Chromium free low expansion superalloys Incoloy 900 series based on the system Fe-Ni-Co-Nb-Ti are precipitation-strengthen austenitic alloys with combination of very low thermal expansion coefficient and high tensile strength. The most important problem of these alloys is the susceptibility to stress accelerated grain boundary oxygen embrittlement (SAGBO) due to the absence of chromium. The oxidation resistance of the alloys after they were exposed for long time at high temperatures and the notch bar rupture strength have been improved with appropriate rare earth (RE) addition. With trace yttrium addition, the platelet precipitates become smaller and denser and the phase of precipitate transforms from ε phase to H phase. The crystal structures of the platelet phases in the conventional and Y-containing superalloys have been investigated and determined. The interface between the H phase and the matrix has also been demonstrated.