Abstract
An oxygen free high conductivity (OFHC) copper was subjected to maximum 13 ECAP passes to achieve a sufficiently high degree of deformation. Strength properties (yield stress and ultimate tensile strength) show obvious increase up to the 4^th pass. In the 6^th ECAP pass, the yield stress and ultimate tensile strength are stabilized at a maximum of 449 MPa. Microstructure studies showed that the 1^st ECAP pass caused a significant increase of dislocation density. These dislocations are arranged in cells, forming dense dislocation walls at low-angle subgrains. After the 5^th ECAP pass, well-defined equiaxed ultrafine grains of diameter approximately 500 nm had emerged forming high-angle grain boundaries. The defect density studies carried out by positron annihilation spectroscopy method revealed an increase in a defect density (vacancies, dislocations) up to the 4^th pass, consequently with number of ECAP passes increasing, decrease in defects density (seen by positrons) was observed.

Key words
dislocation, grain boundaries, equiaxed ultrafine grained microstructure, electron microscopy, Equal Channel Angular Pressing (ECAP)

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