Experimental Study On Loading Experiment Of Oxygen - Free Copper Under Different Strain And Deformation Research On Deformation

The mechanism of plastic deformation in metal crystals is always an important subject in the fields of mechanical engineering and materials. The twinning, stacking fault, dislocation slip and the displacive transformations directly affect the process of plastic deformation in metals. For middle or high stacking fault energy metals, the dislocation slip is the main deformation mechanism. The dislocation density is the important internal variable which describes the microstructure of the materials. The study of the change of the dislocation density can establish the bridge between the macroscopic force and the structure evolution of the materials, which is the basis of building accurate constitutive models.In this paper, the high purity oxygen free copper is used as the representative of the middle or high stacking fault energy metals. The mechanical loading experiments in the strain rate range from 10-4 s-1 to 106 s-1 are carried out. The strain and the thermodynamic path of the loading process are controlled through rational design. The TEM and XRD analyses of the recovery samples are carried out. The dislocation microstructure of the samples under different loading paths is observed. The dislocation densities of the samples are measured. The reloading stress-strain relationships of the samples are described by the constitutive model based on the dislocation mechanism. The main conclusions are as follows:(1) The SHPB loading experiments using the restriction ring are carried out. The results show that before the incident bar comes in contact with the restriction ring, the signals of the bars with and without the restriction ring are almost the same. The maximum strains of the samples calculated from the signals of the bars and the formula (L0-L)/L are nearly equivalent. The strain is precisely controlled.(2) The lateral release effect under shock loading is studied. The plastic work during the one dimensional strain loading process and the lateral release process is calculated. The results show that the ratio of the plastic work produced during lateral release to that produced during uniaxial-strain loading decreases as the impact velocity increases. Under a certain impact velocity, decreasing the initial yield stress of the materials reduces the lateral release effects. The sizes of the flyer, sample and the protective ring are designed. The shock loading experiments within 10GPa are carried out. The results show that the shape of the recovery samples is regular and the thickness of the recovery samples is uniform, which means the lateral release effect is controlled.(3) The radial converging wave is generated when the sample is separated from the protective ring under shock loading. The spherical and cylindrical converging waves of the elastic-plastic materials are studied. The results show that for undercritical loading, the induced wave structure is E1← P1← E2. For supercritical loading, the induced wave structure is E1←P1←E2←P2, where Ei and Pi(i=1,2) represent the elastic region and the plastic region respectively.(4) The TEM analyses of the recovery samples under the static and SHPB loading are carried out. The results show that under the same loading strain rate, increasing the maximum strain can improve the integrity of the dislocation cell morphology, increase the thickness of the dislocation cell wall and increase the dislocation density in the cell wall. Under the same maximum strain, increasing the loading strain rate can help the dislocations to change from the network structure to the cellular structure.(5) The X-ray diffraction experiments of the recovery samples under the static and SHPB loading are carried out. The diffraction line shapes of 2θ range from 30 to 150 degree are obtained (θ is the diffraction angle). The results show that eight standard diffraction peaks are observed. The lack of the characteristic peak is not found. There is no obvious texture in the sample. The orientation of the crystallites is random. The Fourier analyses of the diffraction line shapes are carried out. The dislocation densities of the samples are calculated by the theoretical and experimental Williamson-Hall and Warren-Averbach relationships. The results show that the dislocation densities of the samples under the static (maximum strain is 0.3) and the SHPB loading (maximum strain is 0.08) are 1.19×1015 m-2 and 7.63 ×1014m-2, respectively.(6) The quasi-static reloading experiments of the recovery samples are carried out. The reloading stress-strain curves are fitted by the MTS constitutive model. The yield strength of the samples under zero degree is obtained. The results show that the MTS constitutive model can well describe the reloading stress-strain relationship of the sample. The yield strength under zero degree and the dislocation density conform to the Taylor relationship.