The Study Of The Crystalline Phase And Magnetic Structure Of Iron Gallium Alloys By Transmission Electron Microscopy

The macro-physical properties of materials are determined by the microstructure.Precisely analyzing the crystalline phases and magnetic structure,and further establishing the relationship between the microstructure and the macro-properties are of great values to the development and applications of the magnetostrictive materials.In this thesis,Fe-Ga alloy with excellent magnetostriction and complicated heterogeneous structures was taken as the research object.The evolution of the body-centered cubic(BCC)structure under various heat treatments and the effects on the magnetic properties were systematically studied by transmission electron microscopy(TEM),and the relationship between the microstructure and the magnetic properties was further carried out by theoretical modeling and first-principles calculations.The results are of great significance for guiding the adjustment of the magnetostriction and developing functional devices based on iron-gallium alloys.The main contents are summarized as follows:1.Development of the magnetostrictive measurement platform.The magnetostrictive measurement platform with the resolution?2.1 ppm has been developed.It can be successfully applied to measure the magnetostrictive coefficient of Fe-Ga alloy with different heat treatments.2.The Precisely measurement of the quenched BCC lattice parameters.The three-dimensional lattice parameters of the quenched Fe-Ga were measured based on the high-order Laue zone reflection for the first time.The tetragonal structure with c/a<1was confirmed.The density functional theory by KKR-Green's function method was applied to reveal that the formed tetragonal structure was not the intrinsic structure.The results provided a new insight into the magnetostriction mechanism based on the tetragonal structure.3.The investigation on the precipitation of omega phase in quenched Fe-Ga.The transformation from BCC to the omega structure,with the quenched Fe-Ga alloy isothermal aging at series of low-temperature,was revealed by aberration-corrected transmission electron microscopy at atomic-scale resolution.The results showed that the precipitation is driven by the displacive and diffusive transformation,with the atoms in the(211)plane slipping along the<111>direction and the atoms diffusion achieved with the aid of spinodal decomposition.The variants of omega phase,w1 andw2,were observed simultaneously in the BCC matrix,and the orientation relationships between the variants and the long range ordered BCC D0₃ phase were indexed into(?).The first principles calculations revealed that the doping of Ga atoms could promote the evolution of the BCC structure to the omega phase.Finally,the decline of magnetostriction caused by the precipitation of the omega phase has been confirmed in experimental.4.The investigation on the precipitation of L1₂ phase in quenched Fe-Ga.The evolution details were observed by in-situ heating technique in transmission electron microscopy.The Kurdjumov–Sachs orientation relationship rather than Bain orientation relationship between the L1₂ phase and the BCC matrix has been confirmed.The stacking faults promoted the rapid growth of L1₂ phase.Based on the edge-to-edge matching model,the orientation relationship between the BCC matrix and the face centered cubic L1₂ was further explained and confirmed that the tensile and the shear deformation were required during the precipitation of L1₂ phase.5.The investigation on the magnetic domains in Fe-Ga.The lorentz microscopy was applied to analysis the magnetic domains in BCC structure,L1₂ and D0₁₉ phases.The changes of the magnetic domains of D0₁₉ phase during in-situ magnetization and demagnetization was analysised,and the stable domain walls direction along(0001)has been recorded,indicating the obvious magnetocrystalline anisotropy in D0₁₉.The obvious magnetocrystalline anisotropy not conducived to magnetostriction has been confirmed by comparing the magnetostriction in BCC structure,L1₂ and D0₁₉ phases.Based on transmission electron microscopy results,the relationship between the abnormal low temperature precipitated D0₁₉ phase and omega phase has been discussed,which provided a new insight of the complex phase transformation in Fe-Ga alloy.