Effect of rare earth on the microstructures and properties of a low expansion superalloy

A new Fe–Ni–Co–Nb–Ti–Si superalloy containing trace additions of selective rare earths and having with combination of very low thermal expansion coefficient, high resistance to stress accelerated grain boundary oxygen embrittlement and fairly good notch-bar rupture strength has been successfully developed. The resistance to oxidation for long time exposure at high temperatures and the stress rupture life improves significantly with trace yttrium additions. The microstructures of the alloy have been studied by means of analytical electron microscopy and chemical analysis techniques. The results reveal that the rare earth element segregates in the strengthening phase of platelet morphology. Further it helps in transforming A3B type epsilon phase into A5B type H phase. The morphology and crystal structures of the grain boundary phases also change with selective additions of rare earth elements. The platelet precipitates become smaller and denser, and forms within the grains and along the grain boundaries with the additions of rare earth elements. The segregation of the platelet precipitates within the grains is helpful in improving the strength of the alloy. In addition its precipitation along the grain boundaries can improve the resistance to stress accelerated grain boundary oxidation and stress rupture property of the alloy and thereby contributed to its excellent elevated temperature stability.