Strength and Ductility of Bi-Modal Cu

Engineering a microstructure with multiple length scales has been proposed as a strategy to enhance the plasticity of nanostructured materials which otherwise lack extensive dislocation activity, and therefore low ductility. To that effect, various research groups have implemented this concept by promoting the formation of so called bi-modal microstructures (e.g., consisting of a mixture of ultrafine and micro-grains) with balanced combinations of strength and ductility. Despite encouraging results, fundamental, information on important questions remained unanswered. For example, what is the relationship between the volume fraction of the coarse grained phase and the overall ductility? What is the influence of size of the coarse grained phase, and how does its distribution influence ductility? To provide insight into these, and other related questions, in this work, we prepared bimodal Cu with homogeneous distribution of different-volume-fraction micro-grains via isothermal recrystallization of an ultrafine grained Cu matrix at 200 degrees C. Analysis of the tensile results and microstructural characterization suggest that both strength and ductility of the bi-modal Cu follows the rule-of-mixtures, with interesting results related to volume fraction. Our work provides a pathway for optimizing the mechanical properties of multiscale materials.