Since high-entropy alloys (HEAs) of transition metals show outstanding combinations of structural properties, i. e. strength and ductility, a high research interest has risen for developing HEAs based on refractory metals (RHEAs) as promising high-temperature materials and alternative for nickel-based superalloys. HEAs with non-equiatomic compositions and more complex multi-phase microstructures are referred to as complex concentrated alloys (CCAs).
The refractory complex concentrated alloy (RCCA) AlCrMoTaTi shows a high oxidation resistance comparable to nickel-based superalloys and reasonable high-temperature compression strength. In this work, the equiatomic RCCA AlCrMoTaTi is systematically altered in its Al, Cr and Ti fraction in order to improve both material density, oxidation resistance and high-temperature strength. The non-equiatomic chemical derivatives Al_xCr_yMoTaTi_z (with x = 1, 1.5, 2, 2.5, 3, y = 0.5, 0.75, 1 and z = 1, 1.5, 2, 2.5, 3) are investigated with regards to isothermal oxidation resistance at 1000 °C, formed phases, microstructure and hardness.
CALPHAD simulations using Thermo-Calc software predict a high phase fraction of disordered bcc/ordered B2 phase and a low phase fraction of intermetallic sigma phase at the homogenisation temperature of 1300 °C. Increasing Al concentration increases sigma phase fraction whereas decreasing Cr concentration decreases it. A very high Al concentration is predicted to result in stabilising hcp/A3 with respect to bcc/B2 and high Cr concentrations stabilise intermetallic C14 Laves phase with respect to both bcc/B2 and sigma phase. Experimental findings confirm a significant influence of Al concentration on phase composition, i. e. increasing Al concentration results in increased phase fraction of sigma phase.
Oxidation resistance at 1000 °C for 48 h increases from 1.3 mg/cm² for AlCrMoTaTi to 1.0 mg/cm² for Al₃CrMoTaTi and AlCr_0.5MoTaTi and decreases to 1.6 mg/cm² for AlCrMoTaTi₃. Metallographic cross sections are investigated by SEM and EDX, and formed oxides by XRD.
Macrohardness testing at room temperature results in 700 HV10 for AlCrMoTaTi, 840 HV10 for Al₃CrMoTaTi, 580 HV10 for AlCr_0.5MoTaTi and 460 HV10 for AlCrMoTaTi₃, respectively. Hence, all the RCCA compositions under investigation exceed Ni-based superalloys in marcohardness (e.g. 410 HV10 for MAR-M247) and can compete with refractory alloy MoSiBTi (860 HV10).