Microstructural evolution and mechanical properties in a Zn-Al eutectoid alloy processed by high-pressure torsion

Experiments were conducted to examine the evolution of microstructure and mechanical properties in a Zn-22% Al eutectoid alloy processed by high-pressure torsion (HPT). Measurements of the Vickers microhardness revealed significant weakening in the alloy after HPT, and microstructural analysis showed that the initial duplex structure, consisting of equiaxed grains and lamellae, was retained at the disk centers after HPT, whereas equiaxed fine grains were observed at the disk edges. Direct evidence is presented for a transformation of the lamellae into equiaxed very fine grains and subsequent dynamic recrystallization in the early stages of HPT. The reduction of the lamellar structure and the loss of Zn precipitates account for the weakening in the alloy in HPT processing. Excellent high strain rate superplasticity was recorded after HPT, with elongations up to similar to 1800% at 473 K at a strain rate of 10(-1) s(-1). The experiments show that the maximum elongations are displaced to faster strain rates with increasing numbers of HPT turns. (C) 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.