Dynamic strain aging of austenitic stainless steels and Ni-base alloys

Abstract

Dynamic strain aging (DSA) affects mechanical properties of materials and promotes strain localization. DSA results in serrated plastic flow, which was observed by means of constant extension rate tensile (CERT) tests in different grades of AISI 316 austenitic stainless steel and Ni-base alloys Alloy 600 and Alloy 690 in the temperature range of 200 - 600 °C and values of strain rate from 10-6 to 10-3 s-1. Negative strain rate sensitivity was reported for the studied materials at temperatures and strain rates where serrated plastic flow appeared. The map for the occurrence of serrated flow as a function of strain rate and temperature was built for the materials. The activation enthalpies of dynamic strain aging appearance were found to be 120 kJ/mol for the austenitic stainless steel and 159 kJ/mol for both Ni-base alloys. The internal friction (IF) peak associated with the interstitial atoms of nitrogen and carbon in the solid solution of AISI 316NG austenitic stainless steel and Alloy 600 was reported. The activation enthalpies of nitrogen diffusion in AISI 316NG steel (140 kJ/mol) and carbon diffusion in Alloy 600 (162 kJ/mol), obtained by means of internal friction, correspond well to the activation enthalpies of DSA appearance. The height of the internal friction peak increases depending on the pre-straining conditions in the similar way for AISI 316NG steel and Alloy 600. Annealing in situ at the IF peak temperature results in the decay of the IF peak enhanced by pre-straining for both materials. At the initial stage, the peak annealing processes can be described as an exponential decay function with characteristic times of 0.95 and 0.92 ks for AISI 316NG steel and Alloy 600, respectively. Transmission electron microscopy was performed on the specimens of AISI 316NG steel after CERT tests at temperatures of 400 and 200 °C, where serrated and smooth plastic flow was observed, respectively. Long-range planarity was observed in the dislocation structures of the specimen tested at 400 °C. The microstructure of the specimen strained at 200 °C exhibited cellular dislocation structure. It was concluded, that the diffusive re-distribution of interstitial atoms in the DSA regime affected the deformation behavior of the material by restricting dislocation cross-slip, which in turn promotes strain localization, affecting the mechanical performance of the material.