Influence of segregation on grain boundary heterodiffusion measurements

The influence of G.B. segregation on grain boundary heterodiffusion of very slightly soluble impurities (Ni in AG) is discussed. The tendency to grain boundary saturation, which appears for bulk solute concentration lower than the limit of solubility, does not allow the use of the matching condition Xg. exp b ./X exp b = alpha with alpha = alpha sub o =constant linked to Henry's type isotherm. Thus, owing to the contribution of the G.B. saturation, the grain boundary diffusivity P is more and more underestimated when the temp. decreases and the curve log P = f(1 /T) exhibits a downward curvature. It should be pointed out that this effect will be screened if only a few measurements are carried out in a large temp. range. This may lead to an activation energy estimation higher than the correct value. 20 ref.--AA