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Characterization of the hydroextruded oxygen-free copper via microindentation in the temperature range 77–300 K

The micromechanical properties of annealed coarse-grained (CG, average grain size of 1–10 μm) and ultrafine-grained (UFG) oxygen-free copper have been studied in the temperature range of 77–300 K. UFG samples were obtained in two ways: by direct hydroextrusion with a change in the diameter of the billet from 50 mm to 13 mm (DE, average grain size of 0.6 μm) and as a result of equal channel angular hydroextrusion of the billet with a diameter of 13 mm (ECAE, average grain size of 0.5 μm). The microhardness at the periphery of the extruded billets was 25% less than in the center. The maximum hardening as a result of extrusion expressed as the ratio of the microhardness of the UFG samples to the microhardness of the annealed CG sample was approximately 2 and 2.3 for the DE and ECAE cases, respectively. The temperature dependence of the microhardness of CG and UFG copper indicates that, upon indentation, plastic flow at temperatures of 77–300 K occurs without a change in the mechanism which is evidently the intersection of mobile dislocations with statistically stored (as forest) dislocations, and small values of the activation volume coincide with their high density. It is shown that the UFG structure of copper is stable; the micromechanical characteristics remain unchanged during long-term storage of extruded samples at room temperature. The results of indentation tests are compared with the data on the tensile deformation of similar samples. The correlation between microhardness and yield strength of DE and ECAE copper in the temperature range 77–300 K is considered.