EFFECT OF CD ON SUPERPLASTIC FLOW IN THE PB-62 W-PERCENT SN EUTECTIC

A detailed investigation of the superplastic deformation of two grades of the Pb-62 Sn eutectic (where the compositions here and subsequently are in weight per cent) was conducted under identical experimental conditions of grain size (about 8 mum), temperature (390-420 K) and stress (0.1-10 MPa). The first grade (grade 1) is Pb-62 Sn containing 890 wt p.p.m. Cd while the second grade (grade 2) is very-high-purity Pb-62 Sn. The experimental results on grade 1 (Pb-62 Sn doped with Cd) show the presence of a sigmoidal relationship between strain rate and stress. This sigmoidal behaviour exhibits firstly an intermediate-stress region (region II) which is characterized by a stress exponent n of about 1.7 and an activation energy Q similar to the activation energy Q(gb), anticipated for grain-boundary diffusion and secondly a low-stress region (region 1), where n > 2.6 and Q > Q(gb). By contrast, the experimental results on grade 2 (high-purity Pb-62 Sn) show that the characteristics of superplastic flow at intermediate and low stresses are identical with n = 1.7 and Q almost-equal-to Q(gb); region I is not observed. The difference between superplastic behaviour of the two grades of Pb-62 Sn at low stresses is attributed to a threshold stress for Pb-62 Sn doped with Cd which strongly depends on temperature and whose origin is related to impurity segregation at boundaries. The results on Pb-62 Sn are consistent with earlier findings for the superplastic Zn-22 Al alloy, indicating that the observed effect of impurities is most probably a general characteristic of superplastic flow.