Study On Diffusion And Phase Transformation Of Nd-Cu-Ag Eutectic Alloy In Nd-Fe-B Permanent Magnet

Nd-Fe-B is an essential functional material in modern society,which is widely used in many important fields such as information and national defense.The coercivity of Nd-Fe-B material deteriorates significantly as the temperature increases,and Dy and Tb have to be used to compensate such performance decline.However,the limited natural resources and high cost of heavy rare earth has always been a problem to the factories.Finding a way to enhance the coercivity of Nd-Fe-B sintered magnets without heavy rare earth has received intense research interest recently.Based on thermodynamic analysis,the paper studied the atomic diffusion and migration of Nd-Cu-Ag eutectic in Nd-Fe-B,expecting to improve the coercivity of Nd-Fe-B without using heavy rare earth.The typical characteristics of elements in grain boundaries,the strengthening mechanism of grain boundary to coercivity were studied.The main results are as follows:（1）With the increase of the tempering temperature,the volume of grain boundary（GB）phases is constantly increased in the sample with the addition of Nd₇₀Cu₃₀ eutectic and 1wt.%Nd₇₀Cu₃₀+4 wt.%Nd₈₀Ag₂₀.When the Nd₇₀Cu₃₀ eutectic samples are tempered at 550℃,630℃,710℃,the GB phases are distributed mostly in the triple junction;Thin ribbon-like GB phases have formed in the sample with 1wt.%Nd₇₀Cu₃₀+4wt.%Nd₈₀Ag₂₀ although tempered at 550℃,and are more continuous at 790℃and 870℃.（2）The Ag-included eutectic exhibited better fluidity and wettability,which increasing the surface energy of magnetic grains,and caused the growth of the magnetic grains.There are still a small amount of Cu and Ag atoms remaining inside the magnetic grains after the addition of Ag.（3）A solid phase-transition occurred in NdCu phase at above 530℃.When the temperature is lower than 530℃,the lattice constants a,b and the volume of NdCu phase decreasing with the temperature increasing,indicating that NdCu phase is a negative expansion phase at low temperature.It is found that the lattice constants of the NdCu phase in the（Nd+NdCu）eutectic is smaller than that of the NdCu single phase.The main reason is that the Ndphase in the eutectic structure exert a strong pressure on the NdCu phase.（4）After the magnet is diffused by Nd₇₀Cu₃₀ eutectic for 15 h,the coercivity increases to6550 Oe from 5503 Oe,it is about 19.1%increasement.After the Nd-Cu-Ag(80wt.%Nd₈₀Ag₂₀)alloy diffused,the coercivity is increased to 6650 Oe,which is increased by20.8%.The Nd-Cu-Ag diffusion magnet exhibits better coercivity than the Nd₇₀Cu₃₀ diffusion magnet.