Study On Temperature Dependence Of Elastic Modulus And Magnetoelastic Applications Of Fe-Ga Alloys

Elastic modulus of magnetic materials tends to increase with temperature rising,as the result of gradual loss of magnetic order with approaching Curie point.Such effect was first discovered in Fe-Ni based alloys by Guillaume over one century ago,and now it has been developed into a series of constant elastic alloys.With the development of precision instruments,constant elastic alloy has been applied more and more widely,and higher requirements are put forward for such materials.The origin of the thermo-elastic anomaly(also known as the Elinvar effect)is considered to be the result of the magneto-mechanical coupling effects,namely?E effect.Since their discovery in 2000,Fe-Ga alloys attracted growing research interest due to the combination of large magnetostriction coefficient,low saturation magnetic field,high relative permeability,and mechanical strength,which have been promising for applications in sensors,actuarors,and transducers.However,at present,the elastic modulus-temperature behavior in Fe-Ga alloys was lack of concern.In this study,the temperature dependences of elastic modulus in Fe-Ga alloys were investigated systemtically;therein,Ga content,orientation,and phase structure have been considered.Meanwhile,the applications of Fe-Ga alloys based on magnetoelasticity were studied.Firstly,the elastic modulus-temperature dependences of as-cast and directional solidified Fe100-xGax alloys(x=18,20,22,24,27 at.%)are studied based on the impluse excitation technique(IET)in the temperature range RT-500?.The elastic modulus various Ga content shows different abnormal behavior with temperature rising;among them,the directional solidified Fe73Ga27 alloy along the<100>preferred orientation shows constant elastic property.However,unlike the previous theory,results of magnetic-temperature and thermal expansion measurements show the absence of magneto-volume effect in Fe-Ga alloys.According to the analysis of in situ heating XRD and TEM,three kinds of structure transformation result in this unusual behavior in Fe73Ga27 alloy:Firstly,the lattice contraction caused by the diffusion-type phase transformation of A2?D03 results in an increases of the Young's modulus,especially along<100>bcc direction due to the enrichment of diffusing atoms.Secondly,the transformation of D03?Al,which is attributed to the Bain strain,decreased the Young's modulus along[001]bcc directions.Finally,transformation from disordered A1 structure to ordered L12 phase completes through the exchanges of atomic position,which results in a rapidly increase of the Young's modulus.Therein,the intermediate face-centered-cubic(A1)structure during the phase transformation of D03?L12 was first identified in this work.The gradual process of atomic diffusion has the potential to get constant elasticity in Fe-Ga alloys.Then,the applications of Fe-Ga alloys based on magnetoelasicity are studied,which consist of magnetostrictive displacement senors,variable stiffness spring,and electromagnetic induction torque sensors.(1)Fe82Ga13.5Al4.5 and(Fe83Ga17)99.4B0.6 wires of 0.5 mm diameter are successfully prepared by heat forging,rolling,and combining hot and cold drawing for the improvement of mechanical strength and ductility with A1 and B additions.A large Wiedemann twist of 222.6 "/cm was obtained at 1350.0 A/m axial magnetic field(Ha)and 1400.0 A/m circumferential magnetic field(Hc)in the AC-annealed Fe82Ga13.5Al4.5 wire.This work presents the measuring method of the propagation speed of torsional wave with rising temperature in magnetostrictive wires for the first time,which is beneficial for sensor applications to ensure consistent performance at various temperatures.(2)Fe82Ga13.5Al4.5 spring and the loading way of the magnetic field were designed and processed.An apparent change in the stiffness coefficient of 10.86%is observed by further increasing the magnetic field to 235.5 Oe,which shows good prospect in a range of technical applications,such as force sensors and suspension systems.(3)Fe8iGa19 rod with a<100>preferred orientation along the length direction is prepared for the torque shaft and as the electromagnetic induction sensitive element.An apparent decrease in the induced voltage signal(peak voltage)of 3.88 mV is observed as the torque loading is 50 N·m in the presence of a sine excitation signal(10 V,1 kHz)and a bias current of 0.5 A.Meanwhile,a good repeatability and stress sensitity are obtained,especially in the low torque range.This work shows the prospect of Fe-Ga alloys for non-contact torque sensing,for the large magnetostriction and high sensitivity of magnetization to stress.