Mechanisms of structural evolution of laminates with immiscible components under high-pressure torsion

The mechanism of structural evolution of a three-layer Cu-Mo-Cu laminate under high-pressure torsion (HPT) was studied using scanning and transmission electron microscopy, atom probe tomography, and nanoindentation, complemented with finite element calculations. The results demonstrate a gradual refinement of Mo phases; a greater degree of refinement is observed in the peripheral part of the disk-shaped HPT specimen, although some heterogeneity of the structure remains even at a gigantic degree of shear deformation accumulated therein. The elemental distribution calculated from STEM EDX mapping as well as 3D reconstruction of atom probe tomography results shows a significant degree of mixing of the sample components at the atomic level, such that the concentration of copper in Mo reaches ~4.3 at. %; the corresponding concentration of molybdenum in Cu is ~6 at. %. These observations correlate with nanoindentation results showing an increase in the hardness of both phases due to strain hardening and solid solution strengthening, as well as grain refinement. Numerical simulations made it possible to provide a detailed description of the stages of the structure fragmentation, including its self-organizing nature, to show the formation of rupture forerunners in the hard Mo layer and the deformation of harder fragments in a softer matrix. The proposed model of the process assumes the development of a fractal self-organizing self-similar structure under the influence of such factors as the distribution of the elemental constituents of the laminate, their mechanical properties, and the conditions of HPT processing.