Instability of a molybdenum layer under deformation of a CuMoCu laminate by high-pressure torsion

Abstract High-pressure torsion of a laminate consisting of a layer of monocrystalline molybdenum sandwiched between two layers of copper was investigated. Computed tomography showed that at sufficiently large angles of rotation of the anvils the molybdenum layer loses planarity. It develops periodic folds and vortices, leading to ruptures. EBSD analysis revealed the formation of a blocky substructure in the molybdenum layer and a pronounced fragmentation of copper sheets, with the formation of a significant proportion of high-angle grain boundaries. The proposed mathematical model of the process accounts for the observed phenomena qualitatively. It is based on the gradient plasticity theory and predicts the loss of stability of the harder molybdenum layer when the shear strain in the laminate exceeds a critical value.