the basis of a magnetoelaatic energy that increases rapidly for small concentratons up to ～19at% Ga, plus a near-linear softening of the shear elastic constant extending at least to 27.5at% Ga. But the report about the effect of Ga content on the microstructure and magnetostriction of Fe-Ga alloy was not found. The magnetostriction and microstructure of Fe-Ga alloys have hypostatic contact. In this paper, we report the research results of Fe-Ga polycrystalline samples and ribbons with different Ga contents.Polycrystal alloys of Fe-Ga were prepared by inductive melting. Alloys were heated to 800 °C, stabilized for 3 hours and furnace cooled, air cooled and quenched in ice water, respectively. The ribbons of Fe-Ga alloys were prepared by melt-spun with linear velocities of copper wheel about 15m/s under an argon atmosphere. Ribbons were heated to 800 °C, stabilized for 3 hours and furnace cooled. In this dissertation, the influence of crystal structure and process of heat treatment to microstructure and magnetostriction of Fe-Ga alloys have been investigated. The results showed that:1. The microstructure and magnetostriction of Fe-Ga alloys are flexible with the composition and process of heat treatment. When the Ga content is a 19at%, disorder A2 phase is easily kept down at room temperature; When the Ga content increased to 23at%, A2 phase can not be retained just by heat treatment, and DO₃ phase is separated out; When the Ga content up to 27.5at%, A2 phase is disappeared and DO₃ phase is the main phase. The process of heat treatment shows great influence on magnetostriction of Fe-Ga alloys. The Fe₈₁Ga₁₉ alloy gain avalue of saturation magnetostriction λ=85ppm at a low saturation magnetic field not only air cooled but also quenched; The saturation magnetostriction of Fe₇₇Ga₂₃ alloys change no much by difference heat treatment, and the values stay at 54ppm or so; In contrast, the magnetostrictive properties of Fe_72.5Ga_27.5 polycrystal were greatly improved for quenched samples. At a low saturation magnetic field, value of saturation magnetostriction reached nearly 101 ppm.2. Fe₈₁Ga₁₉ alloy ribbons were prepared by melt-spinning. Phase structure and magnetostriction of ribbon samples were investigated. XRD and M-TG results showed that the matrix of Fe₈₁Ga₁₉ ribbons was disordered Fe(Ga) solid solution, accompanied with a small amount of second phase maintaining an unsymmetrical DO₃ structure. [100] preferred orientation was formed along the thickness direction. Maximum magnetostricton of-1830 ppm was obtained when magnetic field was applied normal to the ribbon plane. Annealed at 800 ℃, unsymmetrical DO₃ phase changes orderly and [100] preferred orientation randomizes, and they lead the magnetostricton along the longitudinal direction of anneling Fe₈₁Ga₁₉ ribbons discrease. The shape anisotropy of ribbons, the unsymmetrical DO₃ structure and [100] preferred orientation are suggested to be responsible for the giant magnetostriction.3. From the Fei_100-xGa_x(x=19, 23, 27.5)different appearance mutually microstructure, we conclude that it is complete dieorder D03 phase do great help to gain larger magnetostriction not A2 phase. A2 phase plays a negative role to the magnetostriction of Fe-Ga alloys, and then A2 phase must be restrained to obtain giant magnetostriction.