Microstructural instability and superplasticity in a Zr-2.5 Wt Pct Nb pressure-tube alloy
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eutron absorption cross section, good corrosion resistance in aqueous media, low hydrogen pickup, adequate high-temperature strength, good creep resistance, as well as microstructural and irradiation stability, dilute zirconium alloys are used as in-core structural materials for pressurized heavy-water reactors (PHWR).[1–9] The Zr-2.5 wt pct Nb alloy is the standard pressure-tube (also called coolant-tube) material in PHWR. However, one of the important safety concerns is the pressure-tube behavior during a loss-of-coolant accident (LOCA). Depending on the type of LOCA, the pressure tube may be exposed to high temperatures and varying stresses. In the extreme case of feeder-pipe failure and simultaneous failure of the emergency core cooling system, there will be complete loss of coolant but fission energy will continue to be released, which will lead to a rise in temperature of the pressure tube. As per the safety-analysis report,[10] the pressure-tube temperature under such conditions can rise up to 800 ⬚C, and the tube will have to support its dead weight, the weight of the fuel bundle, and the residual steam pressure, if any, before it makes any contact with the concentric calendria tube (which is surrounded by a large heat sink in the form of a moderator). Any catastrophic failure of the pressure tube under such conditions will lead to the release of radioactivity in the R.N. SINGH, Scientific Officer (E), R. KISHORE, Scientific Officer (F), and T.K. SINHA, Head, Mechanical Metallurgy Section, Materials Science Division, and A.K. SINGH, Scientific Officer (D), Metallic Section, Atomic Fuels Division, are with the Bhabha Atomic Research Centre, Mumbai, PIN-400 085, India. B.P. KASHYAP, Professor, is with the Department of Metallurgical Engineering and Materials Science, Indian Institute of Technology, Mumbai, PIN-400 076, India. Manuscript submitted December 6, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS A
open, thereby endangering life. Hence, the study of the deformation behavior of this alloy at elevated temperatures is of great technological importance, and there is a need to develop constitutive relationship(s) having predictive capabilities over a wide range of temperatures, strain rates, and microstructural parameters. The current fabrication route of the pressure tubes (for Indian PHWR similar to the modified route II of Atomic Energy of Canada, Ltd. (AECL)[3]) imparts a duplex microstructure of elongated, hcp ␣-phase grains and a grainboundary network of bcc  phase. Several workers have characterized the elevated-temperature deformation behavior of this alloy.[11–15] The heavily cold-worked fibrous microstructure is unstable at elevated temperatures and transforms to a more stable microstructure.[16] For studying the deformation behavior, a stable microstructure is desirable, as it helps in delineating the effects of various microstructural parameters. However, the constitutive relationships based on results obtained from a stable microstructure will have limited applicability during a LOCA, where th
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