Experimental microindentation of pure copper subjected to severe plastic deformation by combined tension-torsion

摘要

The samples of commercial pure copper bars were severely plastic-deformed by different pretension and subsequently by different turns of torsion. Microstructure evolution of deformed samples was characterized using optical microscopy (OM). Longitudinal observations showed the formation of gradient microstructure with the maximum grain refinement in the surface layer of bars. Micro-indentation tests were conducted in the indenter load range of 100-450 mN and at a loading rate of 9.6841 mN/s. The hardness and Young's modulus were analyzed in the transversal section of samples using the method of Oliver and Pharr. Based on the strain gradient plasticity theory, the densities of statistically stored dislocations (SSDs) and geometrically necessary dislocations (GNDs) were calculated according to the Taylor dislocation model, respectively. Experimental results reveal that the hardness and Young's modulus decrease with the indenter load increasing. Indentation size effect on the hardness is relatively more significant than the counterpart of Young's modulus. In a certain pretension condition, the hardness values increase fast and subsequently slowly with the increase of applied torsion turns. The torsion process seems to effectively increase the material hardening due to the introduced simple shear strain. Compared to the hardness analysis, Young's modulus gently increases with the increase of torsion turns. That is due to the saturation of mechanical damage and/or crack introduced by severe deformation. In the combined tension-torsion loading, both densities of GNDs and SSDs increase with the increase of torsion turns. However, the SSDs density is higher than that of GNDs in spite of the same order magnitude. The torsional shear strain enhances the non-uniform deformation and stimulates the propagation of GNDs.