The thermodynamics of multicomponent high-entropy materials
Human development has been based for millennia on manufacturing materials using a conventional alloying strategy of selecting a principal component for the primary property requirement of the material and then adding one or two minor alloying components at relatively low concentrations (either to enhance the primary property or to provide additional secondary properties).
Writing in Springer Nature, Professor Brian Cantor explains that we have relatively recently discovered that, with sufficiently large numbers of alloying elements added in sufficiently large concentrations, the configurational entropy of mixing can often become large enough to suppress the formation of brittle compound phases, leading instead to the formation of multicomponent high-entropy solid solutions. There are billions of multicomponent high-entropy-single-phase solid-solution materials, with a wide range of exciting new properties, in most cases relatively straightforward to manufacture by conventional processing methods. There does seem to be an upper limit to the effects, so that beyond 10 or 12 alloying components, the inevitable increase in chemical diversity prevents the configurational entropy of mixing from suppressing the increasingly strong chemical reactions between them.
Professor Cantor's paper 'The thermodynamics of multicomponent high-entropy alloys' discusses in more detail the thermodynamic balance between: (i) increasing configurational entropy and promoting high-entropy solid solutions; and (ii) increasing chemical diversity and promoting the formation of brittle compound phases. These are discussed on the basis of regular solution thermodynamics, without the need for more complex, semi-empirical Calphad calculations that can sometimes obscure the underlying key physical and chemical principles.
The results reveal that:
(i) why large single-phase solid-solution regions are relatively common in multicomponent phase space;
(ii) why single-phase solid solutions are often favoured over stoichiometric compounds for large numbers of chemically similar components, in concentrated rather than dilute proportions, at high temperatures, and after quenching to room temperature; and
(iii) the difficulty of detailed thermodynamic analysis in multicomponent materials because of the large numbers of thermodynamic parameters that need to be known and the corresponding lack of underlying thermodynamic measurements.
