Epitaxial Fe Thin Films on {100} Y 2 Ti 2 O 7 : Model Interfaces for Nano-Oxide Dispersion Strengthened Steels
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MATERIALS for nuclear fusion applications must reliably perform at high temperatures and accommodate high levels of helium (He) and displacement damage.[1–7] Nanostructured Ferritic Alloys (NFAs) are a promising class of Fe-Cr-based stainless steels with outstanding properties. They are thermally stable up to 1173 K (900 C) and remarkably irradiation tolerant.[1–4] NFAs typically contain a high number density (5 9 1023/m2) of Y-Ti-O nano-oxides (NOs) with average diameters hdi 2.5 nm.[1,2] The NOs impede dislocation climb and glide, stabilize dislocation and grain structures, and trap transmutation-produced He in fine-scale bubbles at matrix-NO interfaces.[1,4,8] The high density of small bubbles prevents the formation of growing voids, and decreases He accumulation at grain boundaries which otherwise degrade the dimensional stability, creep and fracture properties of most structural alloys. The NOs and He bubbles also act as recombination sites for
T. STAN, Y. WU, P.B. WELLS, and G.R. ODETTE, are with the Materials Department, University of California, Santa Barbara, CA 93117. Contact e-mail: [email protected] H.D. ZHOU, is with the Department of Materials Science and Engineering, University of Tennessee, Knoxville, TN 37916. Manuscript submitted December 21, 2016. Article published online August 14, 2017 5658—VOLUME 48A, NOVEMBER 2017
vacancies and self-interstitial defects, thereby promoting radiation damage self-healing. Research to characterize NO compositions, structures, misfit strains, core shell structures, interface characteristics, and NO-matrix orientation relationships (ORs) is ongoing.[9–13] Most of the smallest NOs are Y2Ti2O7 (YTO) fcc pyrochlore.[11–20] The cuboidal NOs have multiple interfacial ORs, usually with low index {100}, {110}, and {111} YTO planes bonded to the surrounding matrix. Here, we focus on the Fe-{100}YTO type interfaces such as the cube on cube {100}Fe//{100}YTO and h100iFe//h100iYTO observed in References 14 and 16, the cube on 45 deg rotated cube {100}Fe//{100}YTO and h100iFe//h110iYTO observed in References 12 and 16, and the edge on cube {110}Fe//{100}YTO and h100iFe// h100iYTO observed in Reference 16. Detailed characterization and analysis of the NOmatrix interfaces is needed to develop first principles and atomic-scale interface models that are part of multi-scale efforts to predict the behavior of NFAs and NOs during processing and in irradiation service environments. YTO-matrix ORs are of particular interest because they impact selection of compositions and processing paths, service stability, mechanical properties, and irradiation tolerance of NFAs. Interface characteristics include structures, local chemistries, defects, misfit strains, energies, and NO interactions with He. The embedded NOs are challenging to characterize, partly due to their small size.
METALLURGICAL AND MATERIALS TRANSACTIONS A
Comprehensive first principles modeling by Jiang et al.[21–23] indicate that He is deeply trapped in the YTO NOs prior to forming nm-scale bubbles on the
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