This paper presents an overview on the peculiar microstructural design, called ‘harmonic structure’ (HS), for improved mechanical performance of structural materials. A well designed powder metallurgy processing approach has been developed to create a unique three-dimensionally gradient HS with controlled bimodal grain size distribution in metals and alloys. The bulk materials with HS exhibited considerably higher strength and improved toughness as compared to coarse-grained structures. The unique HS design promotes uniformity of deformation by avoiding strain localization during plastic deformation. A possible mechanism of deformation behavior of HS has also been proposed based on the available experimental results. GRAPHICAL ABSTRACT

The harmonic structure is a recently introduced concept paving the way for engineering metallic materials to achieve excellent mechanical performance. They consist of soft coarse-grained regions (Core) that are three-dimensionally surrounded by a connected network of hard ultra-fine grained regions (Shell). The interaction in these Core–Shell regions produces a synergistic effect, during plastic deformation, leading to superior mechanical properties that are extremely important in practical applications. The current review paper is aimed at providing a critical assessment on this novel concept of microstructure design. It also involves the identification and critical discussion of key issues which deserve additional studies. GRAPHICAL ABSTRACT