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

Abstract

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.