Effect of Cd on the boundary sliding behaviour in superplastic Pb-62 wt% Sn alloy

Boundary sliding behaviour in the Pb-62 wt% Sn eutectic was investigated in tension by measuring the individual offsets across boundaries. In conducti ng the investigation, two grades of the alloy were tested under identical e xperimental conditions of grain size (about 14 mum), temperature (415 K), a nd strain rate (5 x 10(-7)-10(-1) s(-1)). Grade 1 is Pb-62 wt% Sn containin g 890 wtppm Cd while grade 2 is high-purity Pb-62 wt% Sn. The experimental results show that at intermediate strain rates (region II), the sliding beh aviours of the two grades of Pb-62 wt% Sn are similar and that the contribu tion of boundary sliding to the total strain I is about 50%. By contrast, t he experimental data reveal that, while xi in grade 2 at low strain rates i s essentially equal to that at intermediate strain rates (region II), the b ehaviour of grade 1 is different; xi in grade 1 at low strain rates is cons iderably lower than that at intermediate strain rates. A comparison between earlier results on the creep behaviours of grade 1 and grade 2 and the pre sent results on their sliding behaviours shows that the presence of Cd infl uences in a parallel way both superplastic deformation and boundary sliding behaviour in Pb-62 wt% Sn. It is suggested that such a correspondence is a reflection of an interaction between the following two roles which boundar ies play during superplastic deformation the ability of boundaries to contr ibute to superplastic deformation through the process of boundary sliding a nd their ability to serve as sites for the accumulation of impurities.