Processing and alloying effects on tensile and impact properties of FeAl alloys

Abstract

Two series of iron aluminide alloys [Fe-36–38Al (at.%)] were produced by ingot metallurgy or powder metallurgy methods, to examine the effects of processing and of slight changes in chemical composition on the microstructure and mechanical properties. For the first series of alloys, the powder metallurgy materials developed a very fine grain size when extruded at 950 or 1000°C, much finer than the initial powder particle size. Extrusion at 1100°C resulted in a coarser grain size. The fine-grained powder alloys showed excellent strength and ductility, and high levels of energy absorption in impact tests. The high levels of energy absorption were caused by extensive crack deflection along the remnant oxide layers from the prior particle boundaries; these boundaries had been elongated and oriented perpendicular to the notch by the extrusion process. The coarse-grained material, which had larger, more isolated oxide particles, had slightly lower ductility, but had much poorer impact properties, with no crack deflection. Ingot metallurgy alloys of similar composition, extruded at 900°C, had lower strength and ductility than the powder materials. The addition of boron (0.021 at.%) resulted in a change of fracture mode in the impact tests from intergranular to transgranular, which significantly increased the energy absorption. A second series of cast FeAl alloys examined the effect of changes in the levels of Zr, C, and B, as compared to the first series of alloys. Increases in the Zr and C resulted in increases in the strength and slight decreases in the ductility. Numerous flaws, apparently created during machining of the cast material, were observed on the surfaces of the impact specimens, which still showed some energy absorption when tested. Different fracture modes, including cleavage, intergranular, and ductile tearing, were observed on the fracture surfaces of various impact specimens.