The Mechanism of Zr and Hf in Reducing Radiation-Induced Segregation in 316 Stainless Steel
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STRUCTURAL materials for nuclear applications suffer from a range of deleterious effects due to radiation damage. In Fe-Cr-Ni alloys, one of these effects is the depletion of Cr and the enrichment of Ni at grain boundaries, referred to as radiation-induced segregation (RIS). Although not the primary cause,[1] grain-boundary (GB) RIS in these austenitic stainless steels may play some role in irradiation-assisted stress corrosion cracking (IASCC).[2–4] Enhancing point defect recombination would help to reduce RIS and improve material performance under irradiation. The addition of oversized solute elements is a strategy for reducing RIS in austenitic alloys. Electron irradiations[5–7] conducted on 316L stainless steel samples containing several different oversized solutes showed that the most significant improvement in RIS behavior resulted from the Hf and Zr addition. Shigenaka et al.[6] irradiated 316L stainless steel containing Zr with 400 keV He+ ions at 500 C to doses up to 3.4 dpa, to show a gradual reduction in RIS with an increasing Zr concentration, until RIS was eliminated at 0.41 wt pct Zr. Sakaguchi et al.[5] made additions of 0.5 at. pct of Ti, Nb, Hf and Zr to 316SS and used RIS measurements following electron irradiation at 500 C M.J. HACKETT, Graduate Student, and G.S. WAS, Professor, are with the Nuclear Engineering and Radiological Sciences, University of Michigan, Ann Arbor, MI 48109-2104. Contact e-mail: mjhacket@ umich.edu J.T. BUSBY, Research Scientist, is with the Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN 37830-6138. This article is based on a presentation given in the symposium entitled ‘‘Materials Issues for Advanced Nuclear Systems,’’ which occurred during the TMS Annual Meeting, February 25–March 1, 2007 in Orlando, FL, under the auspices of the Corrosion and Environmental Effects Committee of ASM-TMS. Article published online September 14, 2007 218—VOLUME 39A, FEBRUARY 2008
to estimate a solute-vacancy binding energy. The largest estimated binding energies were for Zr and Hf and were attributed to the large size of these elements relative to the matrix material. Fournier et al.[8] irradiated Hf- and Pt-doped 316SS with protons to 2.5 or 5 dpa at 400 C. The Hf was the most effective at reducing RIS, in this study. At 2.5 dpa, Cr depletion for the Hf alloy was only 1.2 vs 5.1 at. pct for the reference alloy. The authors concluded that the greater effectiveness of Hf was due to its larger size in the matrix relative to Pt. In a similar study using proton irradiations by Allen et al.,[9] the authors concluded that there was no observed benefit of Zr on Cr depletion, although there was a reduction in the enrichment of Ni. While seeming to suggest that Zr had no effect, it should be noted that the reference alloy was irradiated to a lower dose, as compared to the +Zr alloys. Neutron irradiation results further complicate the assessment of oversized solutes.[10] The 304L doped with 0.25 pct Zr was irradiated with neutrons to doses of 0.38 to 19 dpa, be
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