Research On The Microstructure And Magnetic Properties For MnAl Alloy Prepared By Different Methods

Rare earth permanent magnets have been widely used because of its excellent magnetic properties.Due to the scarcity and resulting high cost of rare earth materials,non-earth-free permanent magnets have become an intensive focus of research.As a non-rare earth permanent materials,Mn Al alloy has drawn much attention due to its excellent magnetic properties?The theoretical maximum energy product is 12MGOe?,superior machinability,high specific strength,high modulus of elasticity,strong corrosion resistance,as well as low cost,which makes Mn Al magnets widely used in wind power,new energy automobile and other fields.As the only ferromagnetic phase,?-phase can be obtained through the followed process:firstly,the high temperature non-magnetic?phase?h.c.p.?transforms into a non-magnetic?^/-phase?orthorhombic?by an ordering reaction,and then a ferromagnetic?-phase?f.c.t.?can be achieved by a martensitic phase transition,i.e.???^/??.Mn₅₄Al₄₆ alloys were prepared by arc melting and melt-spinning tecnique.It was found that the 72.8emu/g saturation magnetization was achieved,which indicated that more than 79wt.%content of?phase was obtained for quenched arc-melt Mn Al magnet.It was shown that quenching can effectively promote the formation of ferromagnetic?phase.As high as 91wt.%content of?phase can be obtained by annealing at 350?for 0.5h.Effects of the melt-spinning and heat treatment processes on the magnetic properties were investigated in details for the Mn₅₂Al₄₆C₂ alloy.It was found that the annealing at 500?can lead to the formation of a large amount of?phase.The maximum coercivity of 198k A/m can be achieved,when melt-spinning speed of 15m/s,the annealing temperature of 500?,and 0.5h annealing time were adopted,respectively.The highest remanence,up to 36.9emu/g,can be obtained by using the melt-spinning speed of 20m/s and the annealing process of 650?/0.5h.Using n-heptane as solvent,oleic acid and oil amine as surfactant,Mn₅₂Al₄₆Cu₂powder with high coercivity was prepared by high energy ball milling technique.It was found that the coercivity enchanced with the increase of milling time until 8h and decreases afterwards,where the maximum coercivity with 366k A/m was obtained.XRD and TEM results show that the amorphous phase distributed around the?-Mn Al phase contributes to the increased coercivity by increasing magnetic isolation effect and pinning effect of the domain walls.The annealing processes shows that the? phase content can be elevated by annealing at 400?⁶50?.The highest saturation magenetizaion of 58.4emu/g can be acquired at 650?,which is due to the fact that the?phase and?₂ phase can form more?-Mn Al phase by diffusion.The optimum magnetic properties with the coercivity of 281k A/m and 49.0emu/g remanence were obtained,respectively,by annealing the sample at 400?for 0.5h.The results show that the non-uniform distribution of the amorphous phase,?phase and?₂ phase can produce non-negligible local demagnetization field with non-uniform distribution,and this leads to the inhomogeneity of magnetic anisotropy in the Mn Al magnets.Therefore,the open recoil loops can be demonstrated.Based on the spark plasma sintering technique,effects of different sintering processes on magnetic properties and microstructure were investigated by using Mn₅₂Al₄₆C₂ melt-spinning ribbons as sintering precursors.It was found that the magnetic phase decomposed as the sintering temperature increased.The optimum magnetic properties with 118k A/m coercivity and saturation magnetization of 0.357T can be obtained under the sintering conditions of 600?/50MPa/5min,respectively.Using spark plasma sintered magnets as precursors,the high density and high performance Mn Al C magnet was prepared by hot deformed technique.It was found that the remanence J_r in the perpendicular and parallel to the pressure directions were0.171T and 0.134T for the hot deformed magnet,respectively,which indicated that anisotropic hot deformed Mn Al C magnet was prepared successfully.The coercivity,up to 231k A/m,was obtained.Compared with the SPSed magnet,the coercivity and remanence were greatly improved.