Development of ferritic Fe-Al-Ta alloys with strengthening Laves phase for high temperature applications

Fe-Al alloys are of interest for being considered for high-temperature applications. Iron aluminides, in particular F3AI and FeAI, have excellent oxidation and corrosion resistance and lower density than steel. The major problems of these alloys are their low ductility and brittleness at ambient temperatures and poor strength at temperatures above 600°C. Improvement in mechanical properties of iron aluminides has been attempted through alloying elements in order to optimize the mechanical properties. The ternary system Fe-AI-Ta allows the formation of the hard and brittle ternary Laves phase Ta(Feo. s+x, Alo.5-x)2 with hexagonal C14 structure. This Laves phase is very attractive for high temperature applications due to its high melting temperature (TM>1700 °C). The homogeneity range of this Laves phase covers between 0 and ""'49 at.% Al and 0 and 33 at.% Ta. Such two-phase Fe-AI-Ta alloys are attractive for materials development since the mechanical properties can be optimised with respect to the particular application through adjustment of the Ta content, which controls the volume fraction ofthe Laves phase, and adjustment of the Al content, which controls the atomic order in the Fe-Al matrix i. e. disordered Fe-Al (u) with A2-structure, Fe3Al- ordered (cc') with D0 3-structure and FeAl-ordered (ß') with B2-structure. The present study concentrates on Fe-AI-Ta alloys with small Ta contents up to about 6 at.% and various Al contents between 0 and 45 at.%. Ta solubility in the ternary Fe-Al-matrices and constitution of phase presents were studied experimentally at 800, 1000 and 1150 °C. Ta dissolves in small amounts in Fe-Al-matrices; its solubility varies with Al-contents, i. e. ,.,1 at.% for Al-contents up to 24 at.%,.,5 for Al-contents up to 33 at.% and ,.,4 at.% for Al-contents up to 49 at.% at 1150 °C and expected to be lower at lower temperatures. The homogeneity range of Laves phase runs parallel to the Fe-Al axis for up to more than 49 at.% AI. Heat treatment or mechanical testing at temperatures between 600 and 800°C leads to the unexpected formation of Heusler-phase with L2 1-structure. This Heusler-phase was also observed during slow cooling. This phase appears as small and round partic1es having less Ta-content compared with Laves phase, and distributed homogeneously in the grains. By prolonged ageing at temperature between 600 and ,.,750 °C the Laves phase is formed at the expense ofthe Heusler-phase dissolution.