A phase-field study on the formation of the intermetallic Al₂Au phase in the Al–Au system

The growth mechanism of the intermetallic Al₂Au phase in the Al–Au system during the process of soldering is investigated by a multiphase phase-field model. With the aid of the assessed thermodynamic data (Li et al., 2004), the binary phase diagram of the Al–Au system is reconstructed by assuming a regular solution model for the intermetallic Al₂Au phase. The simulation setup is composed by placing an Al-liquid droplet on top of an Au-solid substrate. To capture the phase boundary evolution of the droplet and substrate interface with respect to the surrounding vacuum phase, the binary phase diagram is extended to a ternary model system at one particular temperature between the melting points of Al and Au. Due to diffusion of Al and Au at the droplet/substrate interface, intermetallic phase is formed. In simulations, we investigate the lateral and vertical growth velocities of the intermetallic phase as a function of the surface energy of liquid–intermetallic. Further, the instability of the liquid(Al)–solid(Au) interface is analyzed by examining the shrinking speed of the liquid–solid interface affected by the diffusivity of Au