Creation of Harmonic Structure Materials with Outstanding Mechanical Properties

Abstract

Conventional material developments have emphasized ultrafine grain refinement and homogenization. However, “nano- and homo-” materials do not usually satisfy the need to be both strong but ductile, which are of course rather contradictory characteristics. To solve such a problem, we have succeeded in designing a “Harmonic Structure Material” that is both a “nano- and harmonic” material which has overcome that antinomy through use of one of the non-equilibrium powder metallurgy (PM) processes called the severe plastic deformation (SPD) PM process. In the present study, pure Ti, Ti-6Al-4V alloy and SUS316L stainless steel powders are subjected to mechanical milling (MM) for various periods of time. The MM powders have two kinds of microstructure, which can be controlled by the MM conditions. They include ultra fine and coarse grain structures known as “shell” and “core”, respectively. Subsequently, these MM powders are sintered using conventional sintering processes. The sintered materials with the shell and the core have a network structure of continuously connected shells, which we refer to as a harmonic structure. The sintered materials with the harmonic structure simultaneously demonstrate both high strength and elongation. These outstanding mechanical properties are influenced by harmonic structure characteristics such as shell and core grain sizes, shell area fraction and shell network size. Thus, the harmonic structure can be considered as a remarkable design for improving the mechanical properties of materials