Published: Jan 1987
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Sufficient evaluation of the changes in mechanical properties, such as elastic, plastic, and failure properties, due to neutron irradiation in service is required to precisely predict fuel performance. This paper presents the results of the uniaxial tensile tests performed for recrystallized (850 K, 2.5 h) Zircaloy-2 claddings irradiated in commercial BWRs to fluences of 5 × 1023 to 4 × 1025 n/m2 (E > 1 MeV). The material constants of irradiated Zircaloy-2 were obtained precisely, using a high temperature elongation detector in a hot-cell and computer analyses of digital stress-strain data.
The tensile tests were carried out at 298 to 673 K at strain rates of 0.05 to 5%/min, using tubular specimens cut to 135 mm lengths from the claddings. From these tests, Young's modulus, strain hardening exponent, strain rate sensitivity, and fracture behavior were obtained and evaluated.
Young's modulus of specimens irradiated to 1024 n/m2 is nearly equal to that of unirradiated ones, but irradiation to 1025 n/m2 shows values 7% higher than for unirradiated ones. The irradiation-induced increments in yield stress are rapid below 1 × 1024 n/m2, then slow down above this fluence becoming proportional to (φt)0.1 for fluences of 1 × 1024 to 4 × 1025 n/m2, where φt is fluence.
The strain hardening exponent before irradiation is 0.15 and a constant which is independent of strain levels, but after irradiation it decreases depending on strain increase. The strain rate sensitivity at the 0.2% plastic strain decreases from 0.037 to 0.021 by irradiation.
There are two types of fracture morphologies after irradiation: necking and spiral types. In the latter, a shear band propagates along only one direction. Both are accompanied by localized deformation bands caused by dislocation channelling. The spiral type is predominant at about 573 K, in high irradiation and at high strain rate. The fracture surfaces of the unirradiated and irradiated materials are all ductile, while the number of small dimples, whose nuclei appear to be irradiation defects, increases with fluence. The ratio of the number for unirradiated, irradiation to 1 × 1024 n/m2, and irradiation to 1 × 1025 n/m2 is 1:2.7:2.9.
Zircaloy-2, irradiation, tensile properties, Young's modulus, strain rate sensitivity, strain hardening exponent, failure morphology