A Low-Expansion Superalloy for Gas-Turbine Applications

A high-strength, low-expansion alloy can greatly increase the efficiency of gas turbines by permitting decreased clearances between rotating and stationary parts. This paper describes development work on a series of nickel-iron-cobalt alloys having the desired combination of high strength and low thermal expansion. The first attempts to develop alloys of this type resulted in materials that required extensive thermomechanical processing and were susceptible to the phenomenon of stress-accelerated grain-boundary oxygen embrittlement (SAGBO). Further development resulted in INCOLOY alloy 909, the first low-expansion superalloy combining good resistance to SAGBO with high mechanical properties achieved without restrictive thermomechanical processing. Those substantial improvements were brought about by the addition of 0.3% to 0.6% silicon to a low-aluminum, 38% nickel, 13% cobalt, 1.5% titanium, 4.7% niobium (columbium), balance iron composition.