Under the Spinodal Curve by Hanuš Seiner

In the early 1940s, Bradley reported the observation of sidebands around the Bragg peaks in the X-ray diffraction pattern of a Cu-Ni-Fe alloy that had been quenched and then annealed inside the miscibility gap. Willard Gibbs described two criteria for a metastable phase: that it must remain stable against a small change over a large area. That is, another biphasic system becomes lower in free energy, but the homogenous phase remains at a local minimum in free energy, and so is resistant to small fluctuations. In contrast, nucleation and growth occur when a homogenous phase becomes metastable. An unstable phase lies at a maximum in free energy. Spinodal decomposition occurs when a homogenous phase becomes thermodynamically unstable. Furthermore, in spinodal decomposition, the two distinct phases start growing in any location uniformly throughout the volume, whereas a nucleated phase change begins at a discrete number of points. As there is no barrier (by definition) to spinodal decomposition, some fluctuations (in the order parameter that characterizes the phase) start growing instantly. When there is a nucleation barrier to the formation of a second phase, time is taken by the system to overcome that barrier. Spinodal decomposition is fundamentally different from nucleation and growth. The dynamics of spinodal decomposition is commonly modeled using the Cahn–Hilliard equation. When the two phases emerge in approximately equal proportion (each occupying about the same volume or area), characteristic intertwined structures are formed that gradually coarsen (see animation). Spinodal decomposition is observed when mixtures of metals or polymers separate into two co-existing phases, each rich in one species and poor in the other. As a result, phase separation via decomposition does not require the nucleation events resulting from thermodynamic fluctuations, which normally trigger phase separation.

Decomposition occurs when there is no thermodynamic barrier to phase separation. Spinodal decomposition is a mechanism by which a single thermodynamic phase spontaneously separates into two phases (without nucleation).

Microstructural evolution under the Cahn–Hilliard equation, demonstrating distinctive coarsening and phase separation.

Under the Spinodal Curve by Hanuš Seiner