Institute of Metals Division - Interfacial Stability in Solid-Solid Transformations

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 IN studying the kinetics of phase transformations such as solidification or proeutectoid precipitation at high temperatures, it is customary to assume that equilibrium exists across the interface and that the rate of the reaction is determined by the rate at which heat and solute diffuse to and from the interface. In the case of precipitation, such an assumption predicts an almost infinite rate of initial growth. It is generally stated that the rate cannot be infinite and that during some initial period the rate must be determined not by solute diffusion but by an interfacial reaction, i.e., transferring atoms across the interface. The duration of this much weaker stabilizing eFfect and impurities tend to enhance instability. It is also concluded that a sets an upper limit on the wavelength of the perturbation that can give instability. Thus initially all flat or smoothly curved particle interfaces will be stable even if their growth is diffusion-controlled. 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 hut a few orientations between the two phases. initial period has never been clearly established, but it is generally assumed to be too short to affect any measurable results.' Several authors have pointed out that in the case of interfacial equilibrium any bump in the surface will grow out ahead of the planar interface due to the higher gradient in front of it.2j3 Thus the interface will be unstable with respect to an irregular interface with points (plates) propagating rapidly out ahead of the interface into the supersaturated phase. Recently, Mullins and Sekerka have given equations for the stabilizing effect of surface tenion.~ They found that for particles with a radius of curvature of 1 p or greater the stabilizing effect of surface tension will be completely overcome by a supercooling, or supersaturation, of as little as