Phase Transitions in Mechanically Milled Mn-Al-C Permanent Magnets

Abstract

Mn-Al powders were prepared by rapid solidification followed by high-energy mechanical milling. The rapid solidification resulted in single-phase ε. The milling was performed in both the ε phase and the τ phase, with the τ-phase formation accomplished through a heat treatment at 500 °C for 10 min. For the ε-milled samples, the conversion of the ε to the τ phase was accomplished after milling via the same heat treatment. Mechanical milling induced a significant increase in coercivity in both cases, reaching 4.5 kOe and 4.1 kOe, respectively, followed by a decrease upon further milling. The increase in coercivity was the result of grain refinement induced by the high-energy mechanical milling. Additionally, in both cases a loss in magnetization was observed. Milling in the ε phase showed a smaller decrease in the magnetization due to a higher content of the τ phase. The loss in magnetization was attributed to a stress-induced transition to the equilibrium phases, as no site disorder or oxidation was observed. Surfactant-assisted milling in oleic acid also improved coercivity, but in this case values reached >4 kOe and remained stable at least through 32 h of milling.

Study of phases evolution in high-coercive MnAl powders obtained through short milling time of gas-atomized particles

“…Ball milling is a well-known technique used for microstructure refinement and strain inducement, but also for promotion of metastable phase formation. Previous studies show that high coercivities (above 4 kOe) can be achieved in MnAl after milling for a typical time duration of several to dozen of hours [20,21,24,25]. A coercivity of 5.3 kOe has been recently reported by Lu et al [26] in isotropic MnAl powders by starting with melt-spun ribbons, annealing at 410 ºC for 30 min and subsequent milling for 23 hours.…”

“…Additionally, it is noted that carbon additions to MnAl stabilizes the -phase. Recently, a short milling time of 1-1.5 hours applied to ternary MnAl-C alloys was reported to donate powder coercivities of H c =4.1-4.6 kOe with -phase and -phase as starting materials, respectively [24].…”

Phase transformation and magnetic properties of rapidly solidified Mn-Al-C alloys modified with Zr

“…In magnetic materials, the microstructure, particularly the grain size, plays a critical role in maximizing the extrinsic coercivity. 11,20 Grain refinement can be facilitated by two factors: number and potency of the nucleation sites, and solute elements present in the melt that retards grain growth. [21][22][23] Carbon additions could increase the number of nucleation sites or may tend to segregate, restricting grain growth during the solidification process.…”

“…7,8 The highest coercivity in C-added Mn-Al was obtained for carbon content just above the solubility limit of carbon. [9][10][11] Thus, the composition of Mn 54 Al 43 C 3 was used as reference composition in this research.…”

Manganese-based Permanent Magnets

“…Four of the papers are focused on MnAl and MnBi, which exhibit a L10 tetragonal structure. Three of the papers are concerned with processing of MnAl either by rapid solidification to ribbons followed by annealing [1], or by mechanical milling of rapidly-solidified powders followed by either simply annealing [2] or by warm consolidation to bulk material using equal channel angular extrusion [3]. In the paper by Park et al [4], the magnetic moment, magnetocrystalline anisotropy energy, Curie temperature, and electronic structure of MnBi have been calculated.…”