Microstructure and mechanical properties of directionally solidified Fe-Al-Nb eutectic

An Fe-26Al-9.5Nb (in at.%) eutectic alloy produced by directional solidification (DS) has been characterised regarding its microstructure and basic mechanical properties and the latter are compared with an as-cast (AC) alloy of about the same composition. The DS alloy exhibits a regular fibrous eutectic Fe3Al + (Fe,Al)2Nb (Laves phase) microstructure. By electron backscatter diffraction (EBSD) it was found that the growth directions are <111> for Fe3Al and <1120> for (Fe,Al)2Nb. The compressive yield strength and compressive creep strength under constant and step-wise increased load for the DS and AC alloy are compared with each other. During creep testing the DS alloy showed high microstructural stability, i.e. only slight coarsening was observed. At 650 °C the DS alloy exhibits creep properties which are better than for binary Fe-Al alloys and which are comparable to P92, a state-of-the-art steel for steam turbine applications. The alloy showed a low mass gain after long-time creep testing in air indicating a good oxidation resistance.