Anomalous expansion of interatomic distance in liquid Al-Zn alloy during cooling

The structural and physical/chemical properties of metallic materials are closely linked to the composition and configuration of their molten state. In this study, the evolution of the local structure in Al-Zn alloys with varying compositions during the quenching process was investigated using on-the-fly machine learning force field (MLFF) simulations based on ab initio molecular dynamics. The results indicate that the first coordination shell of the Al-Zn alloy melt undergoes an anomalous expansion within a specific temperature range, which deviates from the previously reported linear negative expansion, such as metallic Al, Zn, and Sn. The temperature interval of the anomalous expansion decreases with increasing concentration of Zn. Local structural changes, including the abnormal increase in coordination number and the emergence of a shoulder in the second peak of the bond angle distribution function, further confirmed the presence of the anomalous expansion. In addition, the slope of the energy-temperature relationship and the activation energy of diffusion change upon temperature decrease, which suggests that this phenomenon is associated with atomic diffusion driven by thermal excitation. The anomalous expansion is also confirmed in a larger system (over 104 atoms) by MLFF simulations with first-principles accuracy. We found that an increase in cluster radius induces a decrease in pressure due to the interfacial energy of the cluster at ∼1200-1100 K for Al-38Zn melts, which in turn leads to a looser arrangement of the atoms. This study provides valuable insights into the mechanisms governing atomic structure evolution and liquid-liquid transitions.