Low-stress creep behaviour of superplastic Zn-22% Al alloy

Abstract

Low-stress creep behaviour of microduplex Zn-22% Al alloy was studied using spring specimen geometry. The average phase size in the specimens investigated was 0.87, 1.48 and 1.98μm. Experiments were conducted in the temperature range 393-473 K at stresses below about 1.0 MN m⁻². The present study has established that the stress exponent of the creep rate is unity and, therefore, a viscous creep process dominates the flow in Region I superplasticity. The activation energy corresponds to that for boundary diffusion. However, the phase-size exponent was found to be -2 instead of -3, as predicted by the Coble creep theory. Further, the measured creep rates are three to four orders of magnitude slower than those predicted by the Coble theory. Transmission electron microscopy revealed precipitation, along α/α grain interfaces, whose inhibiting action on plastic flow should at least be partly responsible for the lower values of measured creep rates. There also exist two other interfaces, namely α/β and β/β, whose comprehensive role in diffusion creep is not yet fully understood. Therefore, it seems illogical to describe the creep behaviour of Zn-22% Al by the classical Coble theory, originally developed for single-phase polycrystals.