Fracture Toughness Variations in Alloy 718

Heat-to-heat and product-form variations in the JIc fracture toughness of Alloy 718 were examined at 24°C, 427°C, and 538°C using the multiple-specimen JR-curve method. Five different material heats along with three product forms from one of the heats were tested in the conventional heat-treatment condition. Statistical analysis revealed only two significantly different JIc levels of 48 kJ/m2 and 74 kJ/m2 for these materials. These two mean JIc levels were independent of temperature. A minimum-expected JIc level based on a tolerance interval bracketing 90 percent of the lower JIc population at a 95 percent confidence level was evaluated as 33 kJ/m2. Coarse δ precipitates controlled the fracture properties by initiating secondary dimples that pre-empted continued growth of primary dimples nucleated by broken carbide inclusions.