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

• The transformation of a metastable phase is studied on the eutectic Au–Ge alloy. • The transformation kinetics are determined by Fast Differential Scanning Calorimetry. • The activation energy is lower for heating than for isothermal measurements. • The microstructural origin is investigated using Transmission Electron Microscopy. • Diffusion of Ge along grain boundaries causes different activation energies. 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 Au 72 Ge 28 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.