| The properties of permanent magnets are controlled by the intrinsic magnetic properties and the microstructure, and a large uniaxial magnetocrystalline anisotropy is the prerequisite to make a material suitable for permanent magnet. To transfer the magnetocrystalline anisotropy into effective coercivity, one needs to introduce the coercivity mechanism according to the properties of the materail. Although 93% of the theoretical value for (BH)max has been achieved in Nd2Fe14B experimentally, the gap between the coercivity and anisotropy field is still very large. The conventional sintered magnets are composed of grains with size of several micrometers, which is too large compared with the single domain size for Nd2Fe14B. Thus, it is expected that via grain size refining, it is possible to further enhance the coercivity of Nd2Fe14B permanent magnets. Moreover, it has been proved both theoretically and experimentally that the large uniaxial magnetocrystalline anisotropy can also be obtained in transition metal intermetallics. By far, the magnetocrystalline anisotropy of transition metal intermetallics has not been fully explored and there is still a large gap between the theoretical energy density and the experimentally achieved values. This thesis mainly focuses on the (Nd,Pr)Fe14B, MnAl and (Fe,Co)2B alloy systems and investigate the relationship between the microstructure and magnetic properties. The results are as follows:Based on the research on rare earth transition metal bulk glassy alloy, we prepared Nd7Pr4Fe63Nb3B23 alloy by copper mould casting. Nanocrystalline permanent magnet was obtained after annealing the Nd7Pr4Fe63Nb3B23 as-cast alloy. The alloy composed of (Nd, Pr)2Fe14B and (Nd,Pr)1.1Fe4B4, and the nanosized (Nd,Pr)2Fe14B grains formed within the matrix of (Nd,Pr)1.1Fe4B4 phase. Decoupled by (Nd,Pr)1.1Fe4B4 paramagnetic phase, the (Nd,Pr)2Fe14B grains exhibited single-domain-like characteristics in magnetizaiton and demagnetization process. High coercivity was obtained in this alloy, with μ0iHc=1.7T.Mn53.3Al45C1.7 permanent alloy was prepared by mechanical milling and the relationship between microstructure and magnetic properties was investigated. The milling was conducted on τ-MnAl phase alloy. Magnetically anisotropic flake-shaped powders were obtained and the easy axis lied within the planar direction. The coercivity of Mn53.3Al45C1.7 alloy increased with extended milling time. However, as the milling process reduced the atomic ordering degree in τ-MnAl, the magnetization decreased continuously with increasing milling time. The atomic ordering in τ-MnAl can be recovered by annealing the powders at 773K for 30min. Part of the τ-MnAl decomposed in the annealing process. The optimum magnetic properties were obtained in powders milled for 5h followed by annealing, with Mr= 55 Am2kg-1, μ0iHc= 0.28T, (BH)max=2.05 MGOe. The influence of Ni and Ti addition on the magnetic propertis of Mn53.3Al45C1.7 alloy has been investigated. The optimum magnetic properties were obtained in Mn52.8Al45C1.7Tio.5 alloy after 6h of milling followed by annealing, with (BH)max=2.11MGOe, Mr=51 Am2kg-1,μ0iHc=0.37T. The flake-shaped Mn53.3Al45C1.7 powders could be hot compacted and die-upsetted into anisotropic magnets, with easy axis lying pendicular to the pressing direction.The intrinsic magnetic properties of (Fe1-xCox)2B (x=0.20,0.25,0.30,0.35) alloy in the temperature range of 10K to 1000K were investigated, including magnetocrystalline anisotroy constant K1, spontaneous magnetization Ms and the Curie temperature Tc. (Fe0.75Co0.25)2B alloy exhibited the highest magnetocrystalline anisotropy in the temperature range investigated, with K1=450 kJ/m3, Ms=148 Am2kg-1 at room temperature. Mechanically milling in different media were conducted on (Fe0.75Co0.25)2B alloy and coercivity of 0.03 T can be obtained.Mn53.3Al45C1.7/(Fe0.75Co0.25)2B composite were prepared by mechanical milling, using Mn53.3Al45C1.7 as hard phase and (Fe0.75Co0.25)2B as semi-hard phase. The effect of (Fe0.75Co0.25)2B content and milling condition on the magnetic properties of Mn53.3Al45C1.7/(Feo.75Coo.25)2B composite was investigated. The composite exhibited single phase-like hysteresis loops, indicating that the two phase were exchange-coupled. However, the decomposition of τ-MnAl could not be avoided, which was unfavorable for the ideal exchange coupling between the two phases. |
PDF Full Text Request