EPSILON-PHASE AND ETA-PHASE PRECIPITATED IN AN FE-38NI-13CO-4.7NB-1.5TI-0.4SI SUPERALLOY

An Fe-38Ni-13Co-4.7Nb-1.5Ti-0.5Si alloy is a controlled low thermal ex pansion superalloy with a good resistance to SAGBO (Stress Accelerated Grain Boundary Oxidation) embrittlement at elevated temperatures by a n intensional Si addition. The precipitation of second phases, epsilon phase, eta phase and so on, in this superalloy is investigated by mea ns of optical microscopy, scanning electron microscopy, transmission e lectron microscopy, and X-ray diffractometry. A time-temperature-trans formation diagram for the epsilon phase is presented. The crystal stru cture and the lattice parameters of the epsilon phase and the orientat ion relationship with the alloy matrix gamma are determined. It is sho wn that the epsilon phase is not a stable phase as well as gamma' phas e, but cellular eta phase is the most stable precipitation phase in th is alloy.