Phase equilibria in the Fe–Al–Mo system – Part II: Isothermal sections at 1000 and 1150 °C

Isothermal sections for 1000 and 1150 °C in the Fe–Al–Mo system have been determined experimentally by metallography, X-ray diffraction (XRD) and electron probe microanalysis (EPMA) on equilibrated alloys and diffusion couples. The phase Al8FeMo3 (τ1) was observed at both temperatures. At 1150 °C the phases ε∗ and τ2 are found in addition, which both have considerable homogeneity ranges. ε∗ has the hexagonal Al8Cr5-type structure (R-3m, hR26, D810). It can be considered as the ternary derivate of the binary high-temperature phase Al8Fe5 (ε). τ2 has a body-centred cubic W-type structure (Im3m, cI2, A2) with the lattice constant a0 varying between 0.296 and 0.313 nm. Both phases together with the binary phase Al8Fe5 belong to the γ brass family. By differential thermal analysis (DTA) the formation temperatures for ε∗ and τ2 have been established. Besides the occurrence of these phases the strong increase of the solid solubility for Mo in αFe,Al/FeAl with increasing temperature is the main feature within this system. The μ phase Fe7Mo6 is the only other phase which has a marked solid solubility for the third component. From the established phase equilibria a reaction scheme for the temperature range 800–1150 °C has been set up.