Key Takeaways & Executive Findings
- •• Doping Co into AlNbTiVCr LHEAs transforms the microstructure from a single B2 phase to a dendritic mixture of B2, C14, and C15 Laves phases, with coupled growth of C14 and C15. • Increasing Co content from 0 to 1.5 at.% raises the Laves phase volume fraction, boosting compressive strength from 1,520.8 MPa to 1,844.4 MPa and achieving a maximum Vickers hardness of 699.4 HV. • The mechanical property enhancement is primarily attributed to solid solution strengthening and second phase strengthening from the Laves phases. • The formation of B2 and Laves phases in these LHEAs can be predicted using parameters such as mixing enthalpy, valence electron concentration, atomic radius size, mixing entropy, and electronegativity difference.
Abstract
To develop high-hardness and high-strength lightweight high entropy alloys (LHEAs), a series of CoxAlNbTiVCr 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. CoxAlNbTiVCr (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 (ΔHmix), 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. Co1.5AlNbTiVCr 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.
1. Introduction
Pursuit and expectation for materials with low density, good strength, and high ductility, have never faded in fields of energy conservation and environmental protection, engineering applications, and scientific research [1-3]. Efforts to reach the target applications of lightweight materials with excellent properties have been predominantly through alloying methods [4-12].
In recent years, in the quest for optimal mechanical properties among the seemingly countless choices of alloy compositions, a rapidly emerging paradigm, the so-called high entropy alloys (HEAs), exhibit significant advantages over traditional alloys [13-15]. Among them, AlNbTiV lightweight high entropy alloy (LHEA) designed by Stepanov [16] has attracted widespread attention from materials researchers, which has a low density of 5.59 g·cm-3, but its strength is not high enough.
To further improve the mechanical properties of AlNbTiV LHEA, AlNbTiVCrx LHEAs were prepared. The results demonstrate that introduction of Cr promotes the formation of the C14 Laves phase and enhances the room temperature mechanical properties of the alloys [17]. Similarly, Jiang et al. [18] prepared Al0.8Nb0.5Ti2V2Zr0.5 LHEA with C14 Laves phase by vacuum arc melting, and the alloys exhibit superior mechanical performance to nickel-based superalloy. Choi et al. [19] produced (CoCuFeNi)100-xZrx LHEAs consisting of an FCC1, FCC2 matrix phase, and Laves phase. When x increased from 0 to 7, the compressive strength increased from 232 MPa to 446 MPa without decreasing its plasticity. Some researchers observed the transformation of the C14 Laves phase into the C15 Laves phase during synchronized shearing. Yao et al. [20] found that the compressive yield strength of Ti3V2NbNi0.5 alloy reinforced by the C15 Laves phase present in the interdendritic area reached 1,130 MPa with an ultimate compressive strain of 20%. Recent studies have conclusively verified that the atomic radius of Co is relatively small, and it can form C15 Laves phase with transition group elements with larger atomic radii (such as Zr and Hf) [21, 22].
In general, single-phase solid solution HEAs with equal mole ratios have limitations because of the trade-off relationship between different performance characteristics. To break through the limitations, intermetallic phases in multiphase alloys, especially Laves phases, become an important candidate to improve the performance of alloys [23-25]. Laves phase, which is the largest community of intermetallic compounds with many representatives, has the classic composition AB2 [26-28]. Laves phase exists in three types, which are the hexagonal MgZn2 (C14), cubic MgCu2 (C15), and hexagonal MgNi2 (C36) [29]. Numerous studies revealed that the formation criteria of Laves phase in HEAs are mainly determined by the difference in atomic radius sizes, the mixing enthalpy, and the valence electron concentration [30-33]. However, the synergistic interaction of multi...
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Fang-dong Xu, De-zhi Chen, Rui-run Chen, Bin Gan, Jing-yue Yu (2025). Lightweight Al-Nb-Ti-V-Cr high entropy alloys with high hardness and enhanced mechanical properties via doping Co. China Foundry. https://doi.org/10.1007/s41230-025-4130-1
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Frequently Asked Questions
What is the effect of Co doping on the microstructure of AlNbTiVCr lightweight high entropy alloys?
Co doping transforms the microstructure from a single B2 phase to a dendritic mixture of B2, C14, and C15 Laves phases, with the Laves phases exhibiting coupled growth.
How does Co content affect the mechanical properties of these LHEAs?
Increasing Co content from 0 to 1.5 at.% increases the Laves phase volume fraction, enhancing compressive strength from 1,520.8 MPa to 1,844.4 MPa and achieving a maximum Vickers hardness of 699.4 HV.
What are the main strengthening mechanisms in Co-doped AlNbTiVCr LHEAs?
The improvement in mechanical properties is mainly attributed to solid solution strengthening and second phase strengthening from the Laves phases.
Which parameters are used to predict the formation of B2 and Laves phases in these alloys?
The formation is predicted using mixing enthalpy (ΔHmix), valence electron concentration (VEC), atomic radius size (δ), mixing entropy (ΔS), and electronegativity difference (Δχ).
What is the significance of the C14 and C15 Laves phases in these alloys?
The C14 and C15 Laves phases contribute to the enhanced mechanical properties through second phase strengthening, and their coupled growth is a notable microstructural feature.
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