Precipitation in iron aluminides containing carbon and titanium, zirconium or niobium

Three iron aluminide alloys with the composition Fe-40Al-0.5X-0.2C-0.05B, in at.% (X = Ti, Zr, Nb), have been investigated by means of optical, scanning and transmission electron microscopy. All three alloys were extruded and exhibited small grain sizes ranging from 12 to 40 μm. The precipitate volume fractions and lattice parameters were consistent with the low predicted solubilities in these alloys. While the Ti- and Nb-containing alloys formed, as expected, TiC and NbC precipitates, the Zr-containing alloy contained a ternary intermetallic phase (Fe 6 Al 6 Zr). In addition, an unidentified Fe-Al-Zr phase with a Zr concentration of approximately 23 at.% as well as ZrC were found. The precipitate morphologies were also very different in the three alloys. Whereas in the Ti-containing alloy the TiC phase tended to occur in the form of needles, the precipitates were more equiaxed in the Zr-containing alloy. The NbC particles were also equiaxed and occurred either as isolated particles in the interior of the grains or as clusters at the grain boundaries.