Creep crack growth behavior in Ni-base superalloys in 1,273 K helium gas environment

The present paper investigates creep crack growth behavior in Ni-based superalloys, i.e., Ni-26 % Cr-17 % W-0.5 % Mo alloys, at 1,273 K in a He gas environment. The alloys have been developed as candidate structural materials for heat exchangers in Japanese high-temperature gas-cooled reactors (HTGRs) and shown to have good creep resistance. Creep crack growth tests were made on the alloys having different carbon contents in a simulated HTGR He gas environment. Tests were also made in a low vacuum for a comparison purpose. Creep crack growth rate da/dt was evaluated by the fracture mechanics parameter C*. The addition of carbon suppresses the load-line deflection in the primary and secondary stage of creep in the present alloys, whereas it accelerates the deflection rate in the final stage. The da/dt vs. C* diagrams revealed that, in the He gas, da/dt was much faster in the carbon-added alloy than in the non-carbon-added one. This may be attributed to drastic increase in creep ductility in the He gas. The results also showed that the He gas environment accelerated da/dt, especially in higher C* range; nevertheless, this environmental effect was smaller than the effect of carbon-addition. (author)