Coupled Crystal Plasticity-dynamic Recrystallization Modeling of the Extrusion of Magnesium Alloy
Author:Shao Yi Chuan
Supervisor:li da yong
Owing to the high potential of weight-savings,magnesium alloys exhibit promising perspective of application in aerospace,automotive and electronics industries.Hot extrusion is frequently used as the processing method for magnesium alloys due to the optimized formability caused by hydrostatic pressure.Dynamic recrystallization(DRX)is liable to happen during the extrusion and is responsible for the improvement of plasticity and the modification of microstructures and textures of the extruded alloy.Therefore,it is important to build up a coupled deformation-DRX model to investigate the DRX mechanism during magnesium extrusion and its relationship with deformation conditions,composition of alloy,and texture modification.In this thesis,the microstructure and texture evolution during the magnesium extrusion and the coupled crystal plasticity-DRX modeling are investigated in depth.(1)Experimental study on the microstructure and texture evolution of the magnesium alloys during the extrusion.Magnesium alloys Mg-1Al,Mg-1Y,Mg-4Y and Mg-6Y are casted and prepared for hot compression and extrusion tests.By using X-ray diffraction and EBSD,the microstructures,the distribution of the misorientation angle /axes,and the recrystallization textures of the extruded alloys are studied.It is observed that the recrystallized volume fraction of Mg-Y alloy is lower than that of the Mg-Al alloy during the compression,and the orientations of DRX grains in Mg-Y alloy is more randomly spread as compared with Mg-Al alloy.Moreover,the extruded Mg-Y alloy develops textures different from those of the Mg-Al alloy.The above results show that the yttrium addition not only retards the discontinuous dynamic recrystallization(DDRX)of magnesium alloys,but also interferes with the dislocation rearrangement during the continuous dynamic recrystallization(CDRX)of grains and modifies the texture development of the yttrium containing alloy.(2)The coupling work of the phenomenological DRX model and the polycrystal plasticity model.A phenomenological DRX model is integrated into the visco-plastic self-consistent(VPSC)framework in order to simulate the extrusion texture of the magnesium alloy.The recrystallized volume fraction is controlled by an Avrami-type equation,and the texture development is simulated by the crystal plasticity algorithm.Based on different hypotheses on the initial orientations of the DRX nuclei and on the relative critical resolved shear stress(CRSS)values on the slip systems,the extrusion texture of the alloys Mg-1Al and Mg-6Y are simulated using the polycrystal model.The simulation results can shed light on the formation of recrystallization texture of alloys Mg-1Al and Mg-6Y.(3)The coupling work of physically-based CDRX model and polycrystal plasticity model.By introducing internal variables including dislocation density,misorientation angle distribution function of low angle boundaries,and subgrain sizes,the polycrystal model is allowed to describe the transition of low angle boundaries to high angle boundaries during the DRX.On the other hand,the change of the misorientation angles of boundaries caused by the interactions between slips on different planes can be properly described in the polycrystal model with the multiple slip mechanism modeled.The use of physically-based internal variables makes the polycrystal model less dependent on the parameters fitting,and therefore improves its reliability of texture predictions.The simulated results can help better understand the influence of yttrium addition on the deformation mechanism and microstructural evolution of the magnesium alloys.(4)Simulation of the deformation and texture of the profile extrusion of Mg-6Y alloy.The Eulerian finite element method is combined with polycrystal plasticity approach to simulate the profile extrusion of Mg-6Y alloy.This modeling method can not only prevent element distorsion,but can also appropriately simulate the effects of different strain boundary conditions on the texture evolution of the extruded material on different regions of the cross section of the extruded alloy.The simulated results are in good accordance with the measured textures,which proves the reliability of the polycrystal model combined with the CDRX algorithm and the multiple-slip scheme.