The effect of grain size on the microstructural evolution of copper deformed in rolling

Previous study has established that during tensile deformation the behaviour of large-grained polycrystalline copper is similar to that of single crystals. In rolling, the grains are submitted to a higher degree of constraint and so a coupling effect between the loading mode and the grain size must be considered. In order to reach a better understanding of the microstructural evolution under these conditions, rolling experiments were performed on polycrystalline copper with two grain sizes (35 and 250 μm). Transmission electron microscopy observations of 250 μm grain size specimens showed that microbands started to develop after an equivalent strain of 0.20. With increasing strain another family of microbands developed, leading to the subdivision of grains into domains where the initial cell structure continuously evolved. For a grain size equal to 35 μm the microstructural behaviour was strongly influenced by the interactions between grains only to a strain of 0.30, leading to the development of closed cells. For strains between 0.30 and 0.45 the development of microbands occurred intensively. These microbands resulted from localized shearing in the grains, as was confirmed by determination of the crystallographic orientation and by the observation of shear displacements produced in the initial structure and in the grain boundaries. For both grain sizes no propagation of microbands through the grain boundaries has been detected. The microbands tended to evolve towards subgrain structures.