Continuous phase transformations spinodal decomposition

Spinodal decomposition and certain order-disorder transformations are the two categories of continuous phase transformations. Both arise from an order parameter instability in the case of spinodal decomposition, it is a conserved order parameter for continuous ordering, it is a nonconserved order parameter. 

The thermodynamic differences between spinodal decomposition (a continuous phase transformation) and nucleation and growth (a discontinuous phase transformation) are illustrated in Figure 6.6. At a given temperature, the volume free... 

Spinodal decomposition is an example of a continuous phase transformation. In a spinodal transformation, a single phase separates into two phases via gradual changes in local composition. The spinodal decomposition process gradually occurs everywhere (small in degree, large in extent). 

Figure 18.7 Interfaces resulting from two types of continuous transformation, (a) Initial structure consisting of randomly mixed alloy, (b) After spinodal decomposition. Regions of B-rich and B-lean phases separated by diffuse interfaces formed as a result of long-range diffusion, (c) After an ordering transformation. Equivalent ordering variants (domains) separated by two antiphase boundaries (APBs). The APBs result from A and B atomic rearrangement onto different sublattices in each domain.

In crystalline solids, only coherent spinodal decomposition is observed. The process of forming incoherent interfaces involves the generation of anticoherency dislocation structures and is incompatible with the continuous evolution of the phase-separated microstructure characteristic of spinodal decomposition. Systems with elastic misfit may first transform by coherent spinodal decomposition and then, during the later stages of the process, lose coherency through the nucleation and capture of anticoherency interfacial dislocations [18]. 

We believe this example to be representative of a quite general principle with important implications for a large class of configurational changes in solids Often the kinetics of a phase transformation is described as continuous or homophase by means of a generalized diffusion theory, such as the Cahn-Hilliard theory for spinodal decomposition. The tacit assumption is always that a sufficiently large number of mobile diffusion-mediating defects is available in any small volume element considered in order to allow a continuum description. Whereas this requirement may always be well fulfilled in... 

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