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

Abstract

Boundary sliding behaviour in the Pb–62 wt% Sn eutectic was investigated in tension by measuring the individual offsets across boundaries. In conducting the investigation, two grades of the alloy were tested under identical experimental conditions of grain size (about 14 μm), temperature (415K), and strain rate (5 × 10⁻⁷−10⁻¹ s⁻¹). Grade 1 is Pb–62 wt% Sn containing 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 behaviours of the two grades of Pb–62 wt% Sn are similar and that the contribution of boundary sliding to the total strain Ξ is about 50%. By contrast, the experimental data reveal that, while Ξ in grade 2 at low strain rates is essentially equal to that at intermediate strain rates (region II), the behaviour of grade 1 is different; Ξ in grade 1 at low strain rates is considerably lower than that at intermediate strain rates. A comparison between earlier results on the creep behaviours of grade 1 and grade 2 and the present results on their sliding behaviours shows that the presence of Cd influences 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 boundaries play during superplastic deformation: the ability of boundaries to contribute to superplastic deformation through the process of boundary sliding and their ability to serve as sites for the accumulation of impurities.