Interfacial Stability in Solid-Solid Transformations.

摘要

If interfacial equilibrium is maintained at the interface between a growing precipitate and the supersaturated matrix, so that solute diffusion alone determines the growth rate, the precipitate shape should evolve toward that characteristic of the dendrites found in solidification. However, this shape is rarely observed in precipitation reactions proceeding at high temperatures where interfacial equilibrium is usually felt to prevail. Thus some other stabilizing factor must enter, to keep the interface smooth. It is shown that a slow interfacial reaction (low interfacial mobility) will stabilize the interface while transformation stresses have a much weaker stabilizing effect and impurities tend to enhance instability. It is also concluded that the square root of Dt sets an upper limit on the wavelength of the perturbation that can give instability. Proeutectoid precipitation in the Fe-C system is considered in some detail and it is shown that the experimental results can only be rationalized if the growth rate of ferrite in steel transformations is determined by a slow interfacial reaction for all but a few orientations between the two phases. (Author)