SYMPOSIA PAPER Published: 01 January 1985

Microstructure of Irradiated Inconel 706 Fuel Pin Cladding


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 Experimental Breeder Reactor (EBR-II). Temperatures (time-averaged midwall) and fast fluences for the fractured area range from 447°C and 5.5 × 1022 neutrons (n)/cm2 to 526°C and 6.1 × 1022 n/cm2 (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 γ′ in the matrix near the breach confirmed that the irradiation temperature at the breached area was below 500°C, in agreement with other estimates of the temperature for the area, 447 to 526°C. A hexagonal η-phase, Ni3 (titanium, niobium), precipitated at boundaries near the breach. A more extensive η-phase coating at grain boundaries was found in a section irradiated at 650°C. The η-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 η-plates have a {111} habit relationship with the matrix.

Author Information

Yang, WJS
Westinghouse Hanford Company, Richland, WA
Makenas, BJ
Westinghouse Hanford Company, Richland, WA
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Developed by Committee: E10
Pages: 127–138
DOI: 10.1520/STP37357S
ISBN-EB: 978-0-8031-4935-9
ISBN-13: 978-0-8031-0450-1