Assessment of the Effect of Irradiation Temperature on the Mechanical Anisotropy of the Zr + Ion Irradiated Zr-2.5%Nb
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Assessment of the Effect of Irradiation Temperature on the Mechanical Anisotropy of the Zr+ Ion Irradiated Zr-2.5%Nb Bipasha Bose1 and Robert J. Klassen1 1 Department of Mechanical and Materials Engineering, Faculty of Engineering, University of Western Ontario, London, Ontario, N6A 5B9, Canada ABSTRACT We present here new information on the effect of irradiation temperature on the strength and mechanical anisotropy of Zr-2.5%Nb CANDU pressure tube material. Polished samples aligned normal to the transverse (TN), axial (AN) and radial (RN) directions of the pressure tube were irradiated at 300°C with 8.5 MeV Zr+ ions to assess the effect of concurrent thermal annealing of the irradiation damage. Constant-load micro-indentation creep tests were performed at 25°C at indentation depths from 0.1 to 2.0 ȝm on the ion irradiated samples. The increase in the initial indentation stress with increasing levels of Zr+ ion irradiation at 300°C was lower than that reported earlier for similar samples exposed to Zr+ irradiation at 25°C. While the anisotropy of the indentation stress decreased significantly with Zr+ ion irradiation, the level of the decrease was reduced when the irradiation was performed at 300oC compared to 25oC. The apparent activation energy ΔG0 of the obstacles that limit the rate of dislocation glide during indentation creep did not change with indentation direction but did increase with increasing levels of Zr+ ion damage. The values of ΔG0 were, again, lower for samples that were irradiated at 300°C than for those irradiated at 25oC. The observed differences in the magnitude of, and the anisotropy of, the initial indentation stress and also the decrease in the apparent activation energy of the indentation creep process of Zr-2.5%Nb samples irradiated with Zr+ ions at 300oC compared to those irradiated at 25oC indicate the effect that concurrent thermal annealing has on the accumulation of irradiation damage. The effect of irradiation temperature on reducing the degree of, and the strength of, irradiation induced crystallographic damage must therefore be considered when predicting the strength and thermal creep behaviour of irradiated nuclear materials. INTRODUCTION Extruded and cold drawn Zr-2.5%Nb pressure tubes used in CANDU nuclear reactors are mechanically anisotropic due to their heavily textured hcp grain morphology. Mechanical characterization of these pressure tubes has traditionally been done with conventional uniaxial test techniques and thus, because of geometrical constraints, the mechanical properties in the radial (thickness) direction of the tubes have not been well studied. In previous studies we have overcome this problem by using micro-indentation tests performed on the RN, AN, and TN planes of the Zr-2.5%Nb pressure tubes to characterize the anisotropy of the indentation stress, which is related to the flow stress, and the indentation creep [1, 2]. In previous studies we have also shown that high energy Zr+ ion irradiation is an effective way to simulate the crystallographic damage result
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