| The evolution of flow stress and microstructure during the deformation of two Mg-based (+Al, Mn, Zn) alloys has been studied under various conditions of temperature and strain rate. The tested materials were taken from AZ31 and AM30 extruded tubes. The effect of twinning was investigated by using uniaxial tension, uniaxial compression and ring hoop tension testing. Strain path change testing, i.e. tensile prestraining followed by uniaxial compression, was carried out to study the role of contraction and double twins on the subsequent deformation. Optical metallography and EBSD techniques were employed to study the microstructural development.;The first part of the investigation focused on the flow stresses developed during deformation along the three different strain paths. It was found that at low temperatures (≤200°C), the flow behaviors are mainly controlled by twinning. At temperatures above 200°C but below 350°C, especially at a strain rate of 0.001s-1, the deformation is largely accommodated in partially dynamically recrystallized regions. As a result, the formation of voids in these regions causes premature fracture.;The second part focused on the twinning behavior displayed during deformation to a strain of 0.15. The results indicate that the initial extrusion texture plays an important role in the formation of different types of twins and that the twinning behavior also depends on the strain path. {10-11} contraction and {10-11}-{10-12} double twinning are the dominant twinning mechanisms in uniaxial tension, while {10-12} extension twinning prevails in uniaxial compression and ring hoop tension testing. Grain size, temperature, strain and strain rate all have significant effects on the volume fraction of contraction and double twins. There is a sigmoidal relationship between the volume fraction of extension twins and the strain. The effect of grain size (in this grain size range) on the volume fraction of extension twins is small. For a given strain, at high strain rates, temperature does not have much influence on the volume fraction (below 200°C).;In the third part of the investigation, the deformation texture associated with twinning was examined. The near 100% volume fraction of {10-12} extension twins gives rise to significant texture changes. By contrast, {10-11} contraction and {10-11}- {10-12} double twinning only make a small contribution to texture change due to the limited volume fraction of twinned material. Schmid factor analysis indicates that, for the {10-12} extension twins, all variants that are favorably oriented (i.e. with the highest SF values) can undergo rapid and complete twinning. For the {10-11} contraction twins, only variants in the TD component (11-20)<10-10> follow the SF criterion. The development of internal stresses, which somehow particularly affect the RD component (10-10) <0001> grains, may explain this difference.;In the final part of the investigation, the effects of twinning on the strain hardening behavior and on ductility were studied. Different behaviors are shown to be responsible for the sharply contrasting strain hardening characteristics of the experimental flow curves. When a certain volume fraction (≥20% in the present study) is reached, contraction and double twinning generate net softening effects. Nevertheless, when they interact with extension twins, the hardening effect may predominate. On the other hand, extension twinning generally introduces a hardening effect. The hardening effect due to twin boundaries appears to be stronger than that from the volume fraction of extension twins, since the hardening associated with extension twinning cannot be explained by the volume fraction alone. Depending on the deformation conditions, twinning can be detrimental to the formability on the one hand but it can also be beneficial on the other. |
