Insights into dynamic strain aging under cyclic creep with reference to strain burst: Some new observations and mechanisms. Part-1: Mechanistic aspects

Investigations on the cyclic creep behavior of 316LN austenitic stainless steel (SS) are carried out at 823 K as a function of mean stress (σm), stress amplitude (σa) and stress rate. Strain burst was found to be a common observation at 823 K, which is attributed to the periodic locking and unlocking of dislocations in regular intervals under pronounced influence of dynamic strain aging (DSA). Three distinct stages are identified in the cyclic creep curve based on total accumulated strain or cyclic life. Strain accumulated or life spent in these stages is found to be a strong function of the loading condition (σm - σa - stress rate combinations). Critical strain and threshold cycles for the occurrence of a strain burst were found to have a negative mean stress dependence for fixed σa and stress rate and positive stress rate dependence for fixed σa and σm. Frequent appearance of strain bursts towards the end of life results in excessive geometric softening with clear implications on the failure mode which changes from fatigue failure to creep (necking) in specific cases. Dominant damage modes (cyclic hardening/mean stress dependent hardening or geometric softening/fatigue softening) observed under different loading conditions in cyclic creep in particular where strain burst is prominent is illustrated through a map. Characteristic steps in the form of “spider-web” patterns marking sudden increase in the rate of Stage-II crack propagation is noticed on the fracture surface which is a signature of strain bursts occurring during cyclic creep.