Crack initiation and propagation behavior in γ’-Fe4N layer during monotonic and cyclic tensile deformation in the nitrided ultra-low carbon steel

Abstract

In this study, the crack initiation and propagation behaviors of the gamma' layer in nitrided ultra-low carbon steel subjected to monotonic and cyclic tensile tests were investigated to comprehensively understand the deformation and fracture behavior of the Fe4N (gamma') phase. A gamma' layer with a grain size of approximately 2 mu m was successfully formed on the ferrite matrix through nitriding treatment by controlling the nitriding potential. After the monotonic tensile test until fracture, numerous microcracks were generated in the gamma' layer, and a few macrocracks were detected in the ferrite matrix propagated from the gamma' layer. The microcracks were initiated from the gamma' layer/ferrite matrix boundary and propagated to the surface along the {100}(gamma') in the grain or grain boundary during the monotonic tensile test. During the cyclic tensile test, after applying yield stress up to 10(5) cycles, microcracks were initiated from the voids around the surface and propagated to inside of the specimen along {111}(gamma') in the grain or grain boundary. The results demonstrated that, in the gamma' layer, the cracks propagated with the cleavage fracture on {100}(gamma') during the monotonic tensile test, and slipping-off occurred as a result of activating {111} < 1-10 > slip systems during the cyclic tensile test. It is most likely that the macrocracks propagate in the ferrite grain where {100}(alpha) is parallel to the microcrack plane in the gamma' layer and lies in the direction perpendicular to the tensile direction.