Evolution in hardness and texture of a ZK60A magnesium alloy processed by high-pressure torsion

Abstract

A ZK60A magnesium alloy was processed by high-pressure torsion (HPT) at room temperature at compressive pressures up to a maximum of 6.0GPa for up to 5 revolutions and this produced significant grain refinement and the development of a bi-modal microstructure. The evolution of hardness was evaluated using measurements of the Vickers microhardness and the change in texture was examined by X-ray diffraction (XRD) analysis. The hardness results demonstrated an evolution with increasing equivalent strain and a strain hardening behavior which may be quantified using a hardenability exponent of ~0.07 when processing under a pressure of 6.0GPa. An XRD analysis over the total disk surface showed that the texture changed from a typical extrusion prior to HPT towards a saturated very weak basal fiber texture which developed in the early stages of HPT and remained constant with additional torsional straining up to 5 turns. By contrast, an XRD analysis of the local microstructure taken at the edges of the processed disks demonstrated significant changes towards a strong basal fiber texture via different texture stages with increasing HPT revolutions. This study demonstrates the importance of carefully documenting the measurement areas selected for texture analysis in the ZK60A alloy after HPT because of the occurrence of significant microstructural variations within the samples.