Fundamental Research On The Structures And Performances Of Fe-Ga Alloys With A High Magnetic Field

Magnetostrictive material is a kind of high technology functional material with high conversion efficiencies between energy and information. It has been used widely in underwater sonar acoustic transducers, micro-displacement drivers and some other interrelated fields. Comparing with the commercial Terfenol-D alloys, the novelty Fe-Ga alloys have high mechanical strength, good ductility, large magnetostriction at low saturation fields and high imposed stress levels, and low associated costs. However, the magnetostrictive performances of Fe-Ga alloys are only1/5of the Terfenol-D alloys. The origins of the Fe-Ga magnetostriction and methods to improve its performances are still open issues. The fundamental researches on the structures and performances of Fe-Ga alloys with a high magnetic field are investigated in this thesis under the supports by "the Fundamental Research Funds for the Central Universities"[No. N090309002]. The representative Fe-17at%Ga alloys and Fe-27.5at%Ga alloys were chosen to study the effects of magnetic fields on the solidification and heat treatments. The results are as follows.1) The investigations on the XRD spectrum of Fe-Ga alloys with different conditions (casting, conventional heat treatment, solidification with a magnetic field, heat treatments with a high magnetic field) indicate that the Fe-Ga alloys are composed of disordered α-Fe A2phases. The phases are not preferred textured polycrystal structure. The lattice parameters and relative intensities are changed by the lattice distortion.2) The microstructure of Fe-Ga alloys with different conditions are mainly consisted of A2phases, punctifonn DO3phases and strip Ll1phases. The DO3phases and Ll2phases are changed after different magnetic field treatments. After solidified with a magnetic field, DO3phase and Ll2phase are decreased significantly by the influences of the thermoelectric magnetic convection. However, after heat treatment with a high magnetic field with1000℃, punctiform DO3phase and strip Ll2phase are significantly increased.3) The results of VSM show that Fe-Ga alloys with different magnetic field conditions have almost no magnetic loss with lower cocrcivity and remanence. It is beneficial to the repeated magnetization process. The anisotropy energy of Fe-Ga alloys is very small. The effects of magnetic fields on VSM results are limited.4) The magnetostriction of Fe-Ga casting alloys is lower with22ppm for Fe-17at.%Ga alloy and31ppm for Fe-27.5%Ga alloy. The magnetrictive values after slow cooling are59ppm (17%Ga) and67ppm (27.5%Ga), the quenched are up to74ppm (17%Ga) and99ppm (27.5%Ga). The alloys solidified with a magnetic field have a magnetostrictive of62ppm (17%Ga) and69ppm (27.5%Ga). However, the magnetostrictive values of Fe-Ga alloys after heat treatment under1000℃are reduced, but the saturated magnetic fields are decreased, which is effective to apply with low fields.5) From the point of effects of magnetic fields on magnetic domain, the injected magnetic field during solidification processing is beneficial to form the well-aligned strip magnetic domains.6) From the point of effects of magnetic fields on transform from disordered to ordered phase, the magnetic field during the heat treatments will accelerate the transform, and boost the formation of punctiform DO3phase and strip Ll2phase, wich is the origin of the decreasing magnetostrivtion.