Heat treatment effects for improved creep-rupture resistance of a Fe3Al-based alloy

Abstract

Abstract The iron-aluminide alloy FA-180, an Fe 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 it has improved creep-rupture resistance when compared to other Fe 3 Al and FeAl-based alloys. Previous creeprupture testing at 593 °C and 207 MPa has shown that FA-180 has a rupture life of approximately 100 h in the warm-rolled and stress relieved (l 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). Solution-annealing for 1 h at 1150 °C (followed by air cooling) dramatically improved the creeprupture life of FA-180 to about 2000 h but had little effect on the FA-129 base alloy. Transmission electron microscopy analysis showed that this strengthening was due to the precipitation of fine ZrC in the matrix and along grain boundaries. In the current study, creeprupture data showed that a further improvement in creeprupture life to over 6000 h can be produced by increasing the cooling rate (quenching in oil or water) after solution annealing at 1150 °C. Instead of ZrC precipitates, the microstructure of quenched FA-180 contained many fine dislocation loops which were determined to have a (001) habit plane, b //[001]. They are therefore most likely vacancy in nature. These fine loops evolved into a structure of larger dislocation loops and networks during creep at temperatures of 593–700 °C, acting as obstacles to network dislocation climb and resulting in the observed improvement in creep strength and rupture resistance.