Three-Dimensionally Gradient and Periodic Harmonic Structure for High Performance Advanced Structural Materials

Creation of a unique Harmonic Structure (HS) with controlled bimodal grain size distribution in metals and alloys is a new material design paradigm allowing the improved mechanical performance of structural materials via enhancing strength without sacrificing ductility. A well designed powder metallurgy based processing approach has been developed to create such a controlled microstructure which consists of controlled mechanical milling (MM) of powder particles to create powder particles with bimodal grain size distribution, with a peculiar core-shell structure, followed by their hot consolidation. In the present study, full density compacts with HS were prepared and the effect of such a bimodal microstructure on the mechanical properties of commercially pure Ti with hexagonal close packed (HCP) crystal structure was investigated. The HS pure Ti exhibited considerably higher strength values, without sacrificing ductility, as compared to their coarse-grained (CG) counterparts. The numerical simulation results revealed that the initial stages of deformation and strength of the HS are governed by the characteristics of the interconnected network of the strong fine-grained (FG) shell regions whereas the extent of uniform deformation and overall ductility is governed by the ductile CG core region. It was also demonstrated that the unique HS design promotes uniform deformation very efficiently by avoiding strain localization during plastic deformation.