Direct observation of annealing twin formation in a Pb-base alloy

Behaviors of random high-angle grain boundaries of a Pb-base alloy during thermo-mechanical processing and heat treatment were studied with the electron back-scattered diffraction method. Repetitive thermo-mechanical processing was found to be effective in increasing the fraction of special boundaries, and this was attributed to the ∑3 boundaries associated with annealing twins. Evidence was found that the dissociation of fast moving random high-angle boundaries to ∑3 boundaries correlates with other random boundaries. Also formed during intermittent thermal exposure were ∑3n networks, which evolved mainly into ∑3 boundaries of lower energy. The present work thus supports the ‘growth accident’ model of annealing twin formation. Viable thermo-mechanical treatments for promoting the special boundaries are also proposed.