Investigation On The Phase Transformation,Microstructure And Magnetic Properties Of Fe₇₅Ga₂₅

As a new-generation magnetostrictive material,the magnetostriction and tensile strength of Fe-Ga solid solution are more an order of magnitude than the traditional Ni metals and rare earth-based magnetostrictive materials,respectively.Fe-Ga magnetostrictive material has lots of advantages,e.g.low saturation field,high Curie temperature and good corrosion resistance,which can facilitate numerous high-tech applications,such as high-precision micro-displacement control and precision machining.Because the phase structures and magnetic properties of Fe-Ga alloys are sensitive to heat treatment,it is a very promising route to improve their magnetostrictive performance by tuning the phase composition via appropriate heat treatments.However,the previously-reported phase composition of Fe-Ga alloy cannot match the Fe-Ga equilibrium phase diagram,which makes it difficult to search an effective approach to improve the magnetostrictive performance.Therefore,the purpose of our work is to make the selected Fe7sGa25 alloys gradually approach their equilibrium states by the heat treatments of isothermal aging and slow cooling,respectively,and to compare with the metastable state of as-solution-treated sample.The XRD?TEM?metalloscope?VSM?magnetostriction measurement and hardness test are performed in this work to reveal the evolution of microstructure,magnetic properties and magnetostrictive properties during the transition from metastable to equilibrium state.It provides not only new evidence for understanding the phase transition behavior of Fe-Ga alloys,but also an effective route to tailor the magnetostrictive performance.The main results are as follows:1.In Fe75Ga25 alloys,ordering transformation of the matrix phase and precipitation of the equilibrium phase simultaneously occur when approaching the equilibrium state,which is highly dependent with isothermal aging time and cooling rates.The as-solution-treated state bears average BCC structure with randomly-dispersed short-range ordered D03 nanoclusters.During the isothermal aging at 450?,the order degree of D03 phase gradually increases,together with the increasing of volume fraction of the L12 equilibrium phase.When the isothermal aging time is 10 h,the diffraction peaks of the L12 equilibrium phase can be observed in the XRD pattern.Similarly,when the cooling rate decreases from 8.4?/min to 0.4?/min,the average BCC structure also changes to a bi-phase composite structure with BCC+L12.Compared with the proposal in previous literature that the D03 structure is obtained by slow cooling at 10?/min,our results show that the kinetics of the transformation from metastable BCC phase to equilibrium L12 phase is very slow.When the aging time is sufficiently long or the cooling rate is slow enough,L12 equilibrium phase can be precipitated,which means that there is no contradiction with the equilibrium Fe-Ga phase diagram.2.The transition behavior of Fe75Ga25 alloys from metastable state to equilibrium state has obvious surface effect,which belongs to the diffusion-controlled solid-solid phase transformation.Metallographic analysis,XRD,TEM and magnetostrictive tests results show that volume fraction of the L12 equilibrium phase are the largest on the sample surface layer in in the samples aged at 450? for 24 h and slowly cooled at 0.4?/min.With tending to the internal of samples,size and volume fraction of the L12 equilibrium phase clearly decrease.It indicates that the metastable-equilibrium transition of the alloys is a diffusion-controlled phase transition,and the volume fraction of the equilibrium phase is highly dependent with the diffusion rates at different physical positions of the samples.3.This work investigates and establishes the relationship between the magnetic properties,hardness and the phase transition process in Fe75Ga25 alloy.Magnetic measurements show that the higher the degree of D03 order of the BCC matrix has,the lower the saturation magnetization and saturation magnetostriction performance are;and the larger the volume fraction of the L12 equilibrium phase has,the higher the saturation magnetization and hardness are,associated with the deterioration of magnetostriction performance.It indicates that the L12 equilibrium phase has higher saturation magnetization and hardness than the BCC phase,and the volume fraction of L12 equilibrium phase can be controlled to improve the saturation magnetization and mechanical properties.This result can provide a reliable basis for optimizing the overall performance of Fe-Ga alloys.