Chromium-Rich High-Entropy Alloy Insights

• Chromium-rich high-entropy alloys are multicomponent metals containing ≥25% chromium, with high entropy stabilizing simple BCC or FCC structures.
• These alloys exhibit superior performance with high-temperature stability, exceptional corrosion resistance via protective Cr₂O₃, and robust mechanical properties.
• Advanced synthesis and computational techniques enable tailored microstructures and engineered electrochemical interfaces for energy storage applications.

Chromium-rich high-entropy alloys (HEAs) are multicomponent metallic systems engineered so that chromium is a principal constituent, often at or above 25 atomic percent. Their high configurational entropy stabilizes single-phase solid solutions with simple crystal structures (typically body-centered cubic (BCC) or face-centered cubic (FCC)), even where binary or ternary subsystems would exhibit complex intermetallics or phase separation. These alloys exploit the electronic, chemical, and mechanical functionalities imparted by chromium, conferring high temperature stability, exceptional corrosion and oxidation resistance, and, recently, advanced electrochemical properties. Cutting-edge computational and experimental work continues to illuminate the phase formation, kinetic behavior, and functional applications of Cr-rich HEAs across materials science, mechanics, and energy storage.

1. Thermodynamic Stabilization and Phase Behavior

The thermodynamic foundation of Cr-rich HEA design is the maximization of configurational entropy, typically quantified as $$\Delta S_{\mathrm{config}} \simeq -k_B \sum_i x_i \ln x_i$$, which is maximized at or near equiatomic compositions. In paradigmatic alloys such as equiatomic Cr–Mo–Nb–V, first-principles calculations show that this high entropy stabilizes a single BCC solid solution at $T \gtrsim 1700$ K, offsetting the positive mixing enthalpies that otherwise drive Cr–Nb and Cr–Mo pairs toward phase separation (Feng et al., 2017). The generalized Gibbs free energy for each phase is modeled as

$$G(T, x) = H_0(x) + \Delta H_{\mathrm{mix}}(x) - T\,\Delta S_{\mathrm{config}}(x) + G_{\mathrm{vib}}(T, x) + G_{\mathrm{elec}}(T, x)$$

where $\Delta H_{\mathrm{mix}}$ (mixing enthalpy) arises from first-principles density functional theory (DFT), $G_{\mathrm{vib}}$ reflects vibrational free energy (computed via phonon density-of-states from density-functional perturbation theory), and $G_{\mathrm{elec}}$ is the electronic free energy (extracted from DFT-derived density-of-states and Fermi-Dirac occupations). For Cr–Mo–Nb–V, stable single-phase BCC persists to the melting point (~2,100 K), but below $T \approx 1,700$ K, the entropy is insufficient to counteract the negative enthalpy of formation of a C15-Laves intermetallic, driving two-phase decomposition. Kinetic suppression can render low-temperature predicted phase boundaries metastable—important for practical alloy processing.

2. Alloy Design Strategies and Synthesis Approaches

Chromium-rich HEA design principles include maximizing component count or near-equiatomic ratios to boost entropy, judiciously introducing elements such as Mo or V to reduce $\Delta H_{\mathrm{mix}}$ (keeping pairwise $\Delta H_{\mathrm{mix}} \lesssim 20$ meV/atom per binary), and leveraging small additions (e.g., Ti, W) to tune electronic density of states near the Fermi level for further BCC stabilization (Feng et al., 2017). Spark plasma sintering (SPS) enables scalable synthesis from commodity powders, illustrated by non-equiatomic Co–Cr–Fe–Ni–Mo alloys containing 28.6 at.% Cr (Kumaran et al., 2023). This approach yields fully dense, fine-grained, homogeneous FCC matrices after post-sintering anneal, and is compatible with cost-efficient, flexible processing routes from commercially available or recycled precursors.

3. Microstructure, Phase Constitution, and Mechanical Properties

Chromium-rich HEAs can form either BCC or FCC crystal structures, depending on electronic (VEC), atomic size mismatch ($\delta$), and entropy-driven stabilization. For example, Co₀.₂₃₃Cr₀.₂₈₆Fe₀.₂₅₀Ni₀.₂₁₀Mo₀.₀₂₁ (C2) consolidates as a fine-grained ($\sim$17–19 $\mu$m) single-phase FCC solid solution after 24 h anneal at 1200°C (Kumaran et al., 2023). The VEC for C2 ($\approx$8.04) and atomic size mismatch ($\delta \approx 1.9\%$) satisfy empirical criteria for FCC stability. Mechanical characterization reveals exceptional ambient and high-temperature strengths: UTS of 712 MPa and ductility of 62% at room temperature, and compressive strength exceeding 640 MPa with ductility above 45% at 750°C. Fracture surfaces exhibit microductile dimpling, with no evidence of embrittling precipitates, indicating the dominant influence of homogeneous matrix deformation. High Cr contents ensure the formation of adherent, protective Cr₂O₃ surface films, imparting superior corrosion resistance.

4. Oxidation Resistance Mechanisms in Chromia-forming Cr-rich HEAs

Recent machine-learning-guided investigations in Ni–Co–Cr–Al HEAs have overturned the prevailing view that high Al is universally necessary for thermally grown oxide protection. At 850°C, “Cr-rich, low-Al” compositions (e.g., Ni₃₅Co₃₁Cr₂₈Al₆, with Cr 28 at.% and Al 6 at.%) outperform high-Al analogues, forming continuous, fine-grained α-Cr₂O₃ scales instead of slow-forming α-Al₂O₃ (Boakye et al., 17 Dec 2025). Parabolic growth kinetics are described by $(\Delta m/S)^2 = k_p t + C$, where $k_p$ (parabolic rate constant) for optimized Cr-rich, low-Al compositions is $k_p \sim 4.6 \times 10^{-9}$ mg$^2$ cm$^{-4}$ s$^{-1}$ (corresponding to $\ln k_p \approx -19.2$), yielding 5–20× lower mass gains than CrAl-free or Al-free reference alloys at the same temperature. SHAP-feature analysis in XGBoost models confirms Cr content as a dominant, negative predictor of $\ln k_p$, signifying lower oxidation rates with increasing Cr. Mechanistically, the formation of lattice-matched, adherent Cr₂O₃ scales with low defect concentrations and slow cation/anion transport is central. Cr’s grain boundary segregation further enhances TGO (thermally grown oxide) cohesion, reducing susceptibility to spallation. These findings demonstrate that at intermediate temperatures, chromia-forming Cr-rich HEAs are optimal for oxidation resistance, broadening the design landscape for bond-coat alloys and challenging the high-Al paradigm (Boakye et al., 17 Dec 2025).

5. Functional Applications: Energy Storage and Electrochemical Behavior

The electrochemical utility of Cr-rich HEAs has recently been demonstrated in the context of multivalent ion batteries. A Cr₀.₄₀Bi₀.₁₅Cu₀.₁₅Ni₀.₁₅Sn₀.₁₅ HEA (atomic fractions), synthesized by high-energy ball milling, circumvents the passivating Cr₂O₃ bottleneck that blocks reversible Cr³⁺ intercalation in elemental metals (Anjan et al., 10 Jan 2026). This is achieved via a multi-element native oxide containing a mosaic of heterointerfaces, notably Cr₂O₃/Bi₂O₃ and Cr₂O₃/CuO, that simultaneously (i) reduce the activation barrier for Cr³⁺ migration ($E_b \sim 0.5$ eV at Cr₂O₃/Bi₂O₃, compared to 3.83 eV in pure Cr₂O₃) and (ii) raise the O²⁻ barrier at certain interfaces (e.g., $E_b \sim 3.65$ eV at Cr₂O₃/CuO), suppressing further oxygen ingress. The result is sustained symmetric-cell cycling ($\sim$10,000 h, 0.2 mA cm$^{-2}$, 20 mV overpotential), with reversible Cr insertion/extraction up to $\sim$37 at.% Cr. Full-cell pairing with sulfur cathodes yields initial discharge capacities of $\sim$300 mAh g$^{-1}$ (S-basis), with 83% retention after 150 cycles. This capability derives from HEA-enabled engineering of oxide interfaces—opening a pathway to practical Cr batteries and suggesting a design principle for multivalent-ion storage electrodes (Anjan et al., 10 Jan 2026).

6. Summary Table of Selected Chromium-Rich HEA Compositions and Phenomena

Alloy System / Composition	Structure / Phase	Key Functional Findings
Cr₀.₂₅Mo₀.₂₅Nb₀.₂₅V₀.₂₅	BCC (high T); BCC + Laves (low T)	Entropically stabilized, reversible phase separation
Co₀.₂₃₃Cr₀.₂₈₆Fe₀.₂₅₀Ni₀.₂₁₀Mo₀.₀₂₁	FCC	High-ductility, high-T strength, pitting resistance
Ni₃₅Co₃₁Cr₂₈Al₆	BCC/FCC (matrix), α-Cr₂O₃ scale	Slow oxidation at 850°C via chromia protection
Cr₀.₄₀Bi₀.₁₅Cu₀.₁₅Ni₀.₁₅Sn₀.₁₅	BCC-type HEA, multi-oxide interface	Long-lived Cr³⁺ battery cycling, low overpotential

7. Impact and Future Directions

Cr-rich HEAs exemplify the entropic design paradigm, enabling single-phase stability, suppressing deleterious intermetallic formation, and enhancing performance envelopes in aggressive chemical and electrochemical environments. The theoretical and empirical frameworks established by first-principles modeling and high-throughput synthesis validate generalizable guidelines: maximization of component count and entropy, targeted tuning of pairwise interaction enthalpy, and microstructural control enable the systematic development of next-generation functional materials. The discovery that engineered oxide heterointerfaces unlock efficient Cr³⁺ transport in battery anodes provides a blueprint for extending HEA design principles to electrochemical devices beyond structural applications. Continued advances in machine learning-guided design, in situ characterization, and computational modeling are expected to further expand the compositional and functional spaces accessible to chromium-rich high-entropy alloys (Feng et al., 2017, Boakye et al., 17 Dec 2025, Kumaran et al., 2023, Anjan et al., 10 Jan 2026).