Impact of Thermal Gradient on Interfacial Energy and its Anisotropy in Al-Cu Alloy

The solid-melt interfacial energy and its anisotropy are fundamental parameters that govern the final microstructure of cast metals. The magnitude of the interfacial energy dictates the nucleation frequency, while its crystalline anisotropy determines the growth direction of dendrites. While the effect of solute type and concentration on these properties has been well studied, the influence of steep thermal gradients—typical of rapid solidification processes like metal additive manufacturing and welding—remains largely unexplored.

This work addresses the gap by using molecular dynamics simulations to examine the solid-liquid interface of an Al-4.5 at.% Cu system under thermal gradients from 0 to 30 K/nm. Our analysis shows that increasing the thermal gradient greatly reduces capillary-wave fluctuations at the interface. This decrease in fluctuations results in a clear, linear increase in interfacial stiffness and the average interfacial energy. This trend is similar to what is observed in pure aluminum, indicating that under non-equilibrium conditions, thermal effects become dominant over compositional effects. Conversely, and of significant importance for modeling, the interfacial energy anisotropy shows no dependence on the applied thermal gradient and remains essentially constant. These findings indicate that although the magnitude of interfacial energy needs to be adjusted for thermal effects in phase-field models, using equilibrium anisotropy values may be a reasonable approximation for simulating microstructure evolution in dilute Al-Cu alloys during rapid solidification.