Abnormal orderly transformation in supercooled state of an Al-based alloy

Abstract

To understand the underlying mechanism behind the precipitation of high-density face-centered cubic Al nanoparticles during the annealing or preparation of Al-based metallic glass, we used molecular dynamics simulation technique to study the atomic structure evolution of Al-rich and Al-depleted melts in the Al-Zr binary system under rapid cooling conditions. From the quantitative characterization of thermodynamic properties, kinetic properties, and atomic structure, we found that an abnormal orderly transformation occurs in the supercooled liquid of Al-rich alloy, which includes two basic stages, namely, directional diffusion and stabilization of defect clusters. The thermodynamic essence of this transformation is to reduce the Gibbs free energy of the entire system by improve the cluster packing efficiency of atomic structure, preventing the supercooled liquid from being in an unstable state. An important consequence of the abnormal orderly transformation is the formation of nanoscale Al-rich regions in the supercooled liquid. These regions can readily promote the precipitation of high-density face-centered cubic Al nanoparticles, thereby reducing the complete amorphization ability of Al-based alloy melt.