Lightweight Al-Nb-Ti-V-Cr high entropy alloys with high hardness and enhanced mechanical properties via doping Co

To develop high-hardness and high-strength lightweight high entropy alloys (LHEAs), a series of Co_xAlNbTiVCr alloys were designed. The phase constitution, distribution, and crystal structure of the Laves phase in alloys can be altered by adjusting the composition of HEAs, which in turn influences their mechanical properties. Co_xAlNbTiVCr (x=0, 0.5, 1, 1.5, and 2, atomic ratio percentage) LHEAs were designed and prepared to characterize the microstructure and tailor the mechanical properties. The introduction of Co changes the microstructure of LHEAs from a single B2 structure to a mixture dendrite structure, which consists of B2 phase, C14 and C15 Laves phase. Wherein the C14 and C15 Laves phases exhibit coupled growth. Several parameters including mixing enthalpy (ΔH_mix), valence electron concentration (VEC), atomic radius size (δ), mixing entropy (ΔS), and electronegativity difference (Δχ) are used to predict the formation of B2 and Laves phase in LHEAs. When the Co content increases from 0 to 1.5at.%, Laves phase volume fraction gradually increases, which leads to an enhancement in the compressive strength from 1,520.8 MPa to 1,844.4 MPa. Co_1.5AlNbTiVCr alloy exhibits the maximum Vickers hardness of 699.4 HV. The improvement of mechanical properties mainly originates from solid solution strengthening and second phase strengthening.