Internal Stresses in the High Temperature Deformation of the Pb–Sn Eutectic

Internal stress (σi) was measured by the stress dip technique in the high temperature steady state deformation of the Pb–Sn eutectic over wide ranges of grain size (11.0 to 23.4 μm), temperature (339 to 421 K) and strain rate (10−5 to 10−2 s−1). Stable equiaxed microstructures were produced by the prestrain-annealing treatment of the extruded alloy so as to eliminate the strain effects on flow stress (applied stress σ). It is observed that σi is a large fraction of σ in region II of the σ-ε (strain rate) plot. The ratio σi⁄σ decreases with increasing strain rate and temperature in regions II and III. The internal stress is a smaller fraction of the applied stress in region III than that in region II.The parameters of the constitutive equation on the basis of effective stress (σe=σ−σi) in region II are: strain rate sensitivity, me=0.82; grain size exponent, pe=2.8; and activation energy, Qe=33.5 kJ/mol. The σe-ε data are in agreement with the Ashby-Verrall model for superplasticity. In region III me is 0.2 to 0.25.