| Binary Fe-based Fe-X (X=Ga, Mn) alloys exhibit attractive combination of large magnetostriction and high mechanical strength. They offer a good potential for use in actuator, torque sensors, underwater scanning sonar, and so on. In this dissertation, we have systematically investigated the room-temperature phase constitution, microstructure, magnetic domain configuration, magnetization and magnetostrictive properties of Fe81Ga19 alloy. Phase transformation of Fe81Ga19 alloy at different cooling rates has been studied. A precipitation and dissolution process of metastable fcc phase (A1) was found and the effect of phase transformation on the magnetostriction was also revealed, which may provide a significant insight to understand the origin of the enhanced magnetostriction of Fe-Ga alloy and the dependence of magnetostriction on the thermal history. In addition, phase structure and texture dependences of magnetostriction of Fe-Mn alloy were also investigated. It is found that the singleγphase was essential for achieving high magnetostriction in Fe-Mn alloy. Moreover, an enhancement of magnetostriction up to 148% was obtained inγ-Fe50Mn50 alloy with a<110> orientation through cold-rolling and subsequent thermal treatment. Main results are as follows:Heat treatment is helpful for the improvement of magnetostrictive properties of melt-spun Fe81Ga19 ribbons. The matrix of melt-spun Fe81Ga19 ribbons keeps a single A2 structure at room temperature and a<100> preferred orientation is formed along the thickness direction, which becomes stronger with the increase of ribbon thickness. Quenching after annealing at 800℃for 3 h can inhibit the precipitation of second phases and enhance the<100> orientation. A saturation magnetostriction of -189 ppm is obtained in as-quenched Fe81Ga19 ribbons, which becomes 16% larger than the value of -163 ppm obtained in the melt-spun ribbons. A hanging-type device is used to measure the magnetostriction of Fe81Ga19 samples, which can avoid the bending of ribbons along the direction of the applied filed during magnetization, and measurement values are much closer to the intrinsic magnetostrictive strain of Fe81Ga19 ribbons.The room-temperature phase structure of Fe81Ga19 alloy exhibits a strong dependence on the cooling rate. A single A2 phase can be preserved to room temperature by quenching from 800℃, but after slow cooling, Fe81Ga19 alloy exhibits a mixture of A2/D03 or A2/fcc. Compared with D03, fcc phase can only be obtained at slower cooling rates. Studies on the temperature dependence of phase constitution show that this fcc phase precipitates from A2 matrix at 500℃and its volume fraction exhibits a sharp increase at 400℃. However, it begins to dissolve when further decreasing the temperature, which indicates that the fcc phase is metastable below 400℃. Moreover, according to the calculation from XRD, at the same Ga content, the lattice parameter of the metastable fcc phase observed in our work is much larger than that of L12 equilibrium phase. It is considered that this metastable fcc phase may be the y-Fe solid solution (Al). In addition, the formation of fcc phase leads to an enhancement of resistivity of Fe81Ga19 alloy, but weakens the saturation magnetization and magnetostriction.Magnetostrictive properties of Fe81Ga19 alloys with different thermal histories are very sensitive to their magnetic domain structures. Both the as-quenched and air-cooled samples exhibit a strip domain structure. However, in the air-cooled sample, strip domains have a relatively lower degree of alignment than those in the as-quenched one and zigzag strip domains can be found in some regions. With the decrease of cooling rates, in addition to the strip domains, dendrite and plate domains also can be observed in heat-treated Fe81Ga19 alloys. A nearly parallel stripe domain structure is helpful for the enhancement of magnetostriction in Fe81Ga19 alloy.Single y phase is essential for achieving high magnetostriction in Fe-Mn alloy. The as-cast Fe58Mn42 alloy is a single y phase and the magnetostriction reaches 690 ppm at an applied field of 1350 kA/m. Phase separation occurs in Fe58Mn42 alloy during homogenization for 24 h at 1000℃. Both the furnace-cooled and as-quenched samples contain three phases: y phase as matrix,εand a phases as precipitates. Such phase separation in Fe58Mn42 alloy not only leads to a slight increase in lattice parameter of y phase and Neel temperature, but also results in an obvious enhancement in magnetization due to the presence of paramagneticεphase and ferromagnetic a phase. The magnetostrictive performance, however, deteriorates accompanying with the phase separation. Under 1350 kA/m, the magnetostriction of the furnace-cooled and as-quenched samples decreases to 530 and 325 ppm, respectively.Cold-rolling and subsequent annealing are an effective method to obtain the texture and improve the magnetostrictive performance of y-Fe50Mn50 alloy. After cold-rolling to 70% reduction, the Fe50Mn50 alloy has a single y phase and a rolling texture is formed, including a pronounced{001}<110> and{011}<110> together with a weak{011}<100> component. The magnetostriction of the as-castγ-Fe50Mn50 alloy is 810 ppm at an applied magnetic field of 1440 kA/m, but decreases to 670 ppm after cold-rolling to 70% reduction. It can be attributed to the formation of a high density of crystal defects and the large internal stress in the cold-rolled sample. A single y phase and the preferred<110> orientation can be retained after heat treatment at 600℃for 1 h and a dramatically improved magnetostriction is observed in the annealed sample, with a giant value of 2009 ppm at 1440 kA/m.
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