Creep behaviour in the superplastic Pb–62% Sn eutectic

The creep behaviour of the superplastic Pb-62% Sn eutectic was investigated for grain sizes from 5·8 to 14·5 μm and at temperatures in the range from 336 to 422 K. The results showed a sigmoidal relationship between strain rate and stress. At intermediate strain rates (∼ 10−5–10−2 sec−1), the stress exponent was ∼1·65, the exponent of the inverse grain size was ∼2·3, and the activation energy was similar to the value anticipated for grain boundary diffusion. At very low strain rates (≲10−5 sec−1), the stress exponent was ∼3·0, the exponent of the inverse grain size was ∼2·3, and the activation energy was similar to the value anticipated for lattice self-diffusion. The results are not entirely consistent with either of the two major theories of superplasticity, but suggest instead the sequential operation of two different deformation processes.