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The Mechanism of Radiation-Induced Segregation in Ferritic-Martensitic Steels
(2012)
  • Janelle J. Penisten
Abstract
The objective of this thesis is to determine the mechanism of radiation-induced segregation (RIS) in ferritic-martensitic (F-M) alloys. Toward this goal, experimental results were compared to calculations from models of two potential RIS mechanisms: the inverse Kirkendall (IK) mechanism and the solute drag mechanism. Experiment-model comparisons were performed with respect to the dependence of RIS on the experimental parameters of temperature, nominal Cr concentration, and dose. Irradiation experiments were carried out with 2.0 MeV protons over a variety of experimental conditions. The temperature dependence of RIS was measured in alloy T91 at a dose of 3 dpa over a range of irradiation temperatures from 300°C to 700°C. The concentration dependence of RIS was measured in commercial alloys T91 (~9Cr), HCM12A (~11Cr), and HT9 (~12Cr), as well as in a high-purity Fe-9Cr model alloy, at a dose of 3 dpa at 400°C. Lastly, the dose dependence of RIS was measured in both T91 and the Fe-9Cr model alloy at 400°C to doses ranging from 1 dpa to 10 dpa. In all specimens, RIS at grain boundaries was measured using scanning transmission electron microscopy with energy dispersive x-ray spectroscopy (STEM/EDX). Chromium enrichment at grain boundaries was observed in all but one condition, that being T91 irradiated to 3 dpa at 700°C. The tendency for Cr to enrich can be explained by differences in solute-defect diffusion rates, or the IK mechanism. The observed crossover between Cr enrichment at 600°C to Cr depletion at 700°C in T91 irradiated to 3 dpa, can also be explained by the IK mechanism and the competition between Cr enrichment via the interstitial flux and Cr depletion via the vacancy flux. The solute drag mechanism, on the other hand, fails to predict the crossover behavior. The dependence of Cr RIS on nominal Cr concentration demonstrates further consistency between experimental measurements and the IK mechanism, as well as further inconsistency between experiments and the solute drag mechanism. Limited understanding remains regarding the dose evolution of Cr RIS, which is likely influenced by microstructural evolution and minor/impurity element RIS.
Keywords
  • radiation-induced segregation,
  • ferritic-martensitic,
  • irradiation effects on materials
Publication Date
2012
Citation Information
Janelle J. Penisten. "The Mechanism of Radiation-Induced Segregation in Ferritic-Martensitic Steels" (2012)
Available at: http://works.bepress.com/janelle_wharry/4/