A study on the crack initiation and growth from δ-ferrite/γ phase interface under continuous fatigue and creep-fatigue conditions in type 304L stainless steels

Abstract The extensive microstructural observation has been conducted to investigate the effect of δ-ferrite on the low cycle fatigue properties in terms of its content and directionality, using type 304L stainless steels. Under continuous fatigue test, the crack initiation and growth mechanisms are varied depending on the directionality of the δ-ferrite fibers with respect to the loading direction. In the case of parallel δ-ferrite fibers to the loading direction, the fatigue cracks nucleate from the δ-ferrites due to the impingement of slip bands and propagate along the slip lines, whereas in the case of perpendicular δ-ferrite fibers to the loading direction, the debonding of the δ/γ interface provides the crack nucleation site. The nucleated cracks propagate along planes of maximum tensile stress. Under creep-fatigue conditions, cavities considerably nucleate at the perpendicular δ/γ interface to the loading direction, resulting in the accelerated growth of grain boundary cracks. However, the parallel δ-ferrite fibers have little influence on the creep-fatigue property.