Role of dislocation density and structure in grain boundary segregation of phosphorus and carbon in model Fe-P-C alloy

The present study has investigated the effects of dislocation density, produced by tensile prestraining, on the grain boundary segregation of phosphorus and carbon in a Fe - 0.06P - 0.002C (wt-%) alloy during stress free isothermal annealing at 500degreesC for periods up to 1800 h. Changes in grain boundary segregation were followed using Auger spectroscopy, while changes in dislocation density and structure were observed using transmission electron microscopy techniques. The segregation of phosphorus (but not carbon) was enhanced, compared with unstrained specimens, during initial aging. Analysis of diffusion rates required to cause the observed increase in phosphorus segregation suggested that the kinetics of phosphorus segregation was enhanced by pipe diffusion. At intermediate aging times, desegregation of phosphorus was observed, an effect attributed to a reduction in intragranular solute levels resulting from phosphorus precipitation on dislocations. In the case of carbon this process continued to the longest aging times examined and, essentially, complete desegregation ensued. In contrast, at these long aging times, phosphorus segregation resumed, and this was associated with an increase in the binding energy of phosphorus to the carbon denuded grain boundary.