Application of the inverse Kirkendall model of radiation-induced segregation to ferritic–martensitic alloys

The inverse Kirkendall (IK) model was applied to the binary Fe–Cr b.c.c. system, to determine whether observations of Cr in ferritic–martensitic (F–M) alloys can be explained by the IK mechanism. Irradiations were conducted on alloys T91, HCM12A, and HT9 using energetic protons and Fe++ ions and Cr RIS at prior austenite grain boundaries was measured using STEM-EDS. An input data set relevant to b.c.c. Fe–Cr was assembled and model results were compared to experimental results. It was found that unlike austenitic alloys, in b.c.c. Fe–Cr alloys, interstitials played a strong role in Cr RIS. The IK model reproduced the temperature dependence of proton and Fe++ ion irradiations, as well as the composition dependence of proton irradiations. The temperature dependence of Cr RIS is attributed to the dominance of Cr depletion by the vacancy mechanism at high temperatures, and Cr enrichment by interstitials at low temperatures. Cr enrichment changes to Cr depletion at the temperature where the Cr:Fe vacancy and interstitial diffusion coefficient ratios cross one another. Model results also showed a decrease in grain boundary Cr enrichment with bulk Cr concentration, in agreement with experimental results from T91, HCM12A, and HT9 following proton irradiation at 400 °C. The IK model did not account for the experimentally observed dose dependence. This dose dependence of RIS was attributed to the complex microstructural evolution, which is not accounted for in the IK model.