Texture evolution and mechanical anisotropy of an ultrafine/nano-grained pure copper tube processed via hydrostatic tube cyclic expansion extrusion

Texture evolution and mechanical anisotropic behavior of an ultrafine-grained (UFG) pure copper tube processed by recently introduced method of hydrostatic tube cyclic expansion extrusion (HTCEE) was investigated. For the UFG tube, different deformation behavior and a significant anisotropy in tensile properties were recorded along the longitudinal and peripheral directions. The HTCEE process increased the yield strength and the ultimate strength in the axial direction by 3.6 and 1.67 times, respectively. Also, this process increased the yield strength and the ultimate strength in the peripheral direction by 1.15 and 1.12 times, respectively. The ratio of ultimate tensile strength in the peripheral direction to that in the axial direction, as a criterion for mechanical anisotropy, are 1.7 and 1.16 for the as-annealed coarse-grained and the HTCEE processed UFG tube, respectively. The results are indicative of a reducing effect of the HTCEE process on the mechanical anisotropy. Besides, after HTCEE process, a low loss of ductility was observed in both directions, which is another advantage of HTCEE process. Hardness measurements revealed a slight increment of hardness values in the peripheral direction, which is in agreement with the trend of tensile tests. Texture analysis was conducted in order to determine the oriental distribution of the grains. The obtained 111 pole figures demonstrate the texture evolution and reaffirm the anisotropy observed in mechanical properties. Scanning electron microscopy micrographs showed that different modes of fracture occurred after tensile testing in the two orthogonal directions.