PRECIPITATION OF THE INTERMEDIATE PHASE BETA' IN AN AL-8-PERCENT MG ALLOY

The precipitate structure of the intermediate phase beta', as well as the secondary defect structure, in an Al-8 mass%Mg alloy was observed from a very early aging stage mainly by means of TEM. The nucleation process of beta' was revealed to be strongly affected by the cooling rate during solidification. When the cooling rate during solidification was high enough, i.e., absence of porosity, regular-tetrahedron-shaped voids were newly observed as secondary defects. Some of these voids happened to be nucleation sites for beta' and thus the precipitate density was relatively high. On the contrary when the cooling rate was low, i.e., presence of numerous porosities, voids were replaced by dislocation loops which played no role in the nucleation process, so that the precipitate density was extremely low. In addition, a solutionization under water vapor stream resulted in a substantial increase in precipitate density owing to the enhancement of the efficiency of voids as nucleation sites. Consequently, void formation was thought to be promoted by hydrogen which was dissolved into the matrix during solidification. The nucleation of beta' on a void was suggested to depend on the hydrogen segregation onto the interface between beta' and the matrix, as well as on the morphology of beta'. Finally, the present conclusion putting forward the effect of dissolved hydrogen on precipitation could also provide a successful explanation for the reported difference in precipitate density between the surface and the core regions of a rolled sheet.