Tailoring the microstructure, magnetic properties and interaction mechanisms of Alnico-Ta alloys by magnetic field treatment

Abstract Reducing the spatial dimensions of spinodally decomposed phases and enhancing their magnetization difference is a promising route for enhancing the magnetic properties of Alnico alloys. In this paper we report the effects of various heat treatments on the formation of spinodal phases and magnetic properties of Alnico cast alloys. After optimizing the heat treatment conditions, small amount of refractory element Ta is introduced into the alloys and the effect of Ta on the formation and purity of spinodal phases, magnetic properties and phase transition temperatures are investigated. The addition of Ta element improved the intrinsic coercivity (Hcj) of the alloys from 1.52 kOe for standard alloy to 1.83 kOe for 0.6% Ta, while the remanence (Jr) remained nearly unchanged. Scanning electron microscopy analysis confirmed the rod-like Fe–Co rich (α1) phase embedded in Al–Ni–Cu–Ti rich matrix phase. A volume fraction of higher than 67% is obtained for α1 phase, while an ameliorating effect of γ-phase suppression at grain boundaries and triple junctions was noted. The alloy with optimized composition possessed extraordinary thermal stability in the temperature range 300–800 K, which is attributed to the shape anisotropy and uniformly distributed nano mosaic structure. Finally, the interaction mechanisms in Alnico alloys were investigated by means of Henkel plot and the motor applicability was analyzed by recoil loops.