Hydride-Phase Formation and its Influence on Fatigue Crack Propagation Behavior in a Zircaloy-4 Alloy
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ZIRCONIUM alloys are used extensively in the nuclear industry as the nuclear fuel sheathing material and fuel pressure tubes because of the alloys’ high performance under severe pressure and temperature conditions coupled with high transparency to thermal neutrons.[1–3] During service, these alloys undergo a series of simultaneous processes, including redistribution of stresses, irradiation, changes in temperature, and hydrogen diffusion. The combined effects of these processes can lead to the precipitation of hydrides and thus to hydride-induced embrittlement.[4–6] Hydrogen embrittlement is a complex mechanism, not fully understood, which causes changes in the bulk physical and ELENA GARLEA, formerly Graduate Student, Department of Materials Science and Engineering, The University of Tennessee, Knoxville, TN 37996, is now Technical Staff Member, Applied Technologies Division, Y-12 National Security Complex, Oak Ridge, TN 37831. HAHN CHOO, Associate Professor, is with the Department of Materials Science and Engineering, The University of Tennessee, and with the Neutron Scattering Science Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831. Contact e-mails: [email protected]; [email protected] GONGYAO Y. WANG, Research Associate, PETER K. LIAW, Professor and Ivan Racheff Chair of Excellence, and PHILIP D. RACK, Associate Professor, are with the Department of Materials Science and Engineering, The University of Tennessee. BJØRN CLAUSEN, Technical Staff Member, is with the LANSCE-LC, Los Alamos National Laboratory, Los Alamos, NM 87545. DONALD W. BROWN, Technical Staff Member, is with the MST-8, Los Alamos National Laboratory. JUNGWON PARK, formerly Graduate Student, Department of Materials Science and Engineering, The University of Tennessee, is now Process Engineer, dpiX, LLC, Colorado Springs, CO 80923. EDWARD A. KENIK, Distinguished R&D Staff Member, is with the Material Science and Technology Division, Oak Ridge National Laboratory. Manuscript submitted August 19, 2009. Article published online July 14, 2010 2816—VOLUME 41A, NOVEMBER 2010
mechanical properties, such as diminution of ductility, decrease of fracture toughness, and shortening of fatigue life.[4–6] Throughout the years, several theories have been developed in regard to the mechanisms that govern the formation of metal hydrides and their effects on alloy performance. In particular, the diffusion of hydrogen to high-stress regions, the nucleation and growth of hydrides in such regions followed by the reorientation of hydrides under external stresses, and the subsequent fracture have been studied.[5–7] A specific issue of hydrogen embrittlement that occurs mostly during reactor shutdowns is the crack propagation.[3] For example, during the Pickering fuel pressure tubes incident,[3] it was observed that cracking was induced by the high residual stresses in the tube’s inner wall in combination with the hydrogen (typically about 10 ppm of hydrogen present) in the pressure tube that embrittled the alloy.[3] Subsequently, when the stress intensity factor
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