The effect of the ternary elements B, Ti, Cr, Cu, and Mo on fully lamellar FeAl + FeAl2 alloys

Binary Fe-Al alloys in the composition range of 53-65 at.% Al contain finely lamellar microstructures formed of the intermetallic phases FeAl and FeAl2 as a result of a rapid eutectoid decomposition reaction on cooling. Here, we study the effect of ternary additions of up to 2 at.% B, Ti, Cr, Cu and Mo on a fully lamellar alloy with respect to the resulting microstructures, crystal structures of the phases, thermal stability, and mechanical properties. The ternary alloys containing B or Cu preserve the fully lamellar microstructure while those with additions of Mo and Ti show formation of third, ternary phases (Al8FeMo3 and Al2FeTi, respectively). Melting temperatures as well as the formation temperatures of the lamellar microstructure are either unaffected or decrease by the ternary additions compared to the binary case. The hardness of the FeAl phase is much more affected by the ternary additions than that of the FeAl2 phase. The brittle-to-ductile transition temperatures and the yield stresses were studied for all ternary alloys and compared to the binary case, for which also the effect of variations in the Al content was investigated. Especially in case of B additions, an unexpected increase in the brittle-to-ductile transition temperature was observed. Atom probe tomographic experiments reveal that B does not segregate at the phase boundaries but instead forms fine boride particles inside the FeAl lamellae.