Stabilities and mechanical properties of novel Mg-based light metal compositionally complex alloys

Compositionally Complex Alloys (CCAs) consist of four or more elements alloyed in approximately equal fractions and often crystallize in a simple crystal lattice. In many cases, their mechanical properties like structural stability or ductility exceed that of common modern alloys. Usually they contain heavy d-Orbital metals, but investigations into low density light metal CCAs have been rare up to now due to the complex binding modes of their constituents.

We use both a Cluster Expansion approach, augmented by stochastic prescreening steps, and neural network based pair potentials to scan the large configuration space of the Mg-Al-Cu-Zn system for stable phases. The training data was generated using density functional theory calculations implemented in the VASP code. In conjunction with experiments, we find that while the introduction of Al into the brittle MgZn₂ hexagonal Laves phase leads to phase separation and does not improve the mechanical properties of the alloy, the addition of Cu inhibits this process and leads to the formation of a highly stable cubic phase. We find that further increasing the Cu concentration leads to higher hardness of the samples, which is also reflected in an increase of the calculated bulk modulus.