Grain boundary nucleation of proeutectoid ferrite in an Fe-C-Ni alloy

The nucleation kinetics of proeutectoid ferrite at austenite grain boundaries were studied in an Fe-C-Ni alloy by observing ferrite particles on fracture facets along prior austenite grain boundaries. Whereas a large scatter in the ferrite particle numbers was observed among grain boundary facets, the average particle number appeared to increase with isothermal holding time. The particle number measured on the polished surface of specimens applying Schwartz-Saltykov analysis also increased with holding time. Diffusion-controlled growth of a ferrite nucleus was simulated incorporating Gibbs-Thomson capillary effect to indicate that the time for a nucleus to grow to a detectable size was insignificantly small. These results seem to show that ferrite nucleation occurred in a sizable time range and continuous nucleation, rather than site saturation (instantaneous nucleation), may account better for ferrite nucleation kinetics at temperatures of measurement. The difference in ferrite particle numbers between the two methods of measurement is discussed in terms of the possible omission of small particles and non-sphericity of ferrite particles etc.