Thermodynamic principles of phase equilibrium
Equilibrium diagrams

In thermodynamics, a closed system is defined as a certain quantity of matter with a constant number of moles of each constituent pure substance. The definition of chemical constituents is tricky for some systems, where chemical reactions can modify the list of pure bodies actually present in the system. But in the case of alloys, the definition of constituents is unambiguous: each chemical element in the system represents a constituent. Indeed, in many metallic phases, the structural entity is the atom, even if in some cases there are more complex entities with strong internal bonds.

The closed system is bounded by a system contour, the permeability of which must be defined with respect to exchanges with the outside....