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

The extensive microstructural observation has been conducted to investigate the effect of delta-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 delta-ferrite fibers with respect to the loading direction. In the case of parallel delta-ferrite fibers to the loading direction, the fatigue cracks nucleate from the delta-ferrites due to the impingement of slip bands and propagate along the slip lines, whereas in the case of perpendicular delta-ferrite fibers to the loading direction, the debonding of the delta/gamma 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 8/y interface to the loading direction, resulting in the accelerated growth of grain boundary cracks. However, the parallel delta-ferrite fibers have little influence on the creep-fatigue property. (C) 2002 Published by Elsevier Science Ltd.