| A systematic investigation of the effect of impurity content and type on grain boundary sliding behavior of two-phase micrograined alloys was conducted. These alloys, Zn-22%Al and Pb-62%Sn, are model superplastic materials that often exhibit a sigmoidal relationship between stress and strain rate. For the superplastic Zn-22%Al alloy, five different grades were used: (i) grades 0, 1, and 2 contain 1500, 180, and 100 ppm of impurities, respectively, (ii) grade 3 is a high purity grade containing 6 ppm of impurities, and (iii) grade A contains 1000 ppm of Cu impurities. For the superplastic Pb-62%Sn alloy, grade 1 contains 890 ppm of impurities, whereas grade 2 is a high purity grade. In conducting the investigation, the sliding behavior of all above materials were studied in region I (low-strain rate region), region II (intermediate-strain rate region), and region III (high-strain rate region). The experimental results reveal several important findings.; First, for the Zn-22%Al system, it is found that at intermediate strain rates (region III), the sliding behavior of the five grades of Zn-22%Al are similar, and that the contribution of boundary sliding to the total strain, ξ, is about 50% (for grade A) to 60% (for grades 0, 1, 2, and 3). By contrast, the experimental data show that at low strain rates, grades 0, 1, and 2 exhibit significant differences regarding sliding contribution as compared to grade 3 and grade A. These differences are manifested in the following observations: (i) ξ in grade 3 at low strain rate is essentially equal to that at intermediate strain rates (region II), (ii) ξ in either grade 0, 1, or 2 is considerably lower than that at intermedate-strain rates, and (iii) for the same low strain rates, ξ, is highest in grade 2 and lowest in grade 0.; Similarly, for the Pb-62%Sn system, sliding behavior of grades 1 (890 ppm impurities) and 2 (high purity) at the intermediate-strain rate are about the same (ξ ∼ 50%). However, at low strain rates, ξ in grade 1 is significantly lower (∼30%).; The aforementioned sliding behavior in two different alloys systems, Zn-22%Al and Pb-62%Sn, shows consistent characteristics: (i) sliding contribution in region II (intermediate-strain rate region) is not affected by impurity content, (ii) sliding contribution in region I (low-strain rate region) reduces considerably as the content of impurities in the alloy becomes significant. |
