Structure And Performance Of La/Ce-rich Multi-main-phase Re-Fe-B Permanent Magnets

The global rare earth(RE)criticality,especially for those closely-relied Nd/Pr/Dy/Tb in the 2:14:1-type permanent magnets(PMs),has triggered tremendous attempts to develop new alternatives recently.Prospective candidates La/Ce with high abundance,however,cannot provide an equivalent performance due to inferior magnetic properties of(La/Ce)2Fei4B to Nd2Fei4B.Here we report high figure-of-merit La/Ce-rich RE-Fe-B PMs,based on the new prototype of multi main phase(MMP)approach by mixing the La/Ce-free and La/Ce-rich RE2Fei4B terminal powders.Through optimizing the sintering and annealing process,the magnetic performance of MMP magnets can be further enhanced by maintaining the inhomogeneous La/Ce distribution,reaching appreciable densification,inhibiting abnormal growth of RE2Fei4B grains,and forming a continuous RE-rich phase(with cubic-RE2O3 structure)network simultaneously.Meanwhile,the corrosion resistance and mechanical properties,which are also critical criteria when assessing their applicability,are investiagated.Main results of the work are as follows:By La-Ce co-doping into[(Pr,Nd)1-x(La,Ce)x]2.14Fe14B(0 ? x ? 0.5)compounds,their phase structure,magnetic performance and Ce valence are analyzed.Over the whole doping range,2:14:1 tetragonal structure forms and keeps stable even at 1250 K.With increasing La-Ce content,lattice parameters a,c and saturation magnetization Ms firstly decrease(composition range I with 0 ? x ? 0.3),and then increase abnormally(composition range II with 0.3<x ? 0.5).Such crystallographic and magnetic anomalies are attributed to the desirable Ce valence shift towards the magnetically favorable +3 state,which is probably induced by the co-doping of large radius La3+ into 2:14:1 phase lattice.La/Ce-free terminal with(Pr,Nd)29.8Gd1.7FebalM1.1B1.0 and La/Ce-rich terminal with(Pr,Nd)20.3(La,Ce)9.5Gd1.7FebalM1.1B1.0 are designed to prepare the MMP RE-Fe-B magnets.A perculiar microstructural feature is observed that RE concentration in the main phase differs a lot either over local regions within an individual grain or across grains,being completely different from the single main phase(SMP)magnets where La/Ce/Nd are homogenously distributed in the main phase.The resultant short-range exchange couplings of local regions within an individual 2:14:1 main phase grain,and the long-range magnetostatic interactions among different 2:14:1-type grains in these magnets facilitate superior magnetic performance including Br and Hcj.At 18 wt.%La-Ce,(BH)max of the MMP magnet reaches?41.2 MGOe,basically comparable to that of the La/Ce-free starting magnet(?42.0 MGOe).Meanwhile,Br and Hcj are well maintained at?12.91 kGs and?13.1 kOe,even superior to those of the SMP one with only 9 wt.%La-Ce(Br = 12.65 kGs,Hcj = 9.8 kOe,and(BH)max= 38.3 MGOe).To optimize the magnetic performance of(Pr,Nd)21.3(La,Ce)8.5Gdi.7FebalM1.1B1.0(27 wt.%La-Ce)MMP magnet,chemical heterogeneity,microstructure and magnetic property changes as a function of the sintering and annealing condition have been investigated.High sintering temperature intensifies the interdiffusion of La/Ce and Pr/Nd,leading to a chemically more homogeneous RE distribution among 2:14:1 matrix phase grains.The decrease in chemical heterogeneity reduces the intrinsic magnetic properties including Js,HA,and TC.However,the unwanted magnetic dilution can be compensated at the optimal sintering condition of 1040 ? × 3 h by maintaining the inhomogeneous La/Ce distribution,reaching appreciable densification,inhibiting abnormal growth of RE2Fe14B grains,and forming a continuous RE-rich phase(with cubic-RE2O3 structure)network simultaneously.Further optimizing the annealing condition of 480 ? × 3 h,magnetic performance with Br = 12.56 kGs,Hcj 11.0 kOe,and(BH)max = 38.6 MGOe can be achieved.However,further arising annealing temperature or extending annealing time,the coercivity deteriorates at a high rate due to weakened inhomogeneity of La/Ce distribution and hence strengthened magnetic dilution effect.The corrosion behaviors of MMP RE-Fe-B magnets have been systemically investigated by gravimetric,electrochemical and immersion tests,compared to the SMP and La/Ce-free starting magnets.It reveals that MMP magnets exhibit superior corrosion resistance to the SMP ones,suggesting that MMP approach paves an effective way to enhance the anti-corrosion performance of the La/Ce-containing RE-Fe-B PMs.The functional role of MMP method can be summarized as:i)the formation of intergranular REFe2 phase can benefit the corrosion resistance by stabilizing the intergranular regions and reducing the corrosion activity.ii)The volume fraction of RE-rich triple junctions is significantly reduced,leading to decreased pathways for corrosion propagation and suppressed intergranular corrosion.Besides,a proper La-Ce substitution for Pr-Nd can enhance both the compressive strength ?bc and bending strength ?bb,and strengthen the main phase grains by increasing the micro hardness and Young's modulus.