Spinodal transformations microstructures

Fine-scale, spatially periodic microstructures are characteristic of spinodal decomposition. In elastically anisotropic crystalline solutions, spinodal microstructures are aligned along elastically soft directions to minimize elastic energy. Microstructures resulting from continuous ordering contain interfaces called antiphase boundaries which coarsen slowly in comparison to the rate of the ordering transformation. 

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]. 

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. 

Thus spinodal decomposition proceeds as follows (Cahn (1961, 1965)) while all wave numbers may be present initially, the transformation kinetics are quickly dominated by those wave numbers (bj ) corresponding to the maximum kinetic amplification factor (Eq. 1.31). Thus the spatial composition will be a superposition of sine waves of nearly fixed wavelength (27r/bj ) but with random orientations, phases, and amplitudes. For volume fractions greater than 0.15 0.03 the microstructure will have a three dimensional interconnected morphology. For second phase volume fractions less than 0.15, the microstructure will consist of isolated particles (Cahn (1965)). Similar morphologies can also result from nucleation and growth processes (Haller (1965)). 

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