Phase equilibria in the Fe corner of the Fe-Al-Nb system between 800 and 1150°C

Phase equilibria between alpha(Fe,Al) (disordered A2) or FeAl (ordered B2) and the hexagonal C14 Laves phase have been determined in the Fe-Al-Nb system between 800 and 1150 degrees C. From metallographic observations, X-ray diffraction (XRD) and electron probe microanalysis (EPMA) data three partial isothermal sections of the Fe corner have been established. Within the investigated temperature and composition range the two-phase equilibria alpha(Fe,Al) + Laves phase or FeAl + Laves phase are observed in dependence of the Al content. At 1150 degrees C a three-phase equilibrium with a hitherto unidentified phase is found at higher Al contents. The hexagonal C14-type Laves phase Fe2Nb can dissolve more than 30 at.% Al at all investigated temperatures. This does not lead to any structural changes in the Laves phase. In contrast to previous reports the solid solubility of Nb in alpha(Fe,Al)/FeAl increases with increasing Al content. In disordered alpha(Fe,Al) the Nb content also increases slightly with increasing temperature while for FeAl the opposite effect is observed when the temperature is raised from 800 to 1150 degrees C. (C) 2008 Elsevier B.V. All rights reserved.