Effect of Fe and Mn on structure, mechanical properties, and oxidation resistance of lightweight intermetallic Al55Cr23Ti22 complex concentrated alloy

In this study, we analysed the effect of Fe, Mn, or Fe and Mn additions (5 or 10 at%) on the structure, mechanical properties, deformation behaviour, including microstructure evolution, and oxidation resistance of a lightweight (density of 3.92 g/cm3) intermetallic Al55Cr23Ti22 complex concentrated alloy (CCA) with a L12 + C11b structure. The additions of Fe, Mn, or Fe and Mn (at the expense of Cr) retained the density below 4 g/cm3 and resulted in forming a D8a phase (Th6Mn23-prototype; cF120; Fm-3m) in all the alloys, except for the Al55Cr18Ti22Mn5 alloy. The D8a phase nucleated with a different morphology within or adjacent to the C11b phase, and adopted an orientation relationship of (110)C11b || (44 0)D8a, [3 3 1]C11b || [1 11]D8a, which provided coherent C11b/D8a interfaces. Alloying with Mn or Fe and Mn had either neutral or negative effect on the mechanical properties, while the Fe additions boosted the strength along with some decrease in the compressive plasticity at room temperature. Specifically, the Al55Cr13Ti22Fe10 alloy showed a yield strength of 250 MPa at 1000 degrees & Scy;, which was 60 % higher compared to the Al55Cr23Ti22 alloy. During plastic deformation, all the alloys demonstrated pronounced strain hardening at T <= 800 degrees C and steady state flow at 1000 degrees C. Post-deformation observation of the microstructure of the Al55Cr13Ti22Fe10 alloy showed the inhomogeneous distribution of the strain-induced defects between the L12 matrix and (C11b + D8a) regions at 800 degrees C and partial recrystallisation of these phases at 1000 degrees C. All the alloys exhibited complex oxidation behaviour with multistage oxidation kinetics. The addition of 5 at% of Fe or Mn was beneficial for the oxidation resistance, while the further increase in their contents intensified the mass gain after a certain time. The latter effect was more pronounced in the Al55Cr13Ti22Mn10 alloy than in the Al55Cr13Ti22Fe10 alloy, because of forming a loose Al2O3 oxide layer. All the alloys investigated exhibited a superior synergy of 1000 degrees C-specific yield strength and compressive plasticity at room temperature, as well as a lower mass gain after 100 h at 1000 degrees C, compared to both lightweight and the most oxidation-resistant refractory CCAs.