The Research Of Mn-Al-C Type Magnetic Alloys

Mn-Al-C system alloys have drawn attention for their attractive properties, such as low price, good machinability, corrosion resistance and better magnetic properties. The C element in Mn-Al-C alloys can stable the ferromagneticÏ„-phase, then enhance the coercivity of alloys. Carbon nanotubes (CNTs) are six-ring-based one-dimensional nano-materials. In this paper, the graphite carbon was replaced by CNTs, and the as-cast, solution-treatment, annealed, melt-spun and bonded magnets were investigated using XRD, SEM, VSM and DTA.It was found that the Mn₅₁Al₄₆(CNTs)₃ and Mn₅₁Al₄₆C3 as-cast alloys consisted of four phases. The main phase ?Ï„-phase, Al-rich Al8Mn5-type structureγ2 phase, Mn-richβ-Mn-type and AlMn3C-type phases. The replacement of CNTs for graphite does not change the phase constitution of Mn-Al-C alloys, but changes the crystallization process and the chemical composition and distribution of phases, which results in the formation of cellular surface morphology in the as-cast Mn₅₁Al₄₆(CNTs)₃ alloy. Mn₅₁Al₄₆(CNTs)₃ alloy performs better magnetic properties than Mn₅₁Al₄₆C3 alloy under the same condition. Solid-solution and aging treatments enhance the magnetization of both alloys, and in Mn₅₁Al₄₆(CNTs)₃ alloy a significant enhancement in magnetization and remanence occurs. It was found that the aged Mn₅₁Al₄₆(CNTs)₃ alloy had the highest coercivity Hc=1322.6 Oe, remanence Mr=36.3 emu/g and maximum magnetization M2T=93.8 emu/g under an applied field of 20 kOe(2T). However, the Curie temperature ofÏ„? phase in Mn-Al-C alloys decreases 22-26 oC after the replacement of CNTs for graphite.It was found that the surface of melt-spun ribbons prepared at a speed of 30 m/s was composed of refined lamellar dendrites, whose main phase wasεphase besides littleβ-Mn-typeandγ2-type phase, and the ribbons possessed poor magnetic properties. The main phase of ribbons changed toÏ„phase, whose Curie temperature was 270.6 oC, after annealing at 500 oC for 1h and performed a better magnetic propertis. The surface morphology of annealed ribbons was composed of approximately uniaxial grains and the grain size was smaller than 2μm. Ball-milled ribbon powders were dominated byεphase, while ball-milling can enhance the distortion of lattice and lead to finer grain size. Bonded magnets, whose main phase wasÏ„phase, were prepared by ball-milling method followed by pressing and annealing at 600 oC. The bonded magnet approached the highest coercivity of 3581.3 Oe. While annealed at 500 oC, the bonded magnet obtained the highest M2T (73.4 emu/g), Mr (39.9 emu/g) and Mr/Ms (0.54) respectively.It was found that magnetic properties of Mn53Al45(CNTs)₂ alloy were lower than those of Mn₅₂Al₄₆(CNTs)₂ alloy under the same condition. Effect of the melt-spinning speed on magnetic properties of Mn₅₂Al₄₆(CNTs)₂ alloy has been studied. The coercivity, remanence, remanence ratio of the melt-spun ribbons decreased with increasing the melt-spinning speed from 30 m/s to 50 m/s. The highest coercivity of Mn₅₂Al₄₆(CNTs)₂ ribbons melt-spun at 30 m/s is 2249.2 Oe.