Thermodynamic re-modeling of the binary Al–Mo and ternary Al–Fe–Mo systems based on novel experimental data

In the present paper, the Al–Fe-Mo phase diagram was studied by X-ray diffraction (XRD), differential thermal analysis, differential scanning calorimetry, scanning electron microscopy and electron probe microanalysis. For the first time, isothermal sections at 1450, 1300 and 1200 °C were experimentally determined over the entire range of compositions. A ternary intermetallic compound τ (Mo 3 FeAl 8 ) with a TiAl 3 -type structure was observed at all above temperatures. It has been shown that isostructural disordered bcc (A2) phases of pure elements Fe and Mo (W-type structure, cI 2 - I m 3 ¯ m ) in the ternary system at 1450 °C form a continuous solid solution (Mo,Fe,Al), dissolving up to 50 at.% Al. When cooled, it decomposes into a Mo-rich part and an elongated area (Fe,Al) along Fe-Al side. As the temperature decreases, their homogeneity regions reduce in size and shift toward higher Mo and Fe contents, respectively. The μ (Mo 6 Fe 7 ) phase, which in the Fe-Mo binary system is formed in the solid state at 1368 °C, deeply extends into the ternary system, up to 16 and 15 at.% at 1300 and 1200 °C, respectively, where Al substitutes for Fe. Since the addition of Mo stabilizes the ε (Fe 5 Al 8 ) phase, and the temperature of its formation in the ternary system is higher than in the binary (1331 and 1234 °C, respectively), the isothermal section at 1300 °C contains a narrow homogeneity region of this phase. In contrast, the solubility of Mo in ε at 1200 °C is no less than 14.4 at.%. The boundary Al-Mo binary and Al–Fe-Mo ternary systems are thermodynamically remodeled based on our experimental results and novel information reported in literature. Thermodynamic modeling was carried out by the CALPHAD (CALculation of PHAse Diagrams) approach using Thermo-Calc software. Calculations performed using the proposed thermodynamic description well reproduce most of the experimental data, including data from the available literature. Based on the developed description, phase diagrams of the constituent binary systems, as well as liquidus and solidus projections, selected isothermal and isopleths sections and the reaction scheme of the ternary system were calculated. Graphical Abstract