Characterization of an Al-Cu-Mg-Zn Multi Principal Element Alloy by Experimental and Computational Screening Methods

We present a combined experimental and computational investigation of phase stability and mechanical properties in the Al-Cu-Mg-Zn quaternary system. Samples containing different relative compositions were prepared using magnetron sputtering and investigated by electron microscopic and X-ray based methods. To classify the technical relevance of the samples, the indentation hardness was measured.  The phase stability was studied computationally using a cluster expansion approach based on density functional theory (DFT) methods to allow for a comprehensive screening of the configuration space. Upon decreasing Cu concentration, a transition from an fcc to a mixed fcc/bcc crystal system and significant changes in the mechanical properties depending on Valence Electron Concentration (VEC) and atomic size differences (δr) was observed experimentally. The corresponding crystallographic phases were assigned by XRD and the experimentally observed phase transition could be confirmed by computing formation energies for the corresponding  ground-state structures. Since to date, quaternary complex light metal alloy systems cannot be reliably predicted, this is an important step towards a priori modelling of this class of materials.