Insights into dynamic strain aging under cyclic creep with reference to strain burst: Some new observations and mechanisms part-II: Microstructural aspects

Abstract

Cyclic creep behavior of 316LN austenitic stainless steel (SS) was investigated at 823K at different combinations of mean stress (σm), stress amplitude (σa) and stress rate. Characteristic strain bursts were observed being attributed to a pronounced influence of dynamic strain aging (DSA). Detailed microstructural investigation carried out through transmission electron microscope (TEM) revealed that dislocation substructure evolving under a process of strain burst during cyclic creep mainly consists of planar deformation bands. The number density of bands was found to be strongly sensitive to σm- σa-stress rate combination employed. An important substructural feature found in this study was the formation of microtwins. Either planar slip or twinning was found to dominate the substructure depending on the loading combination, which was demonstrated through a dislocation distribution map. Dislocation substructure was further correlated with evolution of surface relief studied through atomic force microscopy (AFM) and field emission gun-scanning electron microscopy (FEG-SEM), which depicts the formation of slip markings and nucleation of cracks from persistent slip markings during the course of a strain burst. Finally, well-known theoretical models explaining the mechanism of DSA during tensile deformation were suitably modified for load-controlled scenario and the origin of strain burst as a function of σm or stress rate was explained based on the same. Dislocation density measurements were carried out for specimens undergoing strain burst during cyclic creep, which was utilized for reconstituting the models.