Anisotropic behavior and rupture of hydrided ZIRCALOY-4 sheets
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I. INTRODUCTION
IN a French pressurized water reactor (PWR), most of the structural parts of the fuel assembly consist of zirconium alloys (ZIRCALOY-4*). The oxidation of the ZIRCALOY *ZIRCALOY-4 is a trademark of Westinghouse Electric Company, Pittsburgh, PA.
components by water in the reactor produces hydrogen, which diffuses in the bulk material. Hydrogen, which has a low solubility in zirconium, precipitates as zirconium hydrides.[1] The hydride volume fraction remains tolerable with respect to the loads encountered during service (including handling and possible major accidents). It is, however, planned to increase the lifetime of the fuel assembly in order to increase the uranium burnup. It is, therefore, important to characterize and to understand the effects of hydride precipitation on the mechanical behavior and rupture of ZIRCALOY-4. The influence of the hydride precipitates on the mechanical properties of zirconium alloys has been the subject of many investigations since the development of nuclear power plants.[2–7] One of the main issues of these studies is the ductile-brittle transition of these materials as the hydrogen content increases. Previous studies on zirconium alloys showed that cracking of hydrides during straining causes the acceleration of the ductile-failure process.[2,3] The plastic-strain yield for hydride cracking depends on the temperature, the stress state, and the hydride orientation.[4,8–10] The hydrogen content at the ductile-brittle transition strongly depends on the nature of the investigated material.[4,5,7] Puls[4] and Choubey and Puls[8] used acoustic emission to detect hydride failure in tensile specimens at different temperatures; they showed that
M. GRANGE, Engineer, is with Framatome Nuclear Fuel, 69456 Lyon, Cedex 06, France. J. BESSON, Staff Researcher, is with the Centre des Materiaux, 91003 Evry, Cedex, France. E. ANDRIEU, Professor, is with the Laboratoire Materiaux, ENSCT, 31077 Toulouse, France. Manuscript submitted April 8, 1999. METALLURGICAL AND MATERIALS TRANSACTIONS A
hydride embrittlement is reduced with increasing temperature and that embrittlement is suppressed above 300 8C. At room temperature, very different plastic strains for the initiation of hydride cracking have been experimentally determined: these range from about 2[4] to 20 pct.[10] These large variations can account for the different observed ductile-brittle transition behaviors. The effect of notches has also been studied.[4,9] The results show a strong decrease of ductility and a decrease of the yield strain needed to start breaking the hydrides when compared to unnotched specimens. Another way to study the effect of the stress-triaxiality ratio on the rupture behavior of hydrided zirconium is the punch-stretch testing technique used by Yunchang and Koss[10] on thin sheets. This method allows the development of a biaxial stress state. The results show a reduced ductility; however, the yield strain for hydride cracking remains constant. The comparison of the crack densities for uniaxial and b
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