Research On The Microstructure And Magnetic Properties Of Hot-pressed Nd-La-Y-Fe-B Magnets

Nd-Fe-B based rare earth?RE?permanent magnetic materials have been widely used in various fields because of their excellent intrinsic magnetic properties.The ever increasing demand of RE permanent magnets is consuming a large number of key RE elements?Pr,Nd,Dy,Tb?.However,the high abundant rare earth?HARE?elements?La,Ce,Y?are constantly overstocked for a long time.The unbalanced utilization of RE resources restricts the sustainable development of RE industry.The research and development of HARE permanent magnets is an effective way to achieve balanced utilization of RE resources.Compared with the Nd₂Fe₁₄B main phase and Nd-rich phase,the thermal stability,phase composition and metallurgical behavior of RE₂Fe₁₄B?RE=La,Y?main phase and RE-rich phase are different.In this thesis,hot-pressed Nd-La-Y-Fe-B magnets were prepared by melt-quenching and hot-pressing techniques,and their magnetic properties,intergranular interactions,microstructure and elemental metallurgical behavior have been studied methodically.The main research is classified into three parts as follows:?1?The La-substituting(Nd_1-xLa_x)₁₃Fe₈₁B₆?at.%??x=0-0.5?ribbons were prepared by melt-spinning technique.It is shown that the magnetic properties decrease slightly with La substitution upto 30 at.%,and they do not deteriorate further for 40 at.%La substitution.The magnetic properties i.e.,remanence J_r=0.76 T,coercivity H_ci=630 kA/m and maximum magnetic energy product?BH?_max=97 kJ/m³ are obtained in(Nd_0.6La_0.4)₁₃Fe₈₁B₆ alloy ribbon.Compared to the alloy with 30 at.%La substitution,the amount of soft magnetic Fe₃B phase decreased in the sample with 40 at.%La,and the volume fraction of the hard magnetic RE₂Fe₁₄B main phase increased,which is beneficial to the magnetic properties.It is observed that(Nd_0.6La_0.4)₁₃Fe₈₁B₆ sample has smaller average grain size and narrow distribution range.?2?The magnetic properties,phase composition,intergranular magnetic interaction and microstructure of[(Nd_0.6(La_1-xY_x)_0.4]₁₃Fe₈₁B₆?at.%??x=0-0.8?quenched alloy have been studied.It was found that the magnetic properties are slightly improved when x?60 at.%,while they are reduced when x=0.8.The best comprehensive magnetic properties J_r=0.80 T,H_ci=654 kA/m,?BH?_max=107 kJ/m³ are achieved when x=0.6.Furthermore,the[(Nd_0.6(La_0.4Y_0.6)_0.4]₁₃Fe₈₁B alloy with a more uniform microstructure results in an enhanced intergranular exchange interaction.It turns out finally that Y-substituted alloy has higher Curie temperature,slightly improved coercivity at high temperature,and enhanced thermal stability.?3?Nd-La-Y-Fe-B permanent magnets were prepared by hot pressing technology.The hot pressing processes,magnetic properties,microstructure,elemental metallurgical behavior and their relationships were systematically studied.The magnet prepared by sintering at 760^oC at 100 MPa pressure for 5 minutes has higher density and more uniform microstructure.Its magnetic properties are:J_r=0.61 T,H_ci=313 kA/m,?BH?_max=47 kJ/m³.The RE-rich phase in the hot-pressed magnet easily distribute in the powder boundary,which is beneficial to the density of the magnet.As the temperature and the pressure increases,the RE content between the powder boundary increases significantly.Compared with the rare earth Nd element,the La element is more easily distribute in the powder boundary while the Y element is more stable inside the powder.