Stability of metastable phases: pathway-dependent transformation kinetics of the γ phase in eutectic AuGe

The transformation kinetics of metastable phases into more stable phases with simultaneous variation of the phase composition is studied in metallic systems using the binary Au₇₂Ge₂₈ eutectic alloy as model system. This alloy forms a metastable γ phase and a non-equilibrated α-Au and Ge composition after cooling the melt at a rate of 10,000 K/s. The kinetics of the γ-phase decomposition is analyzed using Fast Differential Scanning Calorimetry (FDSC) and Transmission Electron Microscopy (TEM). After correcting for the influence of thermal inertia, non-isothermal measurements of the transformation show an activation energy of 87 kJ/mol for the γ-phase decomposition, whereas isothermal studies reveal an activation energy of 121 kJ/mol. This difference indicates pathway-dependent transformation kinetics, resulting from a phase equilibration of the α-Au phase due to diffusion of Ge along grain boundaries. We propose that for the non-isothermal case Ge diffusion occurs before the γ-phase transition, whereas both processes take place simultaneously during the isothermal transformation. History-dependent equilibration of non-equilibrium phases thus significantly influences the pathways of metastable phase transitions. In fact, studying the pathway-dependent kinetics appears to be important in many solid-state transformations to gain a better understanding of the processes involved and create appropriate models.