Precipitation behaviors of γ', ε and η phases in Fe-Ni-Co-Nb based superalloys

摘要

An Fe-38Ni-l3Co-4.7Nb-l.5Ti-0.4Si alloy (alloy 909) and an Fe-29.5Ni-19.7Co-4.0Nb-l.3Ti-0.6Al-0.4Si alloy (alloy 929) are the latest low thermal expansion superalloys. This study was carried out to elucidate the relation between the age-hardeningand the precipitation behaviors ofγ' ε andη phases in these alloys by micro Vickers hardness test and transmission electron microscopy.The results obtained in this study are as follows;(1) In aging at 893Khardening proceeds gradually with aging time and reaches to the maximum hardness; the hardness after aging for 720 ks in alloys 909 and 929 are HV400 and HV460, respectively. The rate of hardening increases with increaseing aging temperature. Afterreaching to the maximum hardness the hardness decreases gradually because of the coarsening of γ' phase and the precipitation ofε andη, phases. The precipitation and growth rates of these phases in alloy 929 are lower than those in alloy 909.(2) The precipitation in alloy 909 follows in a sequence of supersaturated γ phase→ γ' phase→ Widmanstattenεphase→ cellularη phase. That in alloy 929 follows in a sequence of supersaturated γ phase→ γ' phase→ Widmanstatten (ε+η) phases.(3) The main constituent elements of theε andη phases in these alloys are Ni, Co, Nb, Ti, and Fe and Ni Nb, Co, Fe and Si, respectively. Theε phase contains more Co and less Ni contents than theη phase. There is no distinct difference in thecomposition between the Widmanstatten and cellularη phases.