Microstructure inhomogeneity in ultra-fine grained bulk AZ91 produced by accumulative back extrusion (ABE)

Abstract

This paper focuses on the effects of the accumulative back extrusion (ABE) process on microstructure inhomogeneity and room temperature mechanical properties of AZ91 magnesium alloys. The results show that ABE processing leads to significant grain refinement with an average grain size of similar to 2 mu m in the shear zone from initial grain size of similar to 150 mu m at the central region of a specimen subjected to high shear through 1 pass at 300 degrees C. In the outer region of the sample which is in contact with die surface and central region which is in contact with inner punch land, there is no significant change in the microstructure, but some Mg17Al12 precipitation occurs around the massive Mg17Al12 phase after ABE. In the other regions subjected to deformation channel of the specimen, there is more grain refinement along with increased Mg17Al12 dissolution. It is also shown that the ABE process affects the morphology of the Mg17Al12 phase. Microhardness measurements verified the microstructure inhomogeneity results. (C) 2011 Elsevier B.V. All rights reserved.