Icosahedral Ordering Induced by Cr in Al–Zn Alloy Liquid

Cr atoms can promote the formation of icosahedral short-range order (ISRO) in Al–Zn alloy liquid, which is believed to be the structural origin of the quasicrystal (QC) phase in the assumption of a QC-enhanced nucleation mechanism. However, this claim still lacks experimental evidence. In this work, we investigate the liquid structure of Al–Zn and Al–Zn–Cr alloys through high-energy X-ray diffraction at a synchrotron facility. The constrained reverse Monte Carlo simulation method is used to obtain the three-dimensional atomic configuration. The results show that the Cr addition reduces the formation of Al-centered ISRO and increases the formation of Zn-centered ISRO. Large amounts of ISRO or high five-fold symmetry are found around Cr atoms. Furthermore, the correlation between Al and Zn atoms is significantly changed due to the Cr addition. The pentagon-rich Al-centered and Cr-centered clusters show a strong tendency to aggregate and build interpenetrating structures on a medium-range scale. The icosahedral medium-range order (IMRO) will form through the linkage of vertex-, edge- and intercross-shared types of the Cr-centered icosahedra or icosahedra-like clusters. In particular, an IMRO structure with a composition similar to the Al 45 Cr 7 phase is found, which is considered the intermediate phase of fcc-Al nucleation. These findings may provide structural hints of the QC-enhanced nucleation mechanism.