Study On Structure And Magnetic Properties Of La,Ce-contained R-Fe-B Alloys

Since Nd-Fe-B based permanent magnets were discovered in 1980s',they have been widely used in electronic devices,automotive industry and wind generators due to their excellent magnetic properties.The global resources of Nd,Pr and Dy,which are consumed excessively,are inadequate to meet the application demand.However,the rare earth elements La and Ce with abundant reserves are overstocked.In recent years,researches have been focused on the La,Ce-contained permanet magnets for balancing the utilization of rare earth resources.There are two methods for applying high abundance rare earth elements in permanent magnetic materials.Firstly,developing the Ce-Fe-B magnets;Secondly,developing La,Ce contained Nd-Fe-B magnets for decreasing the usage of Nd.These two methods are taken in this study,and the melt spun technology was applied to prepare the Ce-Fe-B,Ce-La-Fe-B and Nd?LaCe?-Fe-B alloys.The phase structure,microstructure and magnetic properties are studied,and the first principle calculations are performed to reveal the mechanisms of phase decomposition and the improvement of intrinsic magnetic properties.The main research results are as follows:?1?CexFe14B?x = 2,2.2,2.4,2.6,2.8,3?melt spun ribbons were prepared.The TbCu7 metastable phase forms in Ce-Fe-B melt spun ribbons,but the formation of TbCu7 is suppressed by increasing the Ce content.After annealing process,the TbCu7-type phase transform into 2:14:1 phase and the CeFe2 phase is generated in x>2 cases.The magnetic properties of Ce-Fe-B is improved with excessive Ce content.there three reasons which can attribute for the enhancement of magnetic properties:? The exchange coupling interaction between CeFe2 soft magnetic phase and 2:14:1 hard magnetic phase;?The formation of TbCu7 metastable phase was suppressed by increasing the Ce content,which is benefitial for micro structure with fine grain size;?Because of the pining effect of grain boundary,the mechanism of coercivity turns form nucleation to pining with increasing Ce content,which is in favor of the enhancement of coercivity Hcj.For the composition of Ce2.6Fei4B alloys,the optimum properities were obtained:Br=5.219kGs,Hcj=4.078kOe,?BH?max=4.685MGOe.?2?(Ce_1-xLa_x)2Fe14B?x = 0,0.1,0.2,0.3,0.4,0.5,0.6,0.7?melt spun ribbons were prepared.It is indicated that the formation of TbCu7 is suppressed while the formation of 2:14:1 phase is promoted through a certain amount of La addition,which is leading to fine microstructure.However,when the La-doping content reaches to 50at%,the 2:14:1 phase begins to decompose.The remanence and energy product are promoted significantly with La substitution but the intrinsic coercivity is more stable,it is relevant to the enhancement of the saturation magnetization Ms.The optimum magnetic properities are obtained in the?Ce0.7La0.3?2Fe14B alloy:Br=6.690kGs,Hcj=2.325kOe,?BH?max=4.593MGOe??3?[Nd1-x?LayCe1-y?x]2Fe14B?x = 0,0.1,0.2,0.3,0.4,0.5,0.6,0.7;y=0,0.2,0.4,0.5,0.6,0.8,1.0?melt spun ribbons were prepared.The La-Ce co-doped Nd-Fe-B alloys are compared with the La-doping and Ce-doping respectively by controlling the doping ratio of La/Ce.It was concluded that:?The alloys remain single 2:14:1 phase with Ce-doping;In contrast,the 2:14:lphase begin to decompose when the doping content of La reaches 30at%.The 2:14:1 phase decomposed seriously with the La-doping content over 50at%,accompanied by the rising of secondary phases.?The magnetic properties of Nd-Fe-B alloy decrease with the increasing of La,Ce doping content.The performance of La-doped Nd-Fe-B is superior to that of Ce-doped when the addition of La was less than 50at%.However,the severe separation of 2:14:1 phase when x reaches 0.5 leads to the sharp decline of magnetic properties of La-doped Nd-Fe-B.? In general,the magnetic properties,especially for the Br and?BH?max of La-Ce co-doped Nd-Fe-B are better than single doped cases.And the Co-doped Nd-Fe-B show greater enhancement of?BH?max with the increasing amount of total doping content.For example,when the total doping concentration reaches 50at%,the?BH?max of[Nd0.5?La0.6Ce0.4?]2Fei4B still reaches 12.416MGOe,which is much higher than?Nd0.5Ce0.5?2Fe14B?BH?max=8.156MGOe).?4?The density functional theory?DFT?was applied to understand the effect of La substitution on the structure and intrinsic magnetic properties.It is indicated that La energetically prefers to occupy the 4g sites,the calculated substitution energy of Ce by La is positive implying that the substitution of La for Ce will lower the structural stability,which is consistent with the experimental results.The unit cell volume expands slightly with La substitution and the Fe-Fe distance is elongated simultaneously,which lowers the overlap of atomic states and hence the magnetic moments increase.As a consequence,the total magnetic moment of unit cell is enhanced.The calculation results reveal the mechanism of the enhancement of Ms which is founded in experiment.