Study of the FCC+L12 two-phase region in complex concentrated alloys based on the Al–Co–Cr–Fe–Ni–Ti system

Abstract New face-centered cubic (FCC) multicomponent alloys designed through the high-entropy (HEA) concept and strengthened with L12 ordered precipitates are promising material solutions for high temperature (HT) structural applications. However, as the design strategy is based on multi-principal elements, the research of alloy compositions exhibiting a stable and well-controlled FCC+L12 microstructure at HT is particularly challenging. Among the critical issues, those relative to the extent and the stability of the FCC+L12 two-phase region in the wide compositional space have to be addressed. Here, we performed high-throughput Calphad calculations in the senary Al–Co–Cr–Fe–Ni–Ti system in the 800°C-1000°C range to screen alloy compositions exhibiting duplex FCC+L12 microstructures. From the 79 695 analyzed compositions, we show that roughly 6% of the total own duplex microstructure at 800°C and 1000°C. Calculations suggest that Cr and Fe additions destabilize the two-phase region. Interestingly we found that Fe is a good candidate to substitute Co or Ni in the FCC phase and potentially induce some solid solution strengthening effects. Finally, the present results allow to propose an original 2D visualization method of the two-phase FCC+L12 region in the complex compositional space. The method is based on the relative influence of the alloying elements on the formation of such microstructure. To assess the reliability of calculations, six alloys were designed and characterized. A good agreement is found between predictive calculations and experimental results except for Cr- and Al-free alloy compositions in the quaternary Co–Fe–Ni–Ti system, due to the absence of description of the τ phase with (Ni0.5Co0.5)3Ti composition in the selected thermodynamic database.