Effect of Elevated-Temperature Irradiation on Hastelloy N

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Abstract
The tensile properties of Hastelloy N have been determined after irradiation at 700° C to a dose level of 7 × 1020 n/cm2 (E > 1 MeV) and 9 × 1020 n/cm2 (thermal). The strength and ductility of the material were determined as functions of deformation temperature for the range 20 to 900°C. These properties were also examined as functions of strain rate within the limits of 0.002 and 0.2 in./min (0.005 and 0.5 cm/min) for deformation temperatures of 500, 600, 700, and 800°C. The stress-strain relationship is not affected by irradiation at 700°C. Ductility, as measured by the true uniform and fracture strains, is reduced for deformation temperatures of 500°C and above. The loss in ductility results in a reduction in the true tensile strength. This loss is more significant at test conditions resulting in intergranular failure, such as low strain rates at elevated temperature. Postirradiation annealing of the irradiated alloy does not result in improved ductility. These data are compatible with the experiments suggesting helium generation from the (n,α) reaction of boron as the cause of low ductility. The low ductility of irradiated alloys in general is described in terms of the present knowledge of intergranular fracture. Means of improving the ductility are discussed.