Abstract
The effect of fast-neutron irradiation on the JIC fracture toughness of Alloy 718 plate and weld was characterized at 427°C. Two heat treatments were employed: the ASTM B637 conventional heat treatment (CHT) and a modified heat treatment (MHT) designed to improve the impact toughness of Alloy 718 welds. The preirradiation and postirradiation fracture toughness values for the MHT materials were consistently higher than those for their CHT counterparts. Irradiation significantly reduced JIC and tearing resistance for the plate and weld in both heat-treatment conditions. At high exposures (on the order of 8 dpa), the toughness degradation saturated such that higher irradiation exposures caused no additional embrittlement. Saturation JIC levels for the CHT base metal and weld were 30 and 20 kJ/m2, respectively. For the MHT plate and weld, saturation JIC levels were 45 and 53 kJ/m2. Electron fractographic examination of test specimens revealed that the irradiation-induced reduction in toughness was associated with a transition from microvoid coalescence to channel fracture, which was caused by localized separation along planar slip bands.
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