Phase Structure And Magnetic Properties Of Rare-Earth Free MnAl Based Permanent Magnet Alloys

Rare earth permanent magnet represented by Nd-Fe-B are widely used in new energy vehicles,rail transit,wind power and other fields with their excellent comprehensive magnetic properties.However,the limited reserves of rare earth,especially heavy rare earth resources,have led to high prices of rare earth permanent magnets.Therefore,it is one of the needs of the permanent magnet material industry to develop a low-cost,high-magnetic-performance rare-earth-free permanent magnet to replace part of the rare-earth permanent magnet.Rare-earth-free MnAl permanent magnets have high magnetic properties,excellent machining properties and good corrosion resistance,and the cost is low(low raw material prices).Therefore,this paper uses theoretical calculations and experimental synthesis to study the influence of Ga,Ge,In,Sn and other elements doping on the permanent magnetic phase structure and magnetic properties of the alloy;and the influence of melt rapid quenching,hot pressing and hot deformation on the microstructure and magnetic properties of MnAl permanent magnet alloys.The main research conclusions are as follows:(1)Theoretical calculation researcha.Mn16Al15X(X=Ga,Ge,In,Sn)and Mn16Al16-xInx(x=0～4,8)have a very asymmetric spin up and down state density,which indicates that they have strong magnetic properties.The doping of the four elements can stabilize the unit cell structure of τ-MnAl,but at the same time it will increase the difficulty of formation.Among them,the unit cell energy after Sn doping is the lowest(-56720.3864 Ry),the unit cell formation energy after Ge doping is the lowest(-0.6116 Ry),and the unit cell structure after In doping does not change much(c/a value only increases by 0.0006)but its energy is greatly reduced(up to 11280.8235 ’Ry);Ge and Sn doping will reduce the total magnetic moment of τ-MnAl unit cell(reduced by 0.1917 μB and 0.4158 μB respectively),and Ga doping will increase the total magnetic moment of the τ-MnAl unit cell(0.1045 μB increased),the total magnetic moment of the τ-MnAl unit cell does not change much after In doping(only a decrease of 0.012 μB).With the increase of In doping amount,the unit cell energy of τ-MnAl gradually decreases but its formation energy gradually increases.The total magnetic moment of the unit cell first decreases and then increases;when the doping amount of In is x=1,the unit cell energy of τMnAl is significantly reduced while the unit cell structure and magnetic moment change very little,with the smallest formation energy(-0.4831 Ry).(2)Experimental researchb.Among the Mn53A144X3(X=Ga,Ge,In,Sn)and Mn53Al47-xInx(x=1,2,3,4)alloy ingots prepared by electric arc furnace smelting:doping Ga and In is beneficial to the magnetic τ phase form and improve its magnetic properties;doping Ge and Sn is difficult to obtain high-purity τ phase MnAl alloy,their magnetic properties are poor.With the increase of In doping amount,the comprehensive magnetic properties of MnAl alloy gradually decrease(where In doping amount is x=4,the magnetic properties are the worst,and the magnetic τ phase disappears completely)c.Among the Mn53Al44In3 strip samples prepared by melt rapid quenching and heat treatment processes:After heat treatment at 450℃ for 30 min,the coercivity of the alloy strip with a rapid quenching speed of 20 m/s is the highest(1.891 kOe);the comprehensive magnetic performance of the alloy strip with a rapid quenching speed of 25 m/s are the highest(Ms=114.684 emu/g,Mr=38.896 emu/g,Hc=1.732 kOe),and its coercivity is controlled by the domain wall pinning mechanism,the study of the recovery curves shows that the opening of the recovery curve of the sample is related to the non-magnetic phase existing in the alloy(the non-magnetic phase will produce a large demagnetizing field under a large external magnetic field,and its magnetic moment will undergo a large reversible rotation,while the magnetic moment of the magnetic phase will not rotate significantly,which will eventually lead to the opening of the recovery curve).d.In the(Mn0.52A10.48)98C2 alloy obtained by hot pressing(HP)and hot deformation(HD)processes:the density of the magnet is 4.82 g/cm3 and 5.06 g/cm3,and the density is 92.69%and 97.31%,respectively.The comprehensive magnetic properties and the degree of anisotropy(DOA)of the hot deformation magnet are significantly higher than those of the hot pressing magnet.Among the tested hot deformation magnet samples:the Ms and Mr of No.1 sample are the largest(MsPPD=84.082 emu/g,MrPPD=40.419 emu/g),the texture of No.2 sample is the largest(its DOA=0.379),and the Hc of No.3 sample is the largest(HcPLL=3.232 kOe).The above results indicate that the hot deformation process can obtain anisotropic MnAl permanent magnets with high density and high magnetic properties.Scanning electron microscopy results show that the dynamic recrystallization produced during the hot deformation process makes the crystal grains of the hot deformation magnet significantly refined,thereby enhancing the strength of domain wall pinning,and ultimately leading to a substantial increase in its coercivity.The absolute values of a and β of the hot deformation magnet are greater than those of the hot pressing magnet in different temperature ranges,which indicates that the hot deformation process is unfavorable for the temperature stability of the MnAl permanent magnet.