Intergranular creep embrittlement by non-soluble impurity: Pb in precipitation hardened Al–Mg–Si alloys

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.