Characterization of deformation processes in a Zn-22% Al alloy using atomic force microscopy

The Zn-22% Al eutectoid alloy was subjected to equal-channel angular pressing at a temperature of 473 K to give an as-pressed grain size of ∼1.3 μm. Subsequent tensile testing of the as-pressed alloy at room temperature revealed a transition from deformation by a dislocation mechanism at the higher strain rates to superplastic flow at strain rates below ∼5 × 10−3 s−1: this corresponds to the transition from region III to region II in conventional superplasticity. Samples were pulled to relatively low total strains, of the order of ∼0.2–0.5, and the surface topography was then examined using an atomic force microscope (AFM). The AFM observations confirm the transition in deformation mechanisms with decreasing strain rate and they provide direct evidence for the occurrence of grain boundary sliding within the superplastic regime.