Phase separation into 2 phases at compositions where the free energy curve is convex upward, in which composition fluctuations grow without a nucleation barrier. Uphill diffusion produces a periodic structure.
Japanese name: スピノーダル分解
Spinodal decomposition appears in 1 practice question across 1 topic, including Thermodynamics of materials.
Field: Thermodynamics of materials
A solid solution quenched into a miscibility gap can separate into two phases by two routes. At compositions where the G–composition curve is concave upward, small composition fluctuations raise G, so separation does not start until nuclei above a certain size form, and it proceeds by nucleation and growth. At compositions where it is convex upward (d²G/dx² < 0), even the smallest fluctuations lower G, so they simply grow. This is spinodal decomposition: at first the composition undulates smoothly, and with time the amplitude grows and approaches the compositions of the two phases. A feature is that there is no waiting time as in nucleation, and the microstructure forms uniformly throughout.
The theory of spinodal decomposition was built up from the late 1950s into the 1960s by Cahn, Hilliard and others. The growth rate of fluctuations depends on wavelength: long wavelengths grow slowly because atoms must move far, and short wavelengths do not grow because the energy from sharp changes in composition (gradient energy) is large. One wavelength therefore grows fastest, producing microstructures with periods of a few to tens of nanometers. In crystals, the elastic strain energy from differences in lattice constant between regions of different composition also opposes decomposition, so the actual boundary (the coherent spinodal) lies at a lower temperature than the chemical spinodal.
Microstructures from spinodal decomposition often consist of two interwoven phases forming continuous networks, seen in transmission electron microscopy as striped or mottled contrast. In X-ray diffraction, small peaks called satellites appear on both sides of the main reflections, corresponding to the periodic variation in composition. Small-angle scattering also shows a scattering maximum at the position corresponding to the period. Recently, atom probes have been used to examine the three-dimensional distribution of atoms and to distinguish spinodal decomposition from nucleation by whether the compositional amplitude grows with time. Often, however, the form of the microstructure alone cannot decide which mechanism operated.
Spinodal decomposition is used to build properties into materials. In Fe-Cr-Co magnets and alnico magnets, heat treatment in a magnetic field aligns the ferromagnetic phase into elongated shapes, and shape anisotropy provides coercivity. Cu-Ni-Sn alloys are strengthened by spinodal decomposition and used for springs and connectors. Heat-treating borosilicate glass to separate it into two phases and dissolving one with acid gives porous silica glass. On the other hand, the ferrite phase of duplex stainless steels becomes brittle by spinodal decomposition during long service at roughly 300 to 500°C, the so-called 475°C embrittlement, so upper limits are set on the service temperature.
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.
The solubility line is the boundary of compositions where two phases determined by the common tangent coexist in equilibrium; inside it, separation into two phases is more stable. The spinodal is the boundary where d²G/dx² = 0 and lies inside the solubility line. Between the two, phase separation proceeds by nucleation and growth, and inside the spinodal by spinodal decomposition.
It occurs in systems with a miscibility gap in the solid solution. Besides alloys such as Fe-Cr, Cu-Ni-Fe, Cu-Ni-Sn, Au-Ni and Al-Zn, it is known in borosilicate glasses and polymer blends. The condition is that the material can be quenched to a temperature inside the spinodal before nucleation occurs.
At first the compositional amplitude grows and the compositions of the two phases approach their equilibrium values. After that the microstructure coarsens to reduce interfacial area, and the period becomes longer. There is therefore a time and temperature that give the best strength or magnetic properties, and the scale of the microstructure is adjusted through the aging conditions.
The 475℃ embrittlement of Fe-Cr ferrite is an example of spinodal decomposition.
A solid solution with a miscibility gap was quenched from t…Activity Chemical potential Ellingham diagram Entropy of mixing Equilibrium oxygen partial pressure Gibbs energy Regular solution