The Fundamental Properties Of Rapidly Quenched Ce/La-Fe-B Alloys And The Preparation Of Nd-Fe-B Based Permanent Magnets

As an important type of rare earth permanent magnetic materials,Nd-Fe-B based alloys play an important role in the fields of electric motor,electronics,information,energy,automobile etc.With the increase of Nd FeB production,the consumption of rare earth significantly increases,especially for Pr,Nd and Dy.Rare earth elements are extracted from associated ore,and the extensive use of Nd,Pr will inevitablely lead to that La,Ce and other aboundant rare earth resources are not fully utilized.Both La and Ce elements can replace the Nd elements to form the RE2Fe14 B tetragonal phase,recently,a large number of studies have been carried out on La/Ce substituted Nd-Fe-B alloys,and significant progress has been made.In this thesis work,the structure,phase precipitation behavior and magnetic properties of rapidly quenched tenery La-Fe-B and Ce-Fe-B alloys are systematically studied.Dual main phase SPSed Nd/Ce-Fe-B magnets and bonded Nd-Fe-B magnets with desired performance were prepared.This work is important for exploring high price/performance ratio magnets.Firstly,the ternary Ce2Fe14 B and La2Fe14 B alloys were prepared by melt spinning technique.The phase precipitation,microstructure and magnetic properties of Ce2Fe14 B and La2Fe14 B alloys were investigated under different quenching rates and heat treatment conditions.For the Ce2Fe14 B alloy,as the wheel speed varied from 10 to 50 m/s,the hard magnetic Ce2Fe14 B phase precipitated in as spun ribbons prepared at 10 and 20 m/s.Increasing wheel speed leads to the appearance and increase of amorphous phase and the reduced saturate magnetization.For the amorphous ribbons prepared at 50 m/s,when the annealing temperature increased from 500 ℃ to 600 ℃ and 700 ℃,α-Fe phase precipitated first followed by the appearance of Ce2Fe14 B phase and grain growth.Transmission electron microscope analysis revealed that large Ce2Fe14 B grains and the uniformly distributed small α-Fe grains precipitated from amorphous matrix after heat treatment.The good properties with saturate magnetization of 1.17 T,remanence of 0.57 T,coercivity of iHc=221 k A/m,and maximum energy product of 17 kJ/m~3 have been obtained in the optimized sample.For the La2Fe14 B alloys,the hard magnetic La2Fe14B(2:14:1)phase is difficult to obtain in the ternary alloy.The α-Fe and La phases mainly precipitated in as spun ribbons prepared at relatively low wheel speed,and more amorphous phase formed at higher wheel speed.With the annealing temperature increased from 400 ℃ to 500 ℃ and 800 ℃,the α-Fe and La phases also precipitated from the amorphous matrix gradually.A liquid La-rich phase and α-Fe phase could coexist in the samples at the high temperatures.In addition,the effects of Nd substitution for La on the structure and phase precipitation during annealing for the(La1-xNdx)2Fe14B(x=0.1.0.2,0.3 and 0.4)alloys have been also investigated.The substitution of Nd for La can greatly enhance the formation of hard 2:14:1 phase,which led to high permanent magnetic properties.The effects of heat treatment temperature on the structure and magnetic properties of commercial NdFeB magnetic powders and rapidly quenched Ce17Fe78B6 magnetic powders were investigated.Then the spark plasma sintering was carried out to sinter these two types of powders with various volume ratio into dual-main-phase magnets at the temperature of 650 ℃ and 700 ℃.Three type of magnets have been obtained,including Nd-Fe-B magnets,Ce-Fe-B magnets and dual-main-phase Nd-Fe-B/Ce-Fe-B magnets.The results show that the magnetic properties of Nd FeB powders are almost no change after heat treatment,while the coercivity of Ce17Fe78B6 powder is reduced gradually.After spark plasma sintering,hard magnetic phases of 2:14:1 were mainly detected in all samples when sintered at 650 ℃,and no coarse grain zone was found.The peaks of α-Fe phase in the SPSed samples contained Ce17Fe78B6 powders was greatly enhanced when sintering at 700 ℃,and the coarse grain zone was also observed.The good magnetic properties with saturate magnetization of 1.41 T,remanence of 0.68 T,coercivity of iHc=477 kA/m,and maximum energy product of 53 kJ/m~3 have been obtained in the SPSed Nd FeB sample prepared at 700 ℃.After SPSed at 650 ℃,the dual-main-phase magnets showed the magnetic properties lying between the two single-main-phase magnets.At last,the Nd-Fe-B magnets used for compensating the iron core in the power transformer have been prepared.In order to meet the needs of the devices,the magnetic properties of nanocrystalline Nd FeB magnetic powders were modified by heat treatment,and then the bonded magnets were prepared by conventional bonding process.The effects of heat treatment on magnetic properties of magnetic powder were studied.It is found that the heat treatment temperature plays an important role in mangnetic properties,and the heat treatment duration has less effect.The addition of adhesive can effectively improve the density and formability of magnets.The magnetic properties with remanence of 0.39 T,coercivity of iHc=144 k A/m,and maximum energy product of 11 kJ/m~3 have been obtained in the prepared bonded magnets.