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THE energy efficiency of fossil fuel-fired boiler/steam turbine power plants is strongly dependent on the operating temperature and pressure. Hence, it is beneficial to run such power plants at higher temperatures and pressures to enhance efficiency and reduce CO2 emissions. A key aspect for achieving this goal is the materials that are used for construction of the boilers and steam turbines.[1–3] Thus, extensive efforts are underway to develop affordable materials with the necessary strength, oxidation resistance, and corrosion resistance for operation at temperature greater than 973 K (700 C).[4–6] High-nickel alloys and nickel-based superalloys meet all the performance goals, but are too expensive.[7–9] Face-centered cubic (f.c.c.) austenitic stainless steels are potential materials for this high temperature application. They have a combination of good high-temperature creep strength and oxidation resistance with a relatively low cost.[3,10] To that end, a family of inexpensive, high creep strength, alumina-forming austenitic (AFA) stainless steels has recently been developed.[3,11–21] The development is based on the discovery B. HU and G. TROTTER, Ph.D. Students, and IAN BAKER, Sherman Fairchild Professor of Engineering, are with the Thayer School of Engineering, Dartmouth College, Hanover, NH 03755. Contact e-mail: [email protected] M.K. MILLER, formerly Corporate Fellow with the Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, TN 37831, is now Retired. L. YAO, Research Associate, is with the Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831. S. CHEN, Assistant Physicist, and Z. CAI, Physicist, are with the X-ray Science Division, Advanced Photon Source, Argonne National Laboratory, Argonne, IL 60439. Manuscript submitted May 5, 2014. Article published online June 12, 2015 METALLURGICAL AND MATERIALS TRANSACTIONS A

that a protective alumina scale can be formed at 923 K to 1073 K (650 C to 800 C) with 2.5 wt pct Al.[3,13,16,22] The best materials recently developed have creep rupture lifetimes >3000 hours at 1023 K (750 C) and 100 MPa, nearly 20 times longer than commercially available Fe-based Superalloy A286 (Fe-14.5Cr-25Ni2.1Ti-0.15Al). They also show much better oxidation resistance compared to chromia-forming A286 at 1073 K (800 C) in air containing 10 pct water vapor.[20] These recently developed alloys are based on Fe-14Cr32Ni-3Nb-3Al-2Ti (wt pct) with various minor, but important, elemental additions. The addition of Nb is important to improve the stability of alumina scale at high temperature, and results in the formation of Fe2Nb Laves phase.[20] The addition of Ti enhances the stability of Ni3(Al, Ti) phase in AFA alloys.[20] In addition to Fe2Nb Laves phase and L12 (ordered f.c.c.) Ni3Al-type phase precipitates, a B2 (ordered b.c.c.) NiAl phase and MC carbides are observed in these AFA stainless steels.[22] The volume fractions of these phases depend on the alloy composition, mainly the Al and Nb contents. The