Microstructure and tensile properties of Fe-40 At. pct Al alloys with C, Zr, Hf, and B additions

Abstract

The influence of small additions of C, Zr, and Hf, alone or in combination with B, on the microstructure and tensile behavior of substoichiometric FeAl was investigated. Tensile prop-erties were determined from 300 to 1100 K on powder which was consolidated by hot extrusion. All materials possessed some ductility at room temperature, although ternary additions generally reduced ductility compared to the binary alloy. Adding B to the C- and Zr-containing alloys changed the fracture mode from intergranular to transgranular and restored the ductility to ap-proximately 5 pct elongation. Additions of Zr and Hf increased strength up to about 900 K, which was related to a combination of grain refinement and precipitation hardening. Fe₆Al₆Zr and Fe₆Al₆Hf precipitates, both with identical body-centered tetragonal structures, were iden-tified as the principal second phases in these alloys. Strength decreased steadily as temperature increased above 700 K, as diffusion-assisted mechanisms, including grain boundary sliding and cavitation, became operative. Although all alloys had similar strengths at 1100 K, Hf additions significantly improved high-temperature ductility by suppressing cavitation.