A new complex ternary phase in the Al-Cr-Sc push-pull alloy

Abstract

The purpose of this study is to find new ternary intermetallic compounds within the Al-Cr-Sc system, which is typical of a push-pull system in which two constituents (Cr and Sc) are immiscible, whilst they form compounds, respectively, with the third constituent (Al). By arc-melting of the three components aluminium (Al), chromium (Cr) and scandium (Sc) under inert atmosphere, a range of different alloys was produced. The microstructure was observed using scanning electron microscopy (SEM) and the phase compositions were analysed by electron dispersive X-ray spectroscopy (EDS). The samples were measured with X-ray diffraction (XRD) and transmission electron microscopy. The results were confirmed by Rietveld refinement. Three different phases were found in the microstructure of the samples, the two known binary phases Al16Cr10 and Al3Sc and a new ternary phase coined ϕ-Al8Cr4Sc. ϕ-Al8Cr4Sc showed great similarities to other Al8Cr4RE compounds with tetragonal I4/mmm crystal structure. Starting from the structure of already known Al8Cr4RE crystals, ab initio calculations were performed to determine the crystallographic parameters of ϕ-Al8Cr4Sc and also to investigate its electronic structure, which identified the energy band factor as the key factor that determines the stability of this compound. The lattice parameters and atomic positions were found in good agreement with the ones obtained by Rietveld refinement. Experimental atomic resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image analysis of the ϕ-Al8Cr4Sc phase confirmed the predicted atomic model.