Probing the entropy hypothesis in highly concentrated alloys

Abstract

High Entropy Alloys (HEAs) designate a class of multicomponent metallic alloys in nearly equiatomic compositions. According to the constituting postulate, a larger number of constituents in a solid solution increases its configurational entropy, which has a maximal value when constituents exist in equiatomic concentrations and this is sufficient to overcome enthalpic contributions between the alloy components which would otherwise favor compound formation or phase separation. This entropy effect would, thus, stabilize disordered, crystallographically simple, solid solutions. Since then, numerous HEA candidate systems have been experimentally studied. It is unclear, however, if, and to which extent, the configurational entropy can be accessed by the system in the way it is suggested. The present work deals with this question using theoretical/computational methods. First, a series of model Body Centered Cubic (BCC) systems involving strong symmetric interactions between unlike atom pairs (referred to as “equinteracting” systems), containing up to five components, is investigated using the Cluster Variation Method in the irregular tetrahedron approximation. The symmetry of interactions, though artificial, allows for the straightforward presentation of phase diagram sections even for quaternary and quinary systems. Next, a “real” HEA candidate system, VNbTaMoW, which presents the BCC structure, is investigated by ab initio calculations, allowing to extend the conclusions to a realistic case with asymmetric interactions. The results show that configurational entropy has a small, even marginal, effect on phase transitions and the competition between conflicting interactions in the solid solution (i.e. frustration) seems to be the relevant factor behind the observed stabilization in the disordered states in HEA systems.