Heat Treatment Effects for Improved Creep-Rupture Resistance of a Fe{sub 3}Al-Based Alloy

The iron aluminide alloy FA-180, an Fe{sub 3}Al-based alloy with a composition of Fe-28Al-5Cr-0.5Nb-0.8Mo-0.025Zr-0.05C-0.005B (at.%), is of interest because of its improved creep-rupture resistance when compared to other Fe3Al and FeAl-based alloys. Creep-rupture testing at 593 C and 207 MPa shows that FA-180 has a rupture life of approximately 100 h in the warm-rolled and stress relieved (1 h at 700-750 C) condition as compared to about 20 h for the FA-129 base alloy (Fe-28Al-5Cr-0.5Nb-0.2C). This report summarizes studies conducted in the last several years to determine the creep-rupture resistance of FA-180 as a function of the microstructures produced by varying the heat treatment temperature and cooling procedures. Solution-annealing for 1 h at 1150 C (air cooled) dramatically improved the creep-rupture life of FA-180 to about 2000 h. Transmission electron microscopy analysis showed that this strengthening was due to the precipitation of fine ZrC in the matrix and along grain boundaries. A further improvement in creep-rupture life to over 6000 h resulted from increasing the cooling rate (by quenching in oil or water) after solution annealing at 1150 C. The microstructure of the quenched specimen contained many fine dislocation loops instead of precipitates. During creep testing, these small loops evolved into a structure of larger dislocation loops and networks, resulting in significant strengthening at temperatures of 593-700 C.