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Low thermal expansion superalloys have been used for a number of years in a variety of applications, including gas turbine engines. The low thermal expansion characteristics of the most widely used class of materials are derived from the ferromagnetic characteristics of Ni, Fe, and Co- based austenitic matrices containing little or no Cr. Over time, a progression of alloy developments ensued, aimed at improving the oxidation resistance and stress accelerated grain boundary oxygen (SAGBO) attack. While notch rupture tests have been wed to screen for the SAGBO phenomenon, a more sensitive measure of this characteristic is the sustained load crack growth test performed in air. This paper describes some final iterations in the development of a new cIass of low expansion superalloys utilizing high Al content and y, y’, and p phases in the microstructure. Such alloys provide good general oxidation resistance, and rupture strength and ductility, and varying degrees of crack growth resistance. A number of designed factorial experiments were carried out to optimize 538°C crack growth resistance, yet maintain a balance of other important engineering properties. These experiments included examinations of Ni, Fe, Co, Cr, Nb, and Ti content combined with heat treatment studies. Al content remained essentially fixed on the basis of prior development work. Tests performed included thermal expansion, tensile, tensile and Charpy impact stability, stress rupture, creep, 538°C static crack growth, and microstructural analysis.

Development of a Low Thermal Expansion, Crack Growth Resistant Superalloy