Dislocation concepts applied to fatigue properties of austenitic stainless steels including time-dependent modes

Abstract

Dislocation substructures formed in austenitic stainless steels 304L and 316L, fatigued at 673K, 823 K and 873 K under total imposed strain ranges of 0.7 to 22% and their correlation with mechanical properties have been investigated. In addition substructures formed at lower strain ranges have been examined using foils prepared from parts of the specimens with larger cross-sections. Investigation has also been extended to include the effect of intermittent hold-times up to 1.8 × 10⁴ s and sequential creep-fatigue and fatigue-creep. It is shown that, during the early stages of fatigue, dislocation multiplication occurs and that both steels exhibit rapid hardening. Near the end of, or soon after, the rapid-hardening stage, dislocations are condensed in rows of walls, often forming labyrinths. The dimensions of the labyrinth and in particular the density of dislocations in the walls is shown to depend on the strain range, which also governs the saturation stress. In low-cycle fatigue-tested specimens, with interposed hold-time at peak tensile strain, it was found that the dislocation density of the walls decreases with increasing hold-time and that, above about one hour hold-time, the characteristic fatigue dislocation arrangements are not observed. In specimens tested in fatigue-creep or creep-fatigue sequences at 873 K, it was noticed that, in the absence of permanent damage to the specimen (cracking or cavitation), the dislocation structures formed during the first mode of testing are quickly replaced by those of the second mode. The experimental results obtained are analysed and their implications for current dislocation concepts and mechanical properties are discussed.