Mechanism for Supression of Radiation-Induced Segregation by Oversized Solute Addition in Austenitic Stainless Steel.

Abstract

The objective of this thesis is to quantify the effect of oversized solutes on radiation-induced segregation in austenitic stainless steels and to determine the mechanism of this effect. Zr or Hf additions to stainless steels demonstrated a reduction in radiation-induced segregation of Cr and Ni at the grain boundary after proton irradiation at 400°C and 500°C to low doses, but the solute effect disappeared at higher doses. Rate theory modeling of RIS incorporated a solute-vacancy trapping mechanism to predict solute effects on RIS. The model showed that RIS is most sensitive to solute-vacancy binding energy. First principles calculations determined a binding energy of 1.08 eV for Zr and 0.71 eV for Hf. Model and experiment agreed in showing suppression of Cr depletion at 3 dpa at 400°C and 1 dpa at 500°C, and experimental results were consistent with the model in showing greater effectiveness of Zr relative to Hf due to a larger binding energy. The dislocation loop microstructure was measured at 400°C, 3 and 7 dpa, and showed a significant decrease in loop density and total loop line length in oversized solute alloys relative to reference alloys. Loop microstructure results were consistent with RIS results by confirming enhanced recombination of point defects by solute-vacancy trapping. Increases in RIS with dose indicated a loss of solute effectiveness, which was consistent with an observed increase in loop line length from 3 to 7 dpa. The loss of solute effectiveness at high dose is attributed to a loss of oversized solute from the matrix due to coarsening of carbide precipitates. X-ray diffraction identified a microstructure with ZrC or HfC precipitates prior to irradiation. Precipitate coarsening was identified as the most likely mechanism for the loss of solute effectiveness on RIS by the following: 1) diffusion analysis suggested significant solute diffusion by the vacancy flux to precipitates on time scales of proton irradiations, and 2) atom probe measurements confirmed the loss of oversized solute in solution as a function of irradiation dose. RIS measurements and subsequent analysis were consistent with the solute-vacancy trapping process as the mechanism for enhanced recombination and RIS suppression.