Effects Of Alloying Elements On The Magnetic Properties And Microstructure Of Nd₂Fe₁₄B/α-Fe Nanocomposites

Nanocomposite permanent magnets are composed of a high magnetic anisotropy of hard magnetic phase and a high saturation magnetization of soft magnetic phase by exchange coupling interaction on a nanometer scale,have attracted much attention.Because a theoretical maximum energy product above 125 MGOe would be expected in the magnets,which higher than the traditional permanent magnet.However,practically obtained maximum energy products in the nanocomposite magnets are about 20 MGOe,which is far less than the theoretical predicted value.This has been attributed to the difficulty in obtaining the microstructures of theoretical model.Then,there is an inherent trade-off between magnetization and coercivity.Therefore,it is still the key to improve the microstructure of nanocomposite permanent magnet.In this paper,the Nd₂Fe₁₄B/α-Fe-type nanocomposite permanent magnets were prepared by melt quenching technology.The magnetic properties of the Nd₂Fe₁₄B/α-Fe nanocomposite magnets were studied by employing a vibrating sample magnetometer（VSM）and the microstructures of the magnets were studied by using X-ray diffraction（XRD）measurements.In addition,the effect of Titanium（Ti）,Niobium（Nb）elements on the magnetic properties and microstructures of Nd₂Fe₁₄B/α-Fe-type nanocomposite magnets were studied.The Nd₉Fe_85-xTi_xB₆（x=0,0.5,1,1.5）and Nd₉Fe_85-xNb_xB₆（x=0.5,1,1.5）alloy ingots were prepared by using vacuum melting technology,and then prepared the melt-spinning ribbons by melt quenching technology.Experimental results show that adding Ti element in the melt-spinning ribbons have the better magnetic properties at a wheel speed of 16 m/s.With the Ti element content increasing,the maximum energy product（BH）_max of nanocomposite magnets are gradually increased.When the amount of Ti element is 1.5at.%,the maximum energy product is the optimum,and（BH）_max=20.7 MGOe.The melt-spinning ribbons which added Nb element,its optimum magnetic properties are obtained at the wheel speed of 18 m/s.As the amount of Nb element increased,the maximum energy product of nanocomposite magnets shows a trend of increasing first and then decreasing.When the amount of Nb element is 0.5 at.%,the maximum energy product is the highest,and（BH）_max=22.4 MGOe.Microstructural analysis of the magnets indicates that as the amount of Ti element increased,the grain sizes of hard magnetic phase and soft magnetic phase were decreased.When the addition amount of Ti element is 1.5 at.%,the magnets have uniform structure and small grain size.The grain sizes of Nd₂Fe₁₄B phase andα-Fe phase grain sizes are 22.3nm and 16.7 nm,respectively.Adding Nb element in the melt-spining ribbons,the grain size first decreases and then increases with the increase of Nb element content.When the addition amount of Nb element is 0.5 at.%,the grain sizes of Nd₂Fe₁₄B phase andα-Fe phase in the magnets are about 22.8 nm and 18.0 nm,respectively.Studies have shown that Ti,Nb elements enriched at the grain boundaries,which can strengthen the domain wall pinning in the process of reversed magnetization,which is beneficial to improve the coercivity of the magnets.The refinement of grain sizes can increased the number of domain-wall points,thus the domain wall pinning effect is enhanced,and then the coercivity increased.In addition,adding Nb element can promote the formation of interfacial amorphous phase,improving the exchange coupling between hard and soft phases,which is helpful for the improvement of magnetic properties.