Fractographical Analysis on Fracture Mechanism of Stainless Steel Having Harmonic Microstructure
Akira UENO, Hiroshi FUJIWARA, Muhammad RIFAI, Zhe ZHANG and Kei AMEYAMA
Abstract: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 gShellh, and an inner domain which is named as gCoreh. 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. Key Words:Fractography, Harmonic microstructure, Nano grain structure, SUS304L, SUS316L, Mechanical milling, Hot roll sintering, Hot isostatic pressing