Hydride embrittlement and irradiation effects on the hoop mechanical properties of pressurized water reactor (PWR) and b
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ION
ZIRCALOYS*
are used as cladding material in the
*ZIRCALOY is a trademark of INCO Alloys International, Huntington, WV.
nuclear industry for fuel elements. Their mechanical properties can be drastically affected by the presence of hydrides.[1–7] It can be stated that, based on the previous work, the morphology and the mechanical properties of hydrides are the dominant parameters in the description of the embrittlement effect in the ZIRCALOYs. The morphology of the hydrides themselves is a very complicated function of the microstructure (prethermomechanical treatment) of the ZIRCALOYs, the hydriding conditions, and the hydrogen content.[2,4,5, 6] In spite of continual efforts,[1– 20] the knowledge available is still insufficient to provide a quantitative description of the mechanical properties of the hydrides. The behavior of the hydrides during deformation and their interaction with the matrix is still not well known at a microscopic level. The investigation of solid hydride specimens shows that these hydride specimens are brittle at 20 ⬚C. These hydrides are different from the hydrides that are created in a ZIRCALOY matrix and that have a number of defects. The behavior of hydrides in a
SYLVIE ARSENE, formerly Ph.D. Student with Lab. MSS/MAT, Ecole Centrale Paris, is Research Engineer, Pechiney CRV SNMT, 38341 Vorepee, Cedex, France. JINBO BAI, CNRS Researcher, and PHILIPE BOMPARD, Professor, are with the Lab. MSS-MAT, Ecole Centrale Paris, 92295 Chatenay, Malabry, Cedex, France. Contact e-mail:[email protected] Manuscript submitted January 14, 2002. METALLURGICAL AND MATERIALS TRANSACTIONS A
matrix cannot be deduced from that of a solid hydride specimen. Hydride fracture in a ZIRCALOY matrix has been studied by different methods (acoustic emissions and investigation of fracture profiles). In the literature, the plastic deformation of the matrix at which certain hydrides start to fracture (visible cracking) varies from 0 to 20 pct. This value is dependent upon several parameters. Puls has shown that the first cavities appear in the hydrides at matrix deformations that decrease in proportion to the increase in hydride length.[8] Fan and Koss have shown that hydride fracture occurs at an earlier stage when the hydrides are submitted to biaxial tension compared to uniaxial tension.[9] The results obtained from tensile testing conducted on notched sheet specimens confirm that the matrix deformation inducing fracture decreases in proportion to the increase in the axiality of the stress state. This has been found in ZIRCALOY[6] and in titanium.[10] On the other hand, the yield strength of the matrix does not seem to affect the matrix deformation required to induce hydride fracture.[6] In most cases, hydrides fracture at extremely low deformations.[8,11] In certain materials with hydrides shorter than 20 m, the initial hydrides fracture after the onset of plasticity in the matrix. Results from the literature are compiled in Table I. All of the hydrides do not fracture at the same time. Cavities of different
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