Superior strengthening mechanisms in dual phase composites having reinforcement agglomeration

The present authors have developed distinct two-phase composites comprising a spherically-clustered phase on the basis of a numerical crack propagation simulation. The composites showed much higher strength as well as the predicted higher crack propagation resistance than the conventional composite with uniform reinforcement distribution. The present work aims to clarify the superior strengthening mechanisms for the composites with the segregated microstructure. Two-dimensional idealized microstructures in which one phase is continuous and the other isolated are modeled for the case where the clustered phase is continuous and for the opposite case. To predict the effect of the segregated microstructures on the strengthening ratio of composites in comparison to unreinforced materials, elastic-plastic finite element method is applied. As the degree of clustering increases, the strengthening ratio of composites increases within the context of this study. The strengthening ratio of composites with a continuous cluster is superior to that of an opposite case. This result could be attributed to the local relaxation of additional internal stresses in the clusters due to the predominant plastic deformation of a softer phase around the clusters when they are isolated. The regular array of clusters always enhances composite strengthening when each cluster contains a sufficient number of reinforcements and its spatial distribution in the clusters is uniform. Primary mechanism, by which additional strengthening arises due to the clustering, is attributed to the large difference in deformation resistance between the metal and the ceramic reinforcement in MMCs. The present composite models are considered to have a structure that the reinforcement clusters reinforce the softer phase. When the overall volume fraction of reinforcements is held constant, the existence of the optimum ratio of elastic moduli or secant moduli between two phases is predicted for the strengthening ratio of composites. Since the optimum ratio is much lower than that of the case of metal/ceramic, i.e., ordinary MMCs, the introduction of a dual phase structure in which both phases can deform elastic-plastically produces high strengthening ratio of composites.