Hydride embrittlement in ZIRCALOY-4 plate: Part II. interaction between the tensile stress and the hydride morphology
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I.
INTRODUCTION
ZIRCALOY*-4, used as cladding tube material in the *ZIRCALOY is a trademark of Westinghouse Electric Company, Pittsburgh, PA.
nuclear reactors, can be embrittled by hydrides precipitation, especially when they are oriented perpendicularly to the applied load (radial hydrides in the case of tubes under internal p r e s s u r e ) . 11-41 Present fabrication procedures can produce tubes that have very few radial hydrides when hydrided without stress. This is related, according to the texture, to the preferred orientation of the hydrides owing to the anisotropic misfit strains that are developed between the platelets and the hexagonal close-packed zirconium alloy matrix. However, interactions with a high enough applied stress can modify this preferred orientation and produce hydrides perpendicular to the tensile direction, tS'6m Therefore, the factors that lead to preferential orientation of the precipitates have been extensively examined. In these previous works, the oriented hydrides were produced by cooling specimens under stress (called reorientation). Now it is possible that for tests with hydrogen contents higher than the maximum solid solubility at the hydriding temperature, full reorientation cannot be achieved. In the present work, the effect of stress on hydride orientation was studied
instead by hydriding under stress. We show that the nature of stress-oriented hydrides is very different for the three heat-treated states of ZIRCALOY-4 (stressrelieved, recrystallized, and fl-treated) and the two fabrication procedures (classical one and super a). [2'~] The thermomechanical treatment and the microstructure have a strong influence on the interaction between hydrides and stresses, tS,9j In this study, a quantitative technique is developed for evaluating the susceptibility of ZIRCALOY-4 sheets to the formation of preferentially oriented hydrides. Owing to space limitation, we restrict ourselves to the analysis of the results about the recrystallized state. The external applied threshold stress is determined using tapered specimens (similar to those used by Leger and Donner t~~ and an image analysis technique. We also try to correlate the hydride morphology to the effect of micro- and/or macrostresses. For that purpose, an evaluation of residual micro- and macrostresses was attempted by X-ray diffraction (XRD), and these measurements were complemented by transmission electron microscopy (TEM) examinations, aiming particularly at an estimation of the dislocation density. It was then possible to try to calculate the stresses induced by hydrides precipitation and to model the stress effect on hydride orientation. II.
J.B. BAI, CNRS Researcher, and C. PRIOUL and D. FRANCQIS, Professors, are with Laboratory MSS/MAT, CNRS URA 850, Ecole Centrale Paris, 92295 Chfitenay Malabry, Cedex, France. N. JI, Research Engineer, is with LM3, CNRS URA 1219, ENSAM, 151 Bd de l'Hrpital, 75013 Paris, France. D. GILBON, Research Scientist, is with C E A / D T A / C E R E M / D T M / S R M A , C.E.N. Saclay, 91191 Gi
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