Residual grain-interaction stresses in zirconium alloys
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I.
INTRODUCTION
I N contrast to cubic metals, hexagonal (hcp) metals are elastically, plastically, and thermally anisotropic. The transition from elastic to fully plastic flow (five operative slip systems) in cubic metals is abrupt since many independent slip systems are available. In hcp metals, this is not the case, and the elastoplastic transition can require up to 5 pct strain. When fewer than the required five slip systems are operative, compatibility among neighboring grains having differing amounts of plastic strain is maintained elastically. Thus, since the plasticity of a given grain depends on its orientation, textured hcp metals always contain significant residual grain-interaction strains due to the grain-to-grain nonuniformity of plastic deformation. The corresponding residual graininteraction stresses must be taken into account correctly if the deformation behavior of textured hcp alloys is to be predicted accurately. A primary objective of the fuel channel research program at the Chalk River Nuclear Laboratories is to be able to model in-reactor deformation reliably and with true predictive capability. The measurements of elongation, diameter, and sag of pressure tubes in CANDU power reactors, while essential for providing a benchmark for the models, are not ideally suited for the development of mechanistic models, since the measurements invariably represent deformation in flux and/or temperature gradients. Small sample experiments, with wellcharacterized conditions (flux, fluence, temperature, microstructure) are essential for model development. Figure 1 presents irradiation growth data obtained recently by Fidlerist~] in sheet-textured ZIRCALOY-2.* The
specimens were prepared to have their axes parallel to the rolling direction of the sheet. The data clearly show the strong influence of even small amounts of deformation on the longitudinal growth strain induced by subsequent irradiation. All specimens elongated by about 3 x 10 -4 after a small neutron irradiation dose, 4 x 1025 n / m 2 (0.1 dpa), and this initial growth strain appears to be independent of the amount of cold work. Thereafter, the deformation was transient for doses up to about 18 • 1025 n / m 2 (approximately 45 dpa), and the evolution of growth strain with fluence was strongly affected by the thermomechanical history of the specimens. The fully recrystallized samples that had received no prestrain simply elongated but with a continuously decreasing rate. The two cold-worked samples decreased in length but at different rates and by different amounts depending on the imposed strain path (uniaxial tension or rolling). The prestrained and stress-relieved sample behaved very similarly to the recrystallized ones. This evidence illustrates the dramatic effect that prestrain, even as small as 1.5 pct, can have on the irradiation growth behavior of Zr alloys. Results such as these cannot be simply explained in terms of the effects that irradiation has on the defect structure of the material and, therefore, lead to the hypothesis that res
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