Fatigue properties of UFG low C, N, ferrite stainless steel produced by ECAP

Grain fragmentation is essential for higher hardness and strength of metallic materials. As a technique of achieving ultrafine grain (UFG) structures, severe plastic deformation (SPD) which can give materials huge strain has attracted attention, and mechanism of UFG formation by SPD has been studied intensively. Equal Channel Angular Pressing (ECAP) is a type of SPD used to produce UFG material. The majority of papers on SPD materials have been devoted to the face-centered cubic (FCC) materials such as Al, Cu and Ni, but there had been limited papers for body-centered cubic (BCC) metals. For structural materials, fatigue strength is another important mechanical property in addition to strength and ductility from a practical viewpoint. Failure mechanism initiating from a crack formation and propagation in materials produced by ECAP under cyclic deformation has not well been studied. From this background, the final goal of this study is to clarify fatigue behavior and fracture mechanism for ultrafine grained low C, N, Fe-Cr steel produced by ECAP.　It was found that in one-pass ECAPed sample, a crack initiated at a slip band which formed parallel to the shear plane of ECAP. It was contrasted with annealed coarse grained samples, where a crack formed at persistent slip band. These phenomenon can be explained in terms of the difference of annealed and one-pass ECAPed microstructure.