Molecular Dynamics Simulation of Excessive Vacancy Formation in Rapidly Solidified Al–Cu and Fe–Cr Alloys

Vacancy trapping is a key nonequilibrium phenomenon during rapid solidification, yet its behavior in alloy solid solutions has not been fully clarified. In this study, molecular dynamics (MD) simulations of isothermal solidification were performed for Al–Cu (FCC) and Fe–Cr (BCC) alloys to investigate the occurrence of vacancy trapping and the temperature and composition dependence of the nonequilibrium vacancy concentration cneq. Vacancy trapping was observed in both alloy systems. In Al–Cu alloys, cneq increased almost monotonically with decreasing temperature, whereas in Fe–Cr alloys, it exhibited a non-monotonic dependence, closely resembling that reported previously for pure BCC metals. In the high-temperature regime (T/Tm ≥ 0.8), cneq correlated strongly with the interface velocity Vint, indicating that vacancy trapping is governed by interface migration kinetics. Analysis based on the Wilson–Frenkel model showed that solute additions increase the activation energy for diffusion across the interface in Al–Cu alloys, reducing Vint and suppressing vacancy trapping, while such effects were negligible in Fe–Cr alloys.