Microstructure of Irradiated Inconel 706 Fuel Pin Cladding

摘要

A fuel pin from the HEDL-P-60 experiment with a cladding of solution-annealed Inconel 706 breached in an apparently brittle manner at a position 12.7 cm above the bottom of the fuel column with a crack of 5.72 cm in length after 5.0 atomic percent burnup in EBR-II. Temperatures (time-averaged midwall) and fast fluences for the fractured area range from 447 exp 0 C and 5.5 x 10 exp 22 n/cm exp 2 to 526 exp 0 C and 6.1 x 10 exp 22 n/cm exp 2 (E > 0.1 MeV). Specimens of the fractured fuel pin section were successfully prepared and examined in both a scanning electron microscope and a transmission electron microscope. The fracture surfaces of the breached section showed brittle intergranular fracture characteristics for both the axial and circumferential cracks. Formation of gamma ' in the matrix near the breach confirmed that the irradiation temperature at the breached area was below 500 exp 0 C, in agreement with other estimates of the temperature for the area, 447 to 526 exp 0 C. A hexagonal eta-phase, Ni sub 3 (Ti,Nb), precipitated at boundaries near the breach. A more extensive eta-phase coating at grain boundaries was found in a section irradiated at 650 exp 0 C. The eta-phase plates at grain boundaries are expected to have a detrimental effect on alloy ductility. A plane of weakness in this region along the (111) slip planes will develop in Inconel 706 because the eta-plates have a (111) habit relationship with the matrix. (ERA citation 09:037635)