Coble, Orowan Strengthening, and Dislocation Climb Mechanisms in a Nb-Modified Zircaloy Cladding
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TRODUCTION
ZIRCONIUM alloys have found extensive applications in nuclear reactors as fuel rods and structural materials for several decades owing to their attractive properties such as sufficient mechanical strength at high temperatures, good corrosion resistance, and more importantly, low absorption cross section for thermal neutrons. In particular, Zircaloy-2 and Zircaloy-4 with major alloying elements of Sn, Fe, and Cr have been serving boiling water reactors (BWR) and pressurized water reactors (PWR), respectively, for several decades.[1] However, with the advent of using these cladding materials for extended burn-up as well as Generation-IV reactor design concepts, the materials inside the reactors are expected to function reliably at higher temperatures, higher neutron doses, and extremely corrosive environment for far longer times.[2] This critically demands for improved microstructural design in reactor structural materials through various methods such as altering composition and processing routes to attain better properties. One such attempt toward realizing this goal was to add Niobium to Zircaloys for achieving an improved long-term corrosion resistance and mechanical properties without losing the neutron absorption
BOOPATHY KOMBAIAH, PhD Student, is with the Department of Materials Science and Engineering, North Carolina State University, Raleigh, NC 27695. Contact e-mail: [email protected] KORUKONDA LINGA MURTY, Professor, is with the Department of Materials Science and Engineering, North Carolina State University, and also with the Department of Nuclear Engineering, North Carolina State University. Manuscript submitted April 15, 2015. Article published online July 18, 2015 4646—VOLUME 46A, OCTOBER 2015
characteristics, the attributes which are generally ascribed to the distribution of b-Nb precipitates in the microstructure.[3,4] For instance, Zr-2.5 pctNb pressure tubes are currently used in pressurized heavy water reactors (PHWR). As well, Nb-modified Zr cladding alloys such as Zirlo and M5 have been developed in USA and Europe for applications in modern PWRs.[2] Along this line, the Zircaloy fuel cladding team at Korea Atomic Energy Research Institute (KAERI) has recently developed several Zr-Nb cladding alloys, known as HANA (High performance Alloy for Nuclear Applications) alloys, with optimum composition and processing conditions to enhance their structural properties.[5–8] The out-of-pile and in-pile performance evaluations of these alloys demonstrated their improved corrosion resistance as well as mechanical properties than Zircaloy-4 probably due to the effect of finely distributed second-phase precipitates in HANA claddings.[9] Among several possible performance degradation phenomena occurring in nuclear fuel cladding materials such as creep, stress-corrosion cracking, and delayed hydrogen cracking, creep is regarded as the prime degradation mechanism. Further, knowledge on transitions in creep mechanisms becomes essential not only in predicting the life times of fuel claddings conservatively
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