Preparation And Magnetic Properties Of TbDyFe Thin Films For Nondestructive Testing

Magnetostrictive ultrasonic guided wave nondestructive testing is a technique based on the weidmann effect of magnetostrictive materials.It can generate ultrasonic guided waves in an alternating magnetic field,and characterizes the internal damage of the material through the signal-to-wave ratio before and after propagation of the device under test.The technology appeared in the 1990s and had the advantages of easy operation,high sensitivity,and low surface,etc.Nondestructive testing(NDT)requires that the magnetostrictive material has the speciality of certain bending flexibility,high energy density,large magnetostrictive strain,etc.Flexible materials like FeCo and Ni are commonly used in it.However,the traditional energy conversion materials are weak on energy density and magnetostrictive strain,which limits their energy conversion efficiency and detection effect.TbDyFe rare earth giant magnetostrictive thin film(GMF)has the characters of large strain,easy bending and so on,which provides theoretical possibilities for improving the energy conversion efficiency and optimizing the detection effect.It has a good application prospect in the field of NDT,and has received much attention from related researchers.In this experiment,TbDyFe thin films were prepared by direct-current(DC)magnetron sputtering method,the stability of the prepared TbDyFe thin films was studied and optimized.The magnetostrictive properties of the films were characterized by the cantilever beam method,and the influence mechanism of the magnetostrictive strain(λ)was systematically analyzed by means of AES,XRD,VSM,TEM and other equipment.Afterwards,the(Tb0.3Dy0.7)Fe1.95 thin films were deposited on FeCo substrate,and the magnetostrictive guided wave(MSGW)detecting instrument was used to characterize the detection efficiency of the composite material as a transducing material.The TbDyFe target with thickness of 6mm can stably sputter at deposition parameters of 100W and 1.0Pa,and the deposition rate is 100nm/min.The increasing deposition time is usually accompanied by the changing of surface composition,and the ratio of RE/Fe going up until the new equilibrium is established in stable parameters.In this case,the proportion of RE and Fe atoms deposited to the substrate is close to that of the target.The magnetostriction coefficient(λ)of as deposited amorphous Tb0.3Dy0.7Fe1.5 is 115ppm.The Ta layer can effectively avoid the influence of the substrate and the oxidation problem of the film and it is also beneficial to the uniformity and stability of the film composition.The high content of RE in the film is favorable for the precipitation of REFe2 phase,but the phase formation is unstable,and it is easily decomposed and oxidized at higher temperatures.The change of magnetostrictive properties are the result of the decrease of effective magnetic anisotropy constant(Keffect),the deflection of magnetic axis and the precipitate of Laves phase,etc.The λof as-deposited amorphous Tb0.3Dyo.7Fe1.95 film is 90ppm,and shows the easy axis is the vertical direction to the film.Annealing can make the easy magnetization direction of the film change to the parallel to the plane.Different thermal expansion coefficient between the film and substrate causes the tensile stress,which reduce the effective magnetic anisotropy constant and magnetic anisotropy energy.According to the principle of minimum energy,the magnetostrictive strain increases with the annealing temperature.The nanostructured crystalline REFe2 partially separated out in amorphous matrix after annealing at 600℃ for 1h,and the λ increased to 265ppm.High temperature caused the oxide phases appear easily,and the λ dropped to 135 ppm after the annealing at 650℃for 1h.Tb0.3Dy0.73Fe1.95/FeCo composite material has in-plane magnetization direction and low He.As a transducer,Results demonstrate that the detection efficiency roughly show regular consistency with the magnetostrictive strain of the TbDyFe films.The detection signal of traditional FeCo strip was 0.4V at the excitation power of 1.0P(P is the instantaneous power at 300V driving voltage)and frequency of 128kHz and that of the strip increased significantly by depositing one layer of(Tb0.3Dy0.7)Fe1.95 film.The detection signal of the composite material annealed at 600 ℃ was the best,even reaching saturation value of 2.5 V(0.8P,128kHz).The detection efficiency of the composite materials was obviously improved,which means that its energy efficiency and detection efficiency were also significantly optimized.The results may provide us with a method for preparing new materials for NDT.