Microstructure And Hot Deformation Behavior Of Mg-Zn-Zr Based Alloys

Magnesium alloys are considered to have the most promising development outlook as metallic structural materials in the21st century due to their high specific strength and specific stiffness, good damping capacity, excellent machinability and easy recycling. Magnesium alloy has become one of the hot topics in the field of materials research around the world. The principle direction to improve the integrated properties of Mg alloy is to satisfy the requirements of some structural materials, such as aluminum, or even steel. Researches which aim to increase the elongation, decrease the anisotropy and improve the plastic deformation capability, simplify fabrication technics, reduce manufacturing cost and expand the application of magnesium alloy are also paid worldwide attention.The applications of ZK60alloy processed by extrusion have been very limited due to their low extrusion speed. In this study, ZK60alloy was selected as base alloy and we designed the new ZK based Mg alloy by addition of different contents of Ce or Cu elements from0.5wt.%to1.5wt.%. The hot deformation behavior T4-treated ZK60alloy with/without Ce or Cu addition was investigated by compressive test using Gleeble3800thermal-simulator in the temperature range of523-673K and strain rate range of0.001-1s-1. In addition, the strain-dependent constitutive models were established by both regression model and feed-forward back-propagation artificial neural network (ANN) model. The rheological behavior and microstructural evolution during compression test were studied with the processing maps. In addition, this thesis also investigated the effects of alloy elements (Ce or Cu) and extrusion conditions on the microstructure and mechanical properties of ZK60alloy. Based on this, the ZK60-1Ce alloy was successfully extruded at high speed of10m/min.The results reveal that the flow stress of all alloys is significantly affected by both deformation temperature and strain rate. The flow stress increases with either decreasing deformation temperature or increasing strain rate. The flow stress tends to be constant after a peak value at high deformation temperature and low strain rate. The hot deformation behavior of T4-treated ZK60alloy can be described by the hyperbolic sine constitutive equation. The predicted data sets from both of regression model and ANN model showed good agreement with the experimental ones. The stress exponent n equals five, which implies that the controlled deformation mechanism of ZK60alloy is dislocation climb controlled by grain boundary diffusion. The peak stress, critical stress and threshold stress increases with increasing of content of Ce or Cu element in ZK60alloy, but decreases as the increasing of deformation temperature.From the processing maps of ZK60and ZK60-0.5Ce, the instability phenomenon can be found located in low temperature and high strain rate region. Samples deformed in this domain will lead to cracks generated at the surface, and the microstructure consisted of twining and local shear band. In addition, the optimum process parameter can also be obtained from the processing maps. The hot workability of ZK60alloy improves up to the addition of1.0wt.%Ce and then deteriorates. There is no obvious improvement of hot workability by addition of Cu. The formation of interface depends on the process of mechanical recovery by cross-slip of screw dislocations in both ZK60and ZK60-0.5Ce alloy. While the DRX of ZK60alloy and ZK60-0.5Ce is controlled by the interface formation and interface migration, respectively. DRX occurs during the extrusion and a strong basal texture is prone to be formed. The main phase in ZK60alloy is α-Mg and Mg-Zn phase, with addition of Ce or Cu. Mg-Zn-Ce and Mg-Zn-Cu phase are generated. Both Ce and Cu had an obvious influence, reducing the average grain size and weakening the basal fiber texture of the as-extruded alloys; these changes were attributed to the promotion of dynamic recrystallization (DRX) by particle stimulated nucleation (PSN). The yield and tensile strengths were improved by the Ce addition attributable to the combination of dispersion strengthening, precipitate strengthening and grain boundary strengthening, while the elongation was decreased due to the increase of large and fragile particles, which are considered as sources of cracks. The grain size and fraction of DRX increases with increasing of extrusion temperature and extrusion speed. The strength of the alloys decreases due to the presence of twinning and unDRX grains. Ce addition makes the microstructure more homogenized. And Ce added alloys significantly depend on the extrusion conditions. In summary, this thesis gives well understanding to the following aspects:improve the workability of Mg alloy at low temperature; simplify the fabrication process, decrease the production cost with enhanced productivity.