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College Park, Maryland      June 6 - 10 , 2004

MP30: Effect of Internal Stress on Hydride Formation in a Hydrogen-Charged Zircoly-4

E. Garlea (Department of Materials Science and Engineering, The University of Tennessee, Knoxville, TN 37996, USA.), H. Choo (Department of Materials Science and Engineering, The University of Tennessee, Knoxville, TN 37996, USA.; Metals and Ceramics Division, Oak Ridge National Laboratory), D. W. Brown (Los Alamos National Laboratory), S. Park, L. L. Daemen (Los Alamos Neutron Science Center, Los Alamos National Laboratory), B. Yang, M. M. Morrison, R. A. Buchanan, P. K. Liaw (Department of Materials Science and Engineering, The University of Tennessee, Knoxville, TN 37996, USA.), C. R. Hubbard (Metals and Ceramics Division, Oak Ridge National Laboratory), H. F. Letzring (Pretreatment and Specialty Products, PPG Industries, Troy, MI 48083)

Hydride formation is one of the main degradation sources of zirconium alloys in hydrogen-rich environments. When sufficient hydrogen is available in these alloys, zirconium-hydride precipitates can be formed. Cracking of the brittle hydrides near a crack tip can initiate crack growth between hydrides leading to premature failure of the material. Hydride formation is believed to be enhanced by the presence of residual or applied stresses. Therefore, the increase in stress field ahead of a crack tip may promote precipitation of additional hydrides. In order to verify this, we investigated the effect of internal stresses on zirconium-hydride-precipitate formation in a zircaloy-4 alloy using neutron and x-ray diffraction.

First, spatially-resolved internal strain measurements were made on a fatigue pre-cracked compact-tension (CT) specimen using in-situ neutron diffraction under applied loads of 667 and 4,444 newtons. The results provided a basic understanding of the strain profiles near the crack tip. Second, hydride formation was investigated using two different specimens: (1) a CT specimen with a fatigue pre-crack, hence with a built-in internal stress and (2) the same CT specimen but without the pre-crack. The two specimens were electrochemically charged with hydrogen under the same conditions x-ray diffraction results clearly showed that the pre-cracked specimen developed higher hydride fractions suggesting that the enhanced hydride formation was due to the presence of the internal stress. Finally, the profile of internal strains in the hydrogen-charged specimen was measured using neutron diffraction to provide an understanding of the effect of hydride on the strain profile near the crack tip in comparison to the strain data measured from a CT specimen before the hydrogenation.

Acknowledgements: This work benefited from the use of the Los Alamos Neutron Science Center (LANSCE) at the Los Alamos National Laboratory. This facility is funded by the US Department of Energy under Contract W-7405-ENG-36. The authors are grateful to the National Science Foundation International Materials Institutes Program (DMR-0231320), Integrative Graduate Education and Research Training Program (DGE-9987548), the NSF Combined Research and Curriculum Development Programs (EEC-9527527 and EEC-0203415), and Tennessee Advanced Material Laboratory Fellowship Program, managed by Drs. C. Huber, W Jennings, L. Goldberg, M. Poats, and Dr. W. Plummer, respectively.

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