In situ neutron diffraction study of metastable Fe66Ga34 alloy under isothermal annealing

Evolution of the phase composition of the as cast Fe66Ga34 alloy during long-term isothermal annealing at a temperature range of 480 – 600°C for 120 min was studied. To track these complex structural changes in real-time, the study utilized in situ neutron diffraction performed at the General Purpose Powder Diffractometer (GPPD) at the China Spallation Neutron Source (CSNS). The initial state of the alloy was characterized as a multi-phase mixture consisting of metastable Fe13Ga9 and ordered Fe3Ga (D03) phases, and the stable intermetallic compound α-Fe6Ga5. During the annealing process, changes in the volume fractions of these phases and precipitation of the A1 / L12 phase were observed. The study clarifies the interplay between these phases: the minimum D03 / B2 content correlates with the maximum A1 / L12 volume fraction, while the disappearance of Fe13Ga9 triggers the growth of volume fraction of α-Fe6Ga5. The results obtained confirmed that preliminary L12 phase precipitation is necessary for the Fe13Ga9 → α-Fe6Ga5 transformation to occur. The maximum precipitation rate of equilibrium L12 phase corresponds to the temperature of about 540°C. Furthermore, the upper limit of presence of metastable Fe13Ga9 in alloy is heating-rate dependent, shifting from 570°C at 2°C / min to >600°C at 10°C / min. The results also suggest the possibility of a reverse transformation of Fe13Ga9 into a phase with crystal structure based on BCC lattice (A2, B2 or D03).

1. Russian Science Foundation - 19-72-20080