Microanalytical study of the hexagonal phase in an yttrium-containing low expansion superalloy

The resistance to stress-accelerated grain boundary oxygen embrittlement (SAGBO) in Fe–Ni–Co–Nb–Ti controlled low expansion superalloy has been improved by trace yttrium element addition. The platelet precipitates in the alloy have been studied by means of optical microscopy, transmission electron microscopy, convergent beam electron microscopy, high-resolution transmission electron microscopy and X-ray diffraction technique. The crystal structure and the chemical composition of the platelet phase have been determined. The phase has a CaCu5-type 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 (1̄1̄1)γ//(0001)H, [1̄10]γ//[112̄0]H. High-resolution images show that the interface between the platelet phase and the matrix is semicoherent and some stacking faults have been found in the platelet phase.