Harmonic Structured Material Research Articles

調和組織構造を有するハイス鋼／炭素鋼複合材料の微細組織と機械的性質

Harmonic structured materials with fine and coarse grains demonstrate not only enhanced strength but also enough ductility simultaneously. The harmonic structured material consists of a network like a fine grain region and a dispersed coarse grain region. The high strength and enough ductility are attributed to the network fine grain region and the dispersed coarse grain region, respectively. In this study, the harmonic structured metallic composite with high speed steel (HSS) and low carbon steel (LCS) is fabricated by mechanical milling and spark plasma sintering process. The harmonic structured HSS/LCS composite consists of the network like fine grain region with HSS and dispersed coarse grain region with LCS. The hardness of the harmonic structured composite with HSS and LCS is higher than the conventional particle dispersed composite with the same HSS volume fraction.Furthermore, the harmonic structured composite with HSS and LCS indicates high tensile strength without ductility loss, compared to the conventional HSS particle dispersed composite with the same HSS volume fraction. The harmonic structure control of composite materials is effective to improve the mechanical properties with enough ductility maintained.

Creation of Harmonic Structure Materials with Outstanding Mechanical Properties

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

調和組織を有するステンレス鋼のフラクトグラフィによる破壊メカニズムの解明

Mechanically milled stainless steel powder was applied to hot roll sintering (HRS) or hot isostatic pressing (HIP) processes. The mechanically milled powder has a bimodal structure with a severely deformed surface domain which is named as “Shell”, and an inner domain which is named as “Core”. The shell and core microstructure in the milled powder can be maintained even after sintering. As a result, microstructure of the HRS and HIP materials consists of a shell and core bimodal microstructure. Because severe plastic deformation mainly concentrates to the shell domain, a nano grain structure forms in the shell, while a coarse (meso) grain structure form in the core. Such a nano / meso harmonic structured material demonstrated not only superior strength but also a large elongation. The mechanical properties of the HRS and HIP materials were strongly influenced by the nano/meso harmonic microstructure, such as grain size of the shell/core and shell volume fraction. The shell had role of strengthening and core has role of ductility. Thus, the nano/meso harmonic microstructure has been proved to be very effective to improve mechanical properties of structure materials. In present study, a fracture and deformation mechanism of the harmonic microstructure stainless steel have been investigated through a delicate SEM observation. It was confirmed that fracture was initiated at second phase particles and then propagated along the shell/core interface.

Effects of Nano / Meso Harmonic Microstructure on Mechanical Properties in Austenitic Stainless Steel Produced by MM / HRS Process

Mechanically milled austenitic stainless steel powder is applied to hot roll sintering (HRS) process. Microstructure and mechanical properties of the HRS material are investigated in detail. The mechanically milled powder has a bimodal structure with a severely deformed powder surface domain which is named as “Shell”, and an inner domain which is named as “Core”. The shell and core microstructure in the milled powder can be maintained even after sintering. As the result, microstructure of the HRS materials consists of a shell and core bimodal microstructure. Because severe plastic deformation mainly concentrates to the shell domain, a nano grain structure forms in the shell, while a coarse (meso) grain structure forms in the core. Such a nano / meso harmonic structured material demonstrates not only superior strength but also a large elongation. The mechanical properties of the HRS materials are strongly influenced by the nano / meso harmonic microstructure, such as grain size of the shell / core and the shell volume fraction. The shell has role of strength and the core has role of ductility. Thus, the nano / meso harmonic microstructure has been proved to be very effective to improve mechanical properties of structure materials.