Creep-fatigue cavities and their transition to small cracks along grain boundaries in type 304 stainless steel

Creep-fatigue tests of Type 304 stainless steel were carried out using smooth bar specimens. Cavities, which were initiated on grain boundaries inside the specimens, were observed on the cross-section by means of a scanning laser microscope. The results obtained are summarized as follows. In the very early stage of fatigue life, spherical cavities, the radius of which is less than 3 mu m are randomly initiated on grain boundaries one after another. The only cavities that exist on grain boundaries perpendicular to the stress axis direction preferentially grow and change the shape from spherical to oblate (or crack-like). The initiation of a crack on each grain boundary, namely, a complete break of one grain boundary is brought about by immediate growth and coalescence of cavities when the reduction in area on the grain boundary reaches 0.5, which can be recognized as a condition of crack initiation on grain boundaries.