Mechanical behavior and microstructural evolution in ECAP copper

The microstructural evolution during ECAP has been investigated using TEM and EBSD. Quasistatic compression and tensile tests, dynamic compression tests using Reverse Taylor geometry were carried out on ECAP samples after different number of passes (initial, 2, 4 and 8); the results are interpreted in terms of the microstructure. Saturation in hardness after 10 ECAP passes is shown to exist indicating a lower limit to the grain size that can be achieved using this technique (similar to 200-500 nm). The dynamic response is modeled by means of the Johnson-Cook constitutive relationship which successfully predicts the shape of deformed specimens and overall strain and an increased strain-rate sensitivity. The evolution of the microstructure leading to a steady-state grain size which recomposes itself as deformation proceeds is discussed. The results suggest a mechanism for plastic deformation involving dislocation generation and annihilation at the grain boundaries coupled with grain-segment rotations enabling continued plastic deformation at a steady state grain size.