Microstructural evolution and mechanical properties in a Zn–Al–Cu–Mg hypoeutectic alloy processed by multi-directional forging at room temperature

Abstract Experiments were performed to examine the flow behavior and microstructural evolution in a Zn–Al–Cu–Mg alloy processed by up to 23 cycles of multi-directional forging (MDF). After a total strain (e) of ~20.7 the grain sizes in the primary η-Zn, eutectic and eutectoid components were reduced to ~0.25, 0.4 and 0.3 μm, respectively. The metal exhibits profuse shear banding and twinning and undergoes work hardening in the 1st MDF cycle. Further straining up to e ≈ 7 promotes significant grain refinement together with strain softening due to occurrence of grain boundary sliding. Afterwards, the plastic flow occurs under a constant stress and flow softening is again prevalent for e > 12. The individual components in the Zn–Al hypoeutectic alloy display different kinetics of hardening and grain refinement such that the microstructural changes are first observed in the primary η-Zn areas and subsequently in the eutectoid and eutectic domains. Based on the average hardness in each component, the rule of mixtures was used to estimate the overall material strength after MDF. Although the results show similar trends, it is revealed that the rule of mixtures overestimates the flow stresses compared with the in-situ stress-strain curve. This undermines the hypothesis of iso-strain and suggests that deformation concentrates in the weaker areas in the flow softening stages of MDF processing.