Creep Behavior and Microstructural Evolution of a Fe-20Cr-25Ni (Mass Percent) Austenitic Stainless Steel (Alloy 709) at
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AUSTENITIC stainless steels have been widely used as major structural materials in nuclear reactors owing to their excellent oxidation and corrosion resistance along with elevated temperature mechanical properties. However, advanced materials with superior high-temperature performance are needed for the Gen-IV nuclear reactors to endure harsh operating conditions including higher operating temperatures in comparison with the current fleet.[1–3] It is known that Ni-based superalloys
ABDULLAH S. ALOMARI is with the Department of Nuclear Engineering, North Carolina State University, Raleigh, NC and also with the Atomic Energy Research Institute, King Abdulaziz City for Science and Technology, Riyadh, Saudi Arabia. Contact e-mail: [email protected] N. KUMAR is with the Department of Nuclear Engineering, North Carolina State University and also with Metallurgical and Materials Engineering, The University of Alabama, Tuscaloosa, AL. K.L. MURTY is with the Department of Nuclear Engineering, North Carolina State University. Manuscript submitted September 20, 2018. Article published online November 30, 2018 METALLURGICAL AND MATERIALS TRANSACTIONS A
(such as Alloy 617) possess higher creep strength in comparison to ferritic and austenitic stainless steels. However, other design factors have to be considered for materials selection in Gen-IV nuclear reactors such as radiation effects and effective cost. For example, Ni-based alloys suffer from serious irradiation embrittlement due to the formation of bubbles on grain boundaries resulting from the formation of transmuted He mainly generated by (n, a) nuclear reaction of nickel.[1,4] Further, Ni-based superalloys are relatively costly than steels. Thus, developing advanced austenitic stainless steels with superior properties is vital to improve the economics for the next-generation nuclear reactors through the reduction of the commodities and the raw materials required for the structural components, assuming all other design factors are still adequate.[3] The majority of the newly developed steels rely on the fine-scale precipitations for their high-temperature creep strength by adding specific type of substitutional (such as Nb or Ti) and interstitial (C and N) alloying elements in a controlled way.[5] The common types of precipitates employed in the creep-resistant austenitic stainless steels VOLUME 50A, FEBRUARY 2019—641
are Z and MX phases, where M is a carbide- or nitride-forming element such as Ti, Nb, V, or Hf, and X is C or N. One of such advanced alloys is a Nb-containing and nitrogen stabilized Fe-20(mass pct) Cr-25Ni austenitic stainless steel (here afterwards referred to as Alloy 709) which is an excellent candidate material for structural applications for Sodium-cooled Fast Reactors due to the desired set of mechanical properties found during preliminary investigations including high-temperature characteristics, sodium compatibility and thermal stability relative to code-approved reference structural materials such as 304 and 316 stainless steels.[6,7] The Alloy 7
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