Evolution Of Microstructure And Magnetostrictive Performance Of Tb_0.27Dy_0.73Fe_1.95 Alloy Solidified In High Magnetic Field Gradients

Tbo.27Dyo.73Fe 1.95 alloy is an important magnetic functional material which is widely used in the field of advanced technologies,e.g.,sonar,actuators,precision machinery,sensors,and adaptive vibration control systems,for its high magnetostrictive coefficient at room temperature,high electromechanical coupling coefficients,high output power,fast response,high energy density,and noncontact drive.(Tb,Dy)Fe2 phase is the functional phase in Tbo,27Dyo,73Fe1.95 alloy which has a cubic Laves phase structure with a higher magnetostrictive performance when it orients along its easy-magnetization axis(<111>direction).Controlling of orientation and structure is the main way to enhance magnetostrictive performance of Tb0.27Dy0.73Fe1.95 alloy.Applying high magnetic field gradient to the solidification process of Tb0.27Dy0.73Fe1.95 alloy,the solidification structure and crystal orientation of Tb0.27Dy0.73Fe1.95 alloy may be controlled and the magnetostrictive functional grade material may be obtained by the coupling effect of magnetic force and magnetic torque.In this dissertation,high gradient and uniform magnetic fields were both applied to the solidification process of Tb0.27Dy0.73Fe1.95 alloy under various cooling rates.The effects of magnetic gradient,magnetic flux density and cooling rate on magnetostrictive and magnetic performance and(Tb,Dy)Fe2 phase orientation behavior were studied.Magnetic force microscope(MFM)was used to study the relationship between the domain structure and magnetic properties.The orientation relation between(Tb,Dy)Fe2 phase and(Tb,Dy)Fe3 phase was studied by electron backscattering diffraction(EBSD)and the mechanism of(Tb,Dy)Fe2 phase orientation induced by high uniform and gradient magnetic field was discussed.The main research contents are as follows:(1)The magnetostrictive performance,saturation magnetization and the orientation of(Tb,Dy)Fe2 phase uniformly distribute no matter the alloy solidified with and without high uniform magnetic field.With the application of high magnetic field gradient,these properties distribute gradiently along magnetic field direction,and gradient degree increased with increasing |BdB/dz".The magnetostrictive performance,saturation magnetization and the<111>orientation degree of(Tb,Dy)Fe2 phase are much higher in the area near the magnetic field center.The migration of(Tb,Dy)Fe2 phase by magnetization force and the behavior of orientation of(Tb,Dy)Fe2 phase by the interaction of magnetic torque and magnetic force are the main reasons for magnetostrictive performance,saturation magnetization and(Tb,Dy)Fe2 phase orientation gradient distribution.(2)With the application of high magnetic field gradient,the gradient degree of magnetostrictive performance,saturation magnetization and the orientation degree of(Tb,Dy)Fe2 phase decreased with increasing cooling rate.The magnetostrictive performance and saturation magnetization distribute uniformly and the(Tb,Dy)Fe2 phase oriented randomly when the cooling rate reached to 60?/min.The phenomenon is due to the action time of high magnetic field gradient decreased with increasing cooling rate.With increasing cooling rate,the action time of magnetic field decreased,and the migration of(Tb,Dy)Fe2 phase induced by magnetic force and(Tb,Dy)Fe2 phase orientation induced by the interaction of magnetic torque and magnetic force are both decreased.(3)With the application of high uniform magnetic field,the domain structure is changed from strip-like to mixed structure of strip-like and dot-like.The width and contrast of domain decreased with the following order:4.4T,11.5T,8.8T and OT.The magnetostrictive performance,saturation magnetization and the rate of magnetization of the specimen are all decreased with the following order:4.4T,11.5T,8.8T and 0T,too.With the application of high magnetic field gradient,the domain width increased gradient along the magnetic field direction.The magnetostrictive performance,saturation magnetization,the rate of magnetization of the specimen all increased along the magnetic field too.The width and contrast of domain,magnetostrictive performance,saturation magnetization,the rate of magnetization of the specimen solidified in high uniform magnetic field are all decreased with increasing cooling rate.The gradient degree of the width and contrast of domain,magnetostrictive performance,saturation magnetization,the rate of magnetization of the specimen solidified in high magaetic field gradient are all decreased with increasing cooling rate.(4)(Tb,Dy)Fe2 phase oriented along<111>,<110>,<112>and<113>direction respectively,when the alloy solidified in various magnetic field condition,(Tb,Dy)Fe3 phase oriented along<241>,<110>,<221>and<233>direction accordingly.With the application of lower magnetic flux density,(Tb,Dy)Fe2 phase orients from randomly orientation to<110>to<113>to<112>to<111>direction with decreasing cooling rate.With application of higher magnetic flux density,(Tb,Dy)Fe2 phase orients from<110>,<113>to<112>,<111>direction with increasing cooling rate.With the application of high magnetic field gradient,the first precipitation(Tb,Dy)Fe2 phase will gather to the area near the magnetic field center and orient along<111>direction.The migration behavior and<111>-oriented of(Tb,Dy)Fe2 phase are not apparent at the end of solidification for the limited action time of magnetic field.