On the problem of fluctuations near first-order phase transformations in solids

The recent experimental results on the phonon structure of b.c.c. Zr-Nb alloys are reviewed. The central (elastic) phonon peak is discussed in terms of quasi-static heterophase fluctuations which are postulated to form by thermal excitation to a coherent-metastable state and to decay by boundary collapse. This concept is extended qualitatively to the problem of fluctuations associated with first-order compositional transformations - clustering and ordering. It is suggested that quasi-static fluctuations should also occur near the coherent phase boundary for first-order compositional transformations. These fluctuations can be properly interpreted as small particles of the low-temperature phase. The model is used to discuss qualitatively the disorder to long-period and long-period to ideally ordered phase transitions.