PRECIPITATION BEHAVIORS OF GAMMA'-PHASES, EPSILON-PHASES AND ETA-PHASES IN FE-NI-CO-NB BASED SUPERALLOYS

An Fe-38Ni-13Co-4.7Nb-1.5Ti-0.4Si alloy (alloy 909) and an Fe-29.5Ni-1 9.7Co-4.0Nb-1.3Ti-0.6Al-0.4Si alloy (alloy 929) are the latest low the rmal expansion superalloys. This study was carried out to elucidate th e relation between the age-hardening and the precipitation behaviors o f gamma', epsilon and eta phases in these alloys by micro Vickers hard ness test and transmission electron microscopy. The results obtained i n this study are as follows; (1) In aging at 893K hardening proceeds g radually with aging time and reaches to the maximum hardness; the hard ness after aging for 720 ks in alloys 909 and 929 are HV400 and HV460, respectively. The rate of hardening increases with increaseing aging temperature. After reaching to the maximum hardness the hardness decre ases gradually because of the coarsening of gamma' phase and the preci pitation of epsilon and eta phases. The precipitation and growth rates of these phases in alloy 929 are lower than those in alloy 909. (2) T he precipitation in alloy 909 follows in a sequence of supersaturated gamma phase --> gamma' phase --> Widmanstatten epsilon phase --> cellu lar eta phase. That in alloy 929 follows in a sequence of supersaturat ed gamma phase --> gamma' phase --> Widmanstatten (epsilon + eta) phas es. (3) The main constituent elements of the epsilon and eta phases in these alloys are Ni, Co, Nb, Ti, and Fe and Ni, Nb, Co, Fe, and Si, r espectively. The epsilon phase contains more Co and less Ni contents t han the phase. There is no distinct difference in the composition betw een the Widmanstatten and cellular eta phases.