On the role of diffusion in phase selection during reactions at interfaces

It is argued that interdiffusion must precede nucleation of new phases during reactions at interfaces between compositionally different phases. The relative rates at which elemental components diffuse in the reacting phases control the sequence in which phases can form, and can also strongly affect the relative nucleation rates of alloy products, especially in the transient nucleation regime. While detailed predictions of the relative nucleation rates require usually unavailable knowledge of the energies of the relevant interfaces, in some cases, knowledge of the relevant diffusivities, along with a thermodynamic analysis, can lead to predictions of likely phase formation sequences. These concepts are used to explain the association of diffusional asymmetry with systems that undergo solid state amorphization, and to specify semiquantitatively the degree of asymmetry required for solid state amorphization.