Relative stability of the metastable D019 and D03 structures in Fe3Ga-type alloys at elevated temperatures

Abstract

The stability of two metastable structures (D0₁₉ and D0₃) in Fe₃Ga-type alloy was investigated using EBSD analysis on stepwise annealing at elevated temperatures 475 and 550 °C. Initially, the Fe-28 Ga alloy quenched from 640 °C in liquid nitrogen, i.e. from the range of equilibrium D0₁₉ phase, exhibits a dominant metastable D0₁₉ phase (97 % or more) at room temperature. Being subjected to annealing at 475 °C, the D0₁₉ phase demonstrates much higher stability, compared with D0₃ structure, at early annealing stages, with only a little amount transitioning to L1₂ phase after 60 min. With an increase in annealing temperature to 550 °C, the D0₁₉ structure transforms more rapidly to L1₂ over the same annealing period. The little amount of D0₃ phase (∼3%) in the structure of the quenched sample exhibits priority in the transition to L1₂ compared to D0₁₉ at both temperatures, fully transforming within 5 min at 550 °C and disappearing after 60 min at 475 °C, which is additionally proved using a 26 %Ga sample with a mixture of D0₃ and D0₁₉ phases. These findings highlight distinct kinetic differences between D0₁₉ and D0₃ transitions and proposed dependence on their equilibrium temperature, as summarized in the constructed TTT-diagrams. This phase stability study provides insights into tailoring functional properties of Fe-Ga alloys.