Study On Phase Structure And Magnetoelasticity Of Co-Fe Alloy

Traditional Co-Fe magnetostrictive alloys have high Curie temperature,high permeability,low coercivity,and good mechanical properties,but their low magnetostriction value prohibits their use in high-power transducer and actuator applications.Since 2011,Hunter et al.found that the magnetostriction of Co-Fe alloy?65 at.%<Co<72 at.%?can be significantly enhanced via high-density precipitation of nanoparticles.However,structure and thermodynamic conditions of the nanoparticles in Co-Fe alloy are still unclear,which is the aim of this work.In addition,the effects of the precipitation of fct nanoparticles on magnetostriction,Wiedemann,and magnetomechanical coupling of Co-Fe alloy alloy have been studied in this work.The<100>textured Co₇₀Fe₃₀ alloy was prepared by directional solidification technique.Because the nanoparticles in Co₇₀Fe₃₀ alloy were metastable phase at room temperature.So Co₇₀Fe₃₀ alloy with nanoscale particles embedded in the bcc phase matrix was prepared by quenching after heat treatment at 860 ?.Then structure of the nanoparticles was studied using TEM and APT methods.The results showed that the nanoparticles in Co₇₀Fe₃₀ alloy was a face-centered tetragonal?fct?structured and Co-riched phase?72 at%<Co?,with lattice parameters of a=b=0.388 nm,c=0.404 nm,and ?=?=?=90�.The fct phase could existed in Co₇₀Fe₃₀ alloy within the temperature ranged of 780 ? to 860 ?.When the temperature was increased beyond 880 ?,the structure of this secondary nano-phase changed from fct to face-centered cubic?fcc?.Moreover,the fct phase nanoparticles were semicoherent with the bcc phase matrix,exhibiting a?001?fet//?110?bcc and[100]fct//[110]bcc crystallographic orientation relation,which was retained even after the structural transformation.In addition,with the precipitation of fct nanoparticles,the magnetostriction of<100>oriented Co₇₀Fe₃₀ polycrystalline could be increased from 125 ppm to 180 ppm.0.5 mm diameter magnetostrictive Co₇₀Fe₃₀ wires were prepared using forging and drawing processes.The Wiedemann twist of the Co₇₀Fe₃₀ alloy could be increased from 750 "/cm to 1100 "/cm by the precipitation of fct nanoparticle.A strong<100>circumferential texture could be achieved in bcc+fct wires by addition of 0.2%NbC.As a result,the Wiedemann twist could be further increased to 1363 "/cm.In this work,we found that the magnetomechanical coupling of Co₇₀Fe₃₀ alloy could be significantly enhanced by high-density fct nanoparticles.The semicoherent fct nanoparticles embedded within a bcc matrix could promote magnetic domain rotations under applications of stress.When the Co₇₀Fe₃₀ alloy was subjected to a 50 MPa compressive stress,its saturated magnetostriction could be increased from 106 ppm to 140 ppm by precipitation of fct nanoparticles.In addition,the largest AE of Co₇₀Fe₃₀ alloy could be increased from 30.6%to 50.2%via the precipitation of fct nanoparticles.Magnetostrictive Co₇₀Fe₃₀ sheets in 0.3 mm thickness were prepared by forging,hot rolling and cold rolling.In this work,the effect of heat treatment on texture evolution and magnetostriction of the Co₇₀Fe₃₀ sheets was investigated.The strong<110>fiber texture along the rolling direction originated from the cold rolling process was retained in the annealed Co₇₀Fe₃₀ sheets as the annealing temperature was below 600 ?.When the annealing temperature was above 700 ?,the proportion of<100>fiber texture markedly increased,and the magnetostriction increased from 93 ppm to 125 ppm.