Microstructure and magnetic properties of binary Mn54-Al46 alloy particulates obtained by severe plastic deformation processing via end-milling and annealing

Abstract

We utilize the plastic deformation process associated with the machining operation of end-milling for the preparation of ε-phase Mn-Al particulates with micron scale external dimensions and self-similar shapes from binary as-cast feedstock with composition 54 at.% Mn and 46 at.% Al. The structural evolution and development of magnetic properties in response to the single-pass plastic deformation processing by the end-milling and subsequent thermal annealing have been studied. Effects of end-milling process parameter combinations, primarily the rotational speed of the cutting tool, have been identified to permit genesis of morphological self-similar particulates of ε-phase characterized by cold-worked and sub-micron to nanometer scale refined microstructures. The end-milling derived particulates were suitable for preparation of τ-phase based MnAl particulates with enhanced permanent magnet properties via isothermal annealing. Discernibly higher rates for the exothermic ε-phase decomposition reactions in the end-milling derived particulates have been observed for isothermal annealing at temperatures in the range of 673 K ≤ T ≤ 723 K. The enhanced decomposition reaction kinetics have been attributed to reduced effective nucleation barriers for the respective product phases stemming primarily from reduced grain size. Large saturation magnetization and coercivity in combination with reasonable squareness ratios of the magnetization curves resulted in a (BH)max = 4.2 MGOe for the end-milling derived binary Mn54Al46 particulates obtained for the highest rotational tool speed of 4200 RPM used here after annealing for 20 min at 673 K. The ε-phase Mn-Al particulates derived by end-milling have potential as precursors for the fabrication of τ-phase Mn-Al based PM materials.