The resistance to stress-accelerated grain-boundary oxygen embrittlement and notch-bar rupture strength in Fe-Ni-Co-Nb-Ti low-expansion superalloy has been improved significantly by trace yttrium addition. The precipitates in the matrix as well as along the grain boundaries have been studied systematically. The platelet precipitates in the matrix and along the grain boundaries have a hexagonal crystal structure (space group, P6/mmm) with lattice parameters a = 0.498 nm and c = 0.408 nm. The crystallographic orientation relationship between the phase and the matrix is found to be ((1) over bar (1) over bar 1)(gamma) //(0 0 0 1)(H), [(1) over bar 1 0](gamma)//[1 1 (2) over bar 0](H). The semi-continuous discrete precipitates along the grain boundaries have an orthorhombic crystal structure with lattice parameters a = 0.45 nm, b = 0.80 nm and c = 1.20 nm. High-resolution images show that the interface between the precipitates and the matrix is semicoherent.