Transformation Pathways of Ferromagnetic Mn-Al-Ga-Ni

This study investigates the impact of alloying Mn-Al-Ga with 3 at.-% Ni and the stability and formation mechanisms of the (Formula presented.) phase and the resulting magnetic properties. The stabilizing effect of Ga on the (Formula presented.) phase was verified, and the ternary alloy’s magnetization was measured up to (Formula presented.) (Formula presented.). The phase transformation from (Formula presented.) ₂ to (Formula presented.) in ternary Mn-Al-Ga was demonstrated microscopically. The solubility limit of Ni into the (Formula presented.) phase was exceeded at 3 at.-% and a primitive cubic κ phase formed. The Ni addition stabilized the (Formula presented.) phase. The highest magnetization at 2 T for the Mn₅₂Al_39.4Ga_5.6Ni₃ alloy was (Formula presented.). A new transformation pathway was demonstrated by first annealing the Mn-Al-Ga-Ni alloy at 800 °C for 24 h, which forms a nearly single κ phase, which is followed by a second anneal at 500 °C for 24 h at which the (Formula presented.) phase formed with some remaining κ phase. This is a new transformation mechanism since it involves a phase reaction from κ to τ. The energy product of the Mn-Al-Ga-Ni alloy exceeded that of the ternary Mn-Al-Ga alloy by a factor of 4.5. The κ-phase particles in the Mn-Al-Ga-Ni alloy hinder magnetic domain boundary motion, thus providing a method for magnetic hardening and increasing the energy product.