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 D0(3) and equilibrium f.c.c.-derived L1(2) 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 degrees C. It has been revealed that the transition proceeds with alternation of the first- and second-order phase transformations according to a D0(3) -> A2 -> A1 -> L1(2) 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 D0(3) -> L1(2) transition is well described by the JMAK equation. The value of the Avrami exponent corresponds to the constant growth rate of the new L1(2) phase and decreasing nucleation rate in the Fe-27.2Ga alloy and indicates the presence of pre-existing nucleation centres of the L1(2) phase in the Fe-28.0Ga alloy.