Creep of zirconium and zirconium alloys
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conium and zirconium alloys has often been described as ‘‘anomalous.’’ Researchers often report that zirconium and zirconium alloys never reach true steady-state creep (e.g., References 1 through 3). It has also been reported that the stress exponent[4,5,6] and activation energy[4–7] change continuously with stress, not reflective of climb control as most other ‘‘pure’’ metals within the ‘‘fivepower-law’’ regime. Many interpretations have been offered explaining the creep behavior of zirconium. Some have suggested that creep is dislocation-climb-controlled in the ‘‘intermediate-stress’’ regime corresponding to the five-power-law regime,[7–10] as in other metals and alloys, while others maintain that creep is dislocation-glidecontrolled.[4,6] Still others suggest several different controlling mechanisms within the five-power-law regime depending on stress and temperature.[5,11] The creep rate of zirconium at stresses below those associated with five-power-law creep has been reported to vary nearly linearly with stress. The details of low-stress creep behavior of zirconium are discussed in detail elsewhere.[12] A limited amount of lowstress creep data for Zircaloy-2 suggests that the stress exponent, n, of the steady-state creep rate is approximately equal to one in this regime ðn [ d log e_ss =d log sss , where e_ss is the steady-state creep rate and sss is the steady-state creep stress). Recent analysis by Blum and Maier on aluminum, however, suggests that steady-state conditions may not be achieved in this regime and that the true stress exponent, n, may not approach one.[13] Aluminum obeys classic creep behavior and similar arguments may also apply to other pure metals and class M alloys (such as Zircaloys). Conclusions about zirconium and zirconium alloy creep behavior in the intermediate stress regime often reflect the analysis of limited data. Cumulative zirconium and zirconium alloy creep data will be presented here based on an extensive literature review that includes data often not included in earlier analyses and on experimental results for zirconium and Zircaloy-2 creep from the current study. TROY A. HAYES, Managing Engineer, is with Exponent Failure Analysis Associates, Menlo Park, CA 94025. MICHAEL E. KASSNER, Department Chair, is with the Aerospace and Mechanical Engineering Department, University of Southern California, Los Angeles, CA 90089. Manuscript submitted September 19, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS A
It has been shown that oxygen absorption has detrimental effects on the creep strength of titanium,[14,15] which shares many common properties with zirconium. Raff and Meyder[16] suggest that the creep strength of Zircaloy in an oxidizing atmosphere is higher than that of Zircaloy in an inert atmosphere at temperatures greater than 1000 °C, but that the opposite is true at temperatures between 600 °C and 1000 °C as a result of oxide cracking. Raff and Meyder did not discuss the effects of oxidation on the creep strength below 600 °C. The effect of atmosphere on the creep properties of z
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