Modelling the thermodynamics and phase stability of high-entropy alloys: Concentration waves in the multicomponent setting

So-called ‘high-entropy alloys’ (HEAs)—those alloys containing four or more elements combined in near-equal ratios—are of interest not only because their physical properties make them well-suited to a range of next-generation engineering applications, but also because they exhibit a range of interesting physical phenomena, including Fermi surface smearing, complex magnetism, quantum critical behaviour, and superconductivity. From the perspective of theory and simulation, they represent a challenging class of materials to study due to their chemical complexity and the huge spaces both of potential alloy compositions and of possible atomic configurations.

 

In this talk, I will outline a new, computationally efficient modelling approach developed for studying the phase stability of these systems [1-6], which is based on representing atomic-scale chemical fluctuations as ‘concentration waves’ describing a range of potential ordered (and segregated) structures. The approach begins with a calculation of the electronic structure and internal energy of the disordered solid solution as described via the coherent potential approximation (CPA) and the Korringa–Kohn–Rostoker (KKR) formulation of density functional theory (DFT). Subsequently, a perturbative analysis of the alloy’s free energy facilitates assessment of the energetic cost of these fluctuations. It is then possible both to infer phase transitions directly via application of a Landau-type theory, as well as to recover atom-atom effective pair interactions suitable for use in subsequent atomistic simulations.  Here, I will present results from case studies on a range of prototypical high-entropy alloys, demonstrating that the approach captures the phase behaviour of these systems, as well as providing fundamental insight into the electronic (and occasionally magnetic [3]) origins of atomic ordering tendencies. I will make links with experimental observations where appropriate.