HEAT-TREATMENT EFFECTS FOR IMPROVED CREEP-RUPTURE RESISTANCE OF A FE3AL-BASED ALLOY

The iron-aluminide alloy FA-LSO, an Fe3Al-based alloy with a compositi on of Fe-28Al-5Cr-0.5Nb-0.8Mo-0.025Zr-0.05C-0.005B (at%), is of intere st because it has improved creep-rupture resistance when compared to o ther Fe3Al and FeAl-based alloys. Previous creep-rupture testing at 59 3 degrees C and 207 MPa has shown that FA-180 has a rupture life of ap proximately 100 h in the warm-rolled and stress relieved (1 h at 700-7 50 degrees 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 de grees C (followed by air cooling) dramatically improved the creep-rupt ure life of FA-180 to about 2000 h but had little effect on the FA-129 base alloy. Transmission electron microscopy analysis showed that thi s strengthening was due to the precipitation of fine ZrC in the matrix and along grain boundaries. In the current study, creep-rupture data showed that a further improvement in creep-rupture life to over 6000 h can be produced by increasing the cooling rate (quenching in oil or w ater) after solution annealing at 1150 degrees C. Instead of ZrC preci pitates, the microstructure of quenched FA-180 contained many fine dis location loops which were determined to have a (001) habit plane, b/[0 01]. They are therefore most likely vacancy in nature. These fine loop s evolved into a structure of larger dislocation loops and networks du ring creep at temperatures of 593-700 degrees C, acting as obstacles t o network dislocation climb and resulting in the observed improvement in creep strength and rupture resistance. (C) 1998 Elsevier Science Li mited. All rights reserved.