Equilibrium diagrams - Ternary and multiconstituent alloys

In the article , devoted to binary phase diagrams, gave the definition of phase diagrams and showed that the very existence of such diagrams was consistent with the predictions of chemical thermodynamics according to Gibbs' theories. Readers of this article are invited to refer first to the article Binary alloys before turning to this text, which is a logical sequel.

In the case of binary alloys, a single composition variable was sufficient to define the chemical composition of the alloy. With this variable on the abscissa and the temperature variable on the ordinate, a flat graphical representation of the alloy equilibrium could be obtained. In multicomponent systems, the number of composition variables is greater than one. Ternary phase diagrams thus occupy a three-dimensional space; from quaternary systems onwards, diagrams occupy hyperspaces with more than three dimensions. Ternary systems are thus the most complex that can yet be represented in their entirety in space. This three-dimensional representation is of didactic interest for understanding the qualitative evolution of alloys, e.g. solidification paths, but it soon reaches its limits, as it is never quantitative and, what's more, some readers find it difficult to "read in space". In the first half of the 20th century, these spatial representations were used extensively, and some specialists were masters of this now somewhat esoteric graphic art. Today, the tendency is to abandon cavalier perspectives and block out a sufficient number of variables (or impose specific conditions), in order to reduce ourselves to flat representations in all cases. This means giving up the global vision of the system, which in any case would be impossible beyond three elements. The ability to model multiconstituent equilibria using appropriate thermodynamic software makes it possible to work in hyperspaces containing any number of variables, and to extract certain planar representations.