Abstract
Understanding serrated yielding behavior resulting from dynamic strain aging (DSA) is essential for design and safety considerations. In this work, uniaxial tensile tests were carried out at temperatures ranging from 298 to 1073 K and strain rates 10−5 – 10−3 s−1 followed by microstructural examination of Fe-25Ni-20Cr (wt%) austenitic stainless steel (Alloy 709), a candidate structural material for Sodium-cooled Fast Reactors. Serrated yielding was found to occur in this alloy in two temperature regimes; low-temperature serrated flow (LT-SF) from 498 to 648 K and high-temperature serrated flow (HT-SF) from 648 to 973 K (depending on the strain rate) with activation energies of 103 ± 13 kJ/mole and 204 ± 11 kJ/mole respectively. The critical strain for the occurrence of serrations was found to increase with strain rate as an exponential function with exponent (m + β) of 0.78 ± 0.1 and 1.56 ± 0.2 for the LT-SF and HT-SF regimes respectively. Based on the activation energies and m + β values, diffusion of interstitial atoms has been suggested to be responsible for serrated flow in the LT-SF regime while Cr atom migration was inferred to be responsible for DSA in the HT-SF regime. Manifestations of DSA in the Alloy 709 were observed including peaks and/or plateaus in flow stresses along with negative strain rate sensitivity at intermediate temperatures. However, no loss in ductility was observed within DSA regime attributed to the relatively high work hardening rate and strain-hardening exponent. The samples deformed in DSA regime showed planar substructure while equiaxed subgrains formed at higher temperatures. The fraction of low angle grain boundaries after deformation exhibited a valley at intermediate temperatures believed to be another manifestation of the DSA.