Effect of carbon content and helium gas environment on creep crack growth properties of Ni-26 pct Cr-17 pct W-0.5 pct Mo alloy at 1,273 K

Nickel-base superalloys are the most promising candidate structural materials for very high temperature use, e.g., as structural components of piping and heat exchangers in high-temperature gas-cooled reactors (HTGRs) and turbine discs in advanced aircraft engines. They can maintain good creep resistance in a very high temperature range of 1,273 K or higher. Here, creep crack growth tests were conducted on Ni-26 pct Cr-17 pct W-0.5 pct Mo alloys with different carbon contents in air and in helium gas environment at 1273 K using the compact-type (CT) specimen, and the effects of carbon content and environment on creep crack growth rate are discussed. Creep crack growth rate da/dt is evaluated by the C* parameters. The da/dt is faster in higher-carbon alloys than in lower-carbon alloys in each environment. This effect of carbon content is attributed to the lower creep ductility due to the increase of fine trans-granular carbides in higher-carbon alloys. The environmental effect on the da/dt vs C* relations is scarcely observed in higher-carbon alloys. In the 0.003 pct C alloy, however, da/dt is much lower in the He gas environment than in air. Carburization is observed ahead of the crack tip in the He gas environment at 1,273 K. The intergranular carbides precipitated due to carburization have a granular configuration and are considered to prevent the grain boundary sliding in lower-carbon alloys