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 coeff icient, high resistance to stress accelerated grain boundary oxygen embritt lement and fairly good notch-bar rupture strength has been successfully dev eloped. The resistance to oxidation for long time exposure at high temperat ures and the stress rupture life improves significantly with trace yttrium additions. The microstructures of the alloy have been studied by means of a nalytical electron microscopy and chemical analysis techniques. The results reveal that the rare earth element segregates in the strengthening phase o f platelet morphology. Further it helps in transforming A(3)B type epsilon phase into A(5)B type H phase. The morphology and crystal structures of the grain boundary phases also change with selective additions of rare earth e lements. The platelet precipitates become smaller and denser, and forms wit hin the grains and along the grain boundaries with the additions of rare ea rth elements. The segregation of the platelet precipitates within the grain s is helpful in improving the strength of the alloy. In addition its precip itation along the grain boundaries can improve the resistance to stress acc elerated grain boundary oxidation and stress rupture property of the alloy and thereby contributed to its excellent elevated temperature stability. (C ) 2000 Elsevier Science S.A. All rights reserved.