Initiation and growth of cavities and cracks along grain boundaries in type 304 stainless steel under creep-fatigue condition

Creep-fatigue tests of Type 304 stainless steel were carried out using smooth round bar specimens. Cavities and small cracks initiated inside the specimen were observed on the cross-section by means of a scanning laser microscope. The results obtained are summarized as follows. (1) From the beginning of the creep-fatigue test, spherical cavities appear at random locations on grain boundaries, one after another. (2) The cavities on the grain boundaries perpendicular to the stress axis preferentially grow, and change shape from spherical to flat oblate (or crack-like). (3) When the ligament area on a grain boundary plane reduces to a half, the cavities coalesce and bring about a complete break of the grain boundary, which is defined as the initiation of a small crack. (4) Mean growth rate of cavities and cracks locates in the vicinity of the crack propagation law which can be derived from the relationship between the propagation rate of large crack and creep J-integral range for time-dependent fatigue of Type 304 stainless steel.