The first- and second-order isothermal phase transitions in Fe₃Ga-type compounds

Structural features and kinetics of the transition between ordered metastable b.c.c.-derived D0₃ and equilibrium f.c.c.-derived L1₂ phases of Fe–xGa alloys (x = 27.2% and 28.0%) have been analyzed by in situ real-time neutron diffraction during isothermal annealing in the temperature range 405–470°C. It has been revealed that the transition proceeds with alternation of the first- and second-order phase transformations according to a D0₃ → A2 → A1 → L1₂ scheme, where A2 and A1 are disordered b.c.c. and f.c.c. structures. Deformations of the crystal lattice that arise due to these transitions are determined. The kinetics of the L1₂ phase nucleation and growth were analyzed in the frame of the Johnson–Mehl–Avrami–Kolmogorov (JMAK) model; however, only the early stage of the D0₃ → L1₂ transition is well described by the JMAK equation. The value of the Avrami exponent corresponds to the constant growth rate of the new L1₂ phase and decreasing nucleation rate in the Fe–27.2Ga alloy and indicates the presence of pre-existing nucleation centres of the L1₂ phase in the Fe–28.0Ga alloy.