X-ray diffraction line profile analysis of defects in neutron-irradiated austenitic stainless steels at low displacement damage levels

Abstract

Neutron irradiation-induced microstructural changes in variants of austenitic stainless steels were characterized using the Angle Dispersive X-Ray Diffraction (ADXRD) technique with a synchrotron X-ray source of energy 14.968 keV (wavelength ∼0.82833 Å). SS 304 L(N), SS 316 L(N), and SS 316 materials were neutron-irradiated to low displacement damage levels (1–6.75 dpa) in the Fast Breeder Test Reactor at Kalpakkam at temperatures in the range ∼380 °C-400 °C. The effect of the irradiation-induced defects on X-ray diffraction peak profile parameters such as peak broadening, peak shift, and asymmetry were quantified and related to dislocation density. Peak broadening is found to increase as a function of dpa in most of the cases. Considering the highest dpa sample, lattice contraction has been shown by SS 304 L(N) and SS 316 whereas lattice expansion is exhibited by SS 316 L(N). Asymmetry in peak broadening is found to decrease in case of the highest dpa sample. Ferrite volume percent is seen to have increased as a function of dpa in SS 304 L(N) and SS 316 L(N) but not observed in SS 316. SS 304 L(N) has shown a greater propensity towards the austenite to ferrite transformation and 5 dpa sample has about 18 vol% of α-ferrite. However, in case of 5 dpa SS 316 L(N), ferrite content has increased to ∼4 vol% . The increase in yield strength was also related to the dislocation density amongst all three grades of stainless steel. SS 304 L(N) showed saturation in dislocation density and yield strength around 2 dpa whereas SS 316 L(N) showed linear behaviour up to the highest dpa.