Thermodynamics of materials
A solid solution with a miscibility gap was quenched from the single-phase state at high temperature and held at a temperature inside the gap. Which statement about spinodal decomposition is correct?
- AIt occurs at compositions where the free energy curve is convex downward (d²G/dx² > 0), and an energy barrier to form a critical nucleus must be overcome
- BIt occurs at compositions where d²G/dx² < 0; there is no nucleation barrier, and composition fluctuations grow by uphill diffusion
- CIt is a diffusionless shear transformation that changes only the crystal structure, keeping the parent phase's composition
Show answer and explanation
Answer: B. It occurs at compositions where d²G/dx² < 0; there is no nucleation barrier, and composition fluctuations grow by uphill diffusion
Even inside the miscibility gap, within the spinodal, where the free energy curve is convex upward (d²G/dx² < 0), even a slight composition fluctuation lowers the free energy, so the fluctuations are amplified as they are. There is no nucleation barrier, and atoms diffuse from the low-concentration side to the high-concentration side, against the concentration gradient (uphill diffusion). The result is a fine structure, uniform throughout, in which the composition varies periodically with a certain wavelength. At compositions where d²G/dx² > 0 (between the spinodal and the solubility line), small fluctuations raise the free energy, so separation occurs by the nucleation-and-growth mechanism that forms critical nuclei. A diffusionless shear-type transformation is a martensitic transformation. The 475℃ embrittlement of Fe-Cr ferrite is an example of spinodal decomposition.