The Study On Microstructure Evolution , Texture Characteristic And Mechanical Property Based On Equal Channel Angle Pressing

The equal channel angle pressing (ECAP) pure shear deformation that based on the super plastic deformation (SPD) technology can refinment the grains effectively, has received widespread concern in recent years, as yet, the research of the deformation process, especially the microstructure deformation mechanism and the texture changing process during the deformation process, lack of comprehensive understanding. This paper systematically studied the deformation process, strain mechanism, texture characteristic and mechanical property of polycrystalline pure copper and directional solidification pure copper during ECAP. First, use the plastic elastomer model, studied the die structure, backpressure, rheological behaviour, temperature changing and materials strain and recrystallization mechanism, then performed the experiments use polycrystalline and unidirectional solidification pure copper through multipath and multipass, respectively. Comparison and analysis of the deformation mechanism and XRD macro orientation characteristic of different original microstructure after ECAP, on the basis, using EBSD technology contrast studied the texture characteristics of the first few passes. Finally, analyzed the mechanical properties of pure copper that ECAPed by different mould structure, main conclusions are as follows:1. Simulated by the finite element method, found that the sample was in different stress state at different stages in the ECAP shear deformation zone. The press stress was inside of the specimen, and the state of the surface was pressure stress–tensile stress - pressure stress. In the deformation process, temperature of the samples will have a significant changing. This will causing dynamic recrystallization. At the end of the deformation, mixed crystal structure formed inside of the material. Smaller inner-angle and appropriate increase the back pressure can improve the uniformity of the materials microstrusture during deformation. Many times ECAP deformation can effectively refinement the grain size, but different routes has different effects on the grain refinement. After deformation, the grains of the polycrystalline didn't have a clear orientation, but the sub grains that deformated by route A of the solidification pure copper refined by route A, shows a obvious arrangement, and the microstructure that deformated by route Bc shows a cellular structure.2. The deformation process of polycrystalline pure copper during ECAP is: crystal boundary migration, grain rotations, grain was elongated and then the grains were refined. Sliping and twinning accompaning with dynamic recrystallization in the grain interiors. After ECAPing, the microstructure of the polycrystalline copper is non-equilibrium structure at the grain boundaries and a high density dislocations and high angle grain boundaries (HAGBs). With the strain increasing, the oritation differecce of the microstrustrue of polycopper increasing with the strain, with the number of HAGBs increasing, the crystal grain become more uniform and the distribution peak of orientation kept moving to the center of HAGBs.3. During the deformation of ECAP, the plastic deformation of directional solidification pure copper is more easily than polycrystalline copper. When deformated by route A and route C, dislocations moving within grains causing formed the island-like structure, continues to deformation, the area of the island-like structure is enlarged with the stain amount, causeing the ordernering transformation and the subgrain boundaries moving to the outsides, causing a strain unsymmetrical, the column grain refined and the subgrains shows a random distribution characteristic; when deformed by route Bc, the moving of dislocations resulted to cell structure. With the strains increasing, the original microstruture was refined. At small strains, there was no twins occurring in the grains but formed the fake texture.4. The deformation routes and the die structure has different effect on the materials mechanical property, the mechanics changing rapid at the small die angles, with the deformation times increasing, the sample fracture developing from tough to brittle model, the homogeneity increasing, the hardness of the two kinds materials increasing and the high circle fatigue has no obvious fatigued limitation.5. The original microstructure affected the mechanical properties of pure copper ECAP is obviously. The directional arrangement of grains and the increasing of fine-crystal grain in unit volume, causing the plasticity of directional solidification pure copper improved steadily after many times deformation, the dislocations accumulated to the grain boundaries (GBs) when the deformation occurred in polycrystalline copper, in the subsequent extrusion, grain orientations did not changing much, GBs increased greatly after be thinned, then the plastic shows a fluctuate charictristic.Analysis that grains of polycrystalline copper affected each other during the deformation process. The different rotatation mode of the grains resulted the oritation difference in polycrystalline copper. In earlier of plastic deformation, formed the disorderly dislocation tangle, when increasing deformation times, formed the cellular structures, the dislocations tangled and accumulated to the two-dimensional boundaries at the cell boundaries, formed the low angle subgrain boundaries (LAGBs) and HAGBs, causing the refinement of the grains. The refining mechanism can be summed as: shear strain -- dislocations division -- deformated twins strengthen– GBs competition disappearance (the grains were refined).The original grains orientation has an important influence on deformation structure. With the increasing of strain, causing the asymmetry of subgrain boundaries and formed the strain gradient in the internal of the columnar grains, then arosing the strain gradient, some GBs transferred from LAGBs to HAGBs, then gradually formed the mixed distribution characteristic.During the course of plastic deformation of polycrystalline materials, the grains transformed from macroscopic "disorder" to microcosmic "order", the evolution of the texture is from microcosmic "order" to macroscopic "order", the whole changing process as follow: macro "disorder" micro "order" macro "order". Shows a " texture heave" effect.The " texture heave" effect is the materials integrated strain reflect under certain temperature and external force. Its strength and direction are closely related to the materials strain state. The result of this continuously process will causing the uniformity improvement of the material microstructures. The arise and disappearance of new textures in materials is the original states of grains strain were damaged by external force in the processes of the change of partially gathered direction and inner-stresses were transferred to adjacent boundaries.This study will be on influence of texture formation from original microstructure, the evolution rule of the grains during ECAP deformation, the relationship between the texture and the materials mechanical property and material uniformity during machining process. This study has important theoretical significance, especially for the deformation response of plastic anisotropic and the asymmetry changing process of the meteries intensity.