Metallurgical Effects on Long-Term Creep-Rupture in a New Nickel-Based Alloy

A series of creep-rupture studies were conducted on a new nickel-based alloy, Inconel Alloy 740, for use in Advanced Ultrasupercritical (A-USC) Steam boiler applications. The research quantitatively showed, for the first time, that the formation of a small amount of eta phase during long-term creep testing at 750 degrees Celsius does not degrade the rupture strength or ductility of the alloy. A unique full-scale pressurized tubular creep test was conducted, and a new analysis methodology was developed to evaluate the effect of cold-straining on the creep performance of tube bends made from the alloy. The results showed that 15% cold-work was detrimental to creep strength and ductility in the alloy and lower strain limits should therefore be imposed for typical manufacturing processes. A comprehensive study on the influence of composition and grain size on the alloy was completed. The research focused on creep data analysis, microstructural studies, and computational thermodynamics and showed that grain size, not gamma prime volume fraction or eta phase formation, was the critical parameter influencing creep strength in the alloy. The research also showed that small changes in the aluminum to titanium ratio in the alloy dramatically change the eta phase thermal stability. Results suggests that a large amount of eta phase (above 7 volume %) may reduce creep-rupture ductility (but not strength); however, more research is needed to validate this finding. The accuracy of computational thermodynamics was established for volume fraction and thermal stability of eta phase formation, but deficiencies were noted in kinetic (eta phase precipitation) calculations. In summary, the research shows that provided cold-work and grain size are appropriately controlled, Inconel 740 has predictable creep strength and ductility despite the potential for variations in microstructure. This is most likely due to the formation of gamma prime denuded or precipitate free zones along grain boundaries where creep damage is concentrated.