1998 Volume 84 Issue 9 Pages 664-671
An Fe-38 Ni-13 Co-4.7Nb-1.5 Ti-0.4 Si alloy (alloy 909) and an Fe-29.5 Ni-19.7 Co-4.0 Nb-1.3 Ti-0.6 A1-0.4 Si alloy (alloy 929) are the latest low thermal expansion superalloys. This study was carried out to elucidate the relation between the age-hardening and the precipitation be-haviors of γ', ε and η 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 893K hardening 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. After reaching to the maximum hardness the hardness decreases gradually because of the coarsening of γ' phase and the precipitation of ε and η 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 →Widmanstätten ε phase →cellular η phase. That in alloy 929 follows in a sequence of supersaturated γ phase →γ' phase →Widmansätten (ε+ η) phases.
(3) The main constituent elements of the ε and η phases in these alloys are Ni, Co, Nb, Ti, and Fe and Ni, Nb, Co, Fe, and Si, respectively. The ε phase contains more Co and less Ni contents than the η phase. There is no distinct difference in the composition between the Wid-manstätten and cellular η phases.
