Spinodal phase transformation

Figure 6.2 schematically illustrates the differences between a continuous spinodal phase transformation and a discontinuous nucleation and growth transformation. 

Lipson (1943, 1944), who had examined a copper-nickeMron ternary alloy. A few years ago, on an occasion in honour of Mats Hillert, Cahn (1991) mapped out in masterly fashion the history of the spinodal concept and its establishment as a widespread alternative mechanism to classical nucleation in phase transformations, specially of the solid-solid variety. An excellent, up-to-date account of the present status of the theory of spinodal decomposition and its relation to experiment and to other branches of physics is by Binder (1991). The Hillert/Cahn/Hilliard theory has also proved particularly useful to modern polymer physicists concerned with structure control in polymer blends, since that theory was first applied to these materials in 1979 (see outline by Kyu 1993). 

The third and the most common type is complex phase transformations, including the following (i) some components in a phase combine to form a new phase (e.g., H2O exsolution from a magma to drive a volcanic eruption the precipitation of calcite from an aqueous solution, Ca + + COf calcite the condensation of corundum from solar nebular gas and the crystallization of olivine from a basaltic magma), (ii) one phase decomposes into several phases (e.g., spinodal decomposition, or albite jadeite + quartz), (iii) several phases combine into one phase (e.g., melting at the eutectic point, or jadeite +... 

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. 

In the metastable region between the binodal and spinodal curves, phase separation has to occur by the mechanism of nucleation and growth. In this region, the one-phase-state is Indeed stable against small concentration fluctuations but unstable against separation into two phases of more different concentrations. Phase transformations in one-component systems like condensation, evaporation or solidification as well as the crystallization of solutes from solvents occur by the nucleation and growth mechanism. The well known phenomena of oversaturation and hindered-phase transformation can be explained by discussing the nucleation as an equilibrium reaction with the creation of the "critical nucleus" (6, 7). 

The Alnico microstructures are prepared by a process which involves spinodal decomposition. In this phase transformation, the high-temperature phase decomposes into two phases, usually known as and a2. Fig. 6.25. A spinodal curve inside the solvus curve separates the regions where either spinodal decomposition (compositions and temperatures inside the spnodal curve) or normal, nucleation and growth transformation (between solvus and spinodal) occur. Spinodal decomposition occurs by periodic composition fluctuations (Burke, 1965) as transformation proceeds, composition fluctuations increase ( i becomes richer in A and 2 in B, for instance), but the spatial periodicity is conserved. 

Binder, K. (1991). Spinodal decomposition. Phase Transformations in Materials. 

More extensive reviews of various aspects of the subject are as follows amorphous semiconductors, Kolomiets (1964) and Pearson (1964a) glasses, Weyl and Marboe (1962, 1964, 1967) and Rawson (1967) glass formation and glass transition theory, Turnbull and Cohen (1960) and Turnbull (1965a, 1965b, 1969) phase transformations, Turnbull (1956) and spinodal decomposition, Cahn (1968). 

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). 

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