The objective for developing this new alloy is to combine low CTE, high strength, and good oxidation resistance so that it can be used in aircraft gas turbine components such as rings, casings, and shrouds to maintain the tightened blade tip clearances at different turbine operating temperatures and thus improve turbine efficiency. INCONEL alloy 783 is a newly developed low CTE superalloys. However, since the addition of Cr has a strong effect of lowering the Curie temperature, and thereby increasing the overall thermal expansion coefficient, low CTE alloys have very low Cr content and thus poor surface oxidation resistance which prevents their broad employment. Initial research efforts led to successful development of the first generation of commercial low CTE alloys, including Incoloy 903, 907, 909. This class of superalloy has found application as stationary components in gas turbine engines and super-conducting magnets for fusion reactors. Over the past two decades, a further major effort for superalloy research has been the development of superalloys with a low coefficient of thermal expansion (CTE). Developing an improved understanding of the mechanism of SAGBO and the introduction of alloys resistant to such environmental effects have become major challenges to materials scientists.
The materials’ resistance to crack growth is reduced as oxygen in front of the crack tip diffuses into the grain boundary. Stress assisted grain boundary oxygen (SAGBO) embrittlement has been revealed as the principal environmental effect, ,. It is well-established that environmental degradation plays a major role in the crack growth behaviors of the tested alloys. Research results show that the crack growth rates of many of these alloys in air are much faster than those tested in vacuum, ,, ,. For relatively brittle materials such as René95, the stress intensity factor, K, can adequately characterize the growth rate and for relatively ductile materials like Udimet 700 and some stainless steels, the C*-integral appears to be the appropriate characterizing parameter. Creep crack growth rate of the alloys are connected to some fracture mechanics parameters, ,, such as K, CTOD and C*-integral. To improve the performance and to better predict the service life of materials, extensive studies on the fatigue and creep crack growth behaviors of Fe- and Ni-base superalloys have been conducted in last decades,.
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