AbstractIncubation time for nucleation of fatigue cavities at grain boundaries has been introduced to explain the frequency dependence of fatigue-fracture modes at elevated temperatures. It is suggested that for fatigue at relatively low frequencies, cavitation is an intrinsic response to cyclic stress, mainly occurring on grain boundaries perpendicular to the stress axis. The critical frequency below which bulk cavitation occurs is determined by the incubation time for cavity nucleation. Stress-concentration sites on a sliding grain boundary become nucleation sites in fatigue only at relatively high frequencies caused by the comparatively short time for stress relaxation. In both cases, environment and material chemistry play an important role in stabilizing the cavity embryos. Fatigue cavity nucleation schema have been constructed to depict the various dominant mechanisms in different stress-frequency domains.MST/138