Microstructure And Magnetostriction Of Rare Earth-Doped Fe-Ga Alloys

Magnetostrictive material is a kind of functional material which can convert energy.It is widely used in various fields such as transducers,actuators,etc.Fe-Ga alloys,as a new type of magnetostrictive material,with large low-field magnetostriction,good mechanical properties,resistance to severe environment,which have shown promising application.The magnetostrictive properties of polycrystalline Fe-Ga alloys remain to be improved to meet the practical application requirements.It was found that magnetostriction can be improved by dopingof rare earth in Fe-Ga alloys and the mechanism is unclear.In this thesis,the rare earth Tb and Dy were doped in the Fe83Ga17 alloys.The effects of the content and type of rare earth doped on microstructure and magnetostriction of the annealed alloys were studied.In order to measure the magnetostriction of the alloys,a measuring platform was built by using an 1555 data acquisition system,resistance strain gauges and a PEM-50 type electromagnet.The reliability of the magnetostriction measurement was verified by performing measurements on Fe,Co,Ni and Tb-Dy-Fe(Terfenol-D).Alloys of Fe83Ga17Dyx(x=0?1.0)and Fe83Ga17Tby(y=0?1.0)were prepared by arc-melting and annealed under an inert atmosphere.The crystal structure,morphology and micro-constitution of the annealed alloys were studied by the power X-ray diffraction(XRD)and scanning electron microscope(SEM).The magnetostriction were measured using the measurement platform at 0?1.0T DC magnetic field.The main conclusions obtained are as follows:(1)Typical magnetostrictive materials were used for testing the reliability of self-built measuring platform.The results show that the saturation magnetostriction of pure Fe,pure Ni,pure Co are-10 ppm,-40 ppm and-13 ppm(500 mT),and that the magnetostriction of Terfenol-D is 972 ppm(500 mT).These data are in good agreement with the widely reported in the literature.The standard deviations of magnetostriction coefficients of the three pure metals are less than 1.1 and that of Tb-Dy-Fe is less than 7.8,and each of the samples was tested for 7 times.The testing results show that the self-built measuring platform is reliable and meets the accuracy requirement.(2)The results of SEM and XRD show that the main phase of the alloys doped rare earth element Tb and Dy is still A2 phase.In addition,the second phase Tb2(Fe,Ga)17 and Dy2(Fe,Ga)17 was observed.The formation of the second phase causes the content of Ga in the A2 phase to decrease and causes the lattice parameter to decrease.The second phase appears to be isolated dots in the grain boundary and grain interior of the alloys with low Tb and Dy content,but exhibits a network-like morphology around the dendrites of the main phase with high Tb and Dy content.(3)The magnetostriction of Fe83Ga17 alloys doping of low Tb and Dy(x<0.3 or y<0.2)can be improved,while with high rare earth content the magnetostriction coefficient decreases.It is concluded that there are two mechanisms for the effect of rare earth doped.First,forming the second phase,R2(Fe,Ga)17(R=Tb,Dy),with larger magnetostriction coefficient,is benefit to magnetostriction.Second,forming more grain boundaries,which can impede the movement of domain walls,have a negative influence on magnetostriction.The first mechanism dominates the magnetostriction coefficient of the alloys with low rare earth content,while the second mechanism plays a main role with high rare earth content in the alloys.(4)The results show that the effect on magnetostriction of Tb and Dy doped is different along different direction,which main due to the magnetocrystalline anisotropy between Tb2(Fe,Ga)17 phase and Dy2(Fe,Ga)17 phase.The magnetostriction of Tb-doped alloys is improved greatly along the direction parallel to growth direction and a value of 84 ppm is achieved for x=0.3.The magnetostriction of Dy-doped alloys is improved greatly along the direction perpendicular to growth direction and a value of-77 ppm is achieved for x=0.1.