Grain size, misorientation, and texture evolution of copper processed by equal channel angular extrusion and the validity of the hall-petch relationship

The microstructure and texture of copper subjected to equal channel angular extrusion (ECAE) via route B-C for up to 16 passes have been assessed using electron backscatter diffraction (EBSD), transmission electron microscopy (TEM), X-ray diffraction (XRD) peak broadening, and texture analysis. The differences in grain size measured by these techniques allows for an understanding of microstructural evolution. A gradual decrease in grain size occurs with an increasing number of passes, while the subgrain size remains approximately constant. Up to four passes, the fraction of high-angle grain boundaries (HAGBs) (> 15 deg) increases from 15 to 45 pet, but remains constant thereafter. The grain boundary character distribution shows a decrease of 11 boundaries and an increase of 13 boundaries with higher passes. After 16 passes, a few regions of large I to 4 mu m sized grains embedded in a submicrometer sized matrix were observed. These agglomerates of larger grain diameters together with the measured decrease in the dislocation density correlate with the observed decrease in yield strength of samples subjected to four and more passes. Texture evolution is adequately described under conditions of negative simple shear. The effect of increased accumulated strain results in an overall spread of orientation densities due to the absence of stable end positions post-ECAE. Detailed microstructural information suggests that strengthening mechanisms in the material can be sufficiently well described by the classical Hall-Petch relationship by applying it to the subgrain size, while the subgrain size remains smaller than the grain size.