It is demonstrated that a population of non-interacting and non-coalescing defects develops according to a continuity equation ∂ρ∂t+∂∂x(ρ · υ) = 0 where ρ is the partial density of defects of length x at time t and υ is their growth rate. This equation predicts two types of basically different behaviours: in type I, defect nucleation reaches a constant (steady state) value, while in type II it increases exponentially with time. The analysis of the experimental defect length distribution curves with the help of the model's equations allows the growth law υ(x) to be identified and the influence of metallurgical and testing conditions upon nucleation and growth to be determined. The model is applied to the creep of precipitation-strengthened Al-base alloys and to the slow tensile testing of an Fe-base austenitic alloy at elevated temperature; it is demonstrated that the former behaves according to type I and the latter to type II.

Minute amounts of Pb induce intergranular embrittlement of precipitation hardened Al–Mg2Si alloys in creep conditions but not in tension and toughness tests. Auger and Rutherford backscattering analyses of impact-fracture surfaces revealed the presence of submonolayer Pb coverages which were first attributed to a segregation of Pb to grain boundaries. However, in view of the contradictions which arose from the two types of results, further experiments using scanning Auger and X-ray microanalysis revealed that in fact Pb was essentially in precipitated form, its solubility being much smaller than expected, and that it spread on the surface by diffusion immediately after fracture. The preferential association between Pb globules and large AlFeSi inclusions favours this process since the latter particles initiate transgranular dimples. The surface analysis results are discussed on the basis of a semiquantitative description of the possible distribution of globules on fracture surfaces. The surface diffusion process and the Pb–(AlFeSi) association are also responsible for the mechanism of embrittlement by Pb in creep conditions.