Medium Entropy Alloy Refractory Modified Alloy: Compositional Design, Phase Engineering, And Advanced Applications

Fundamental Compositional Design Principles And Entropy Engineering Of Medium Entropy Alloy Refractory Modified Alloy

The design of medium entropy alloy refractory modified alloy systems fundamentally relies on precise control of configurational entropy (ΔS_conf) calculated via the Boltzmann equation: ΔS_conf = -R Σ(X_i ln X_i), where R represents the gas constant (8.314 J·mol⁻¹·K⁻¹) and X_i denotes the molar fraction of element i1716. Medium entropy alloys occupy the entropy range 1.0R ≤ ΔS_conf ≤ 1.5R, distinguishing them from high-entropy alloys (ΔS_conf ≥ 1.5R) and low-entropy alloys (ΔS_conf ≤ 1.0R)51619. This intermediate entropy window enables formation of metastable single-phase or dual-phase microstructures that facilitate transformation-induced plasticity (TRIP) effects and precipitation hardening without excessive phase complexity71017.

Refractory element incorporation follows strategic compositional rules to balance melting point elevation, density reduction, and phase stability. The TiZrCr refractory medium-entropy alloy exemplifies this approach, utilizing Ti and Zr (melting points 1668°C and 1855°C respectively) combined with Cr to achieve spherical powder suitable for additive manufacturing with particle sizes below 200 μm and minimal satellite powder formation6. For Ti-rich compositions, the formula Ti_xAl_aCr_bNb_c (where x = 45–80 at%, a+b+c = 100-x, and differences between a, b, c remain within 0–0.1 at%) produces refractory medium entropy alloys with enhanced oxidation resistance and specific strength exceeding 200 MPa·cm³·g⁻¹4.

The Fe-based medium entropy alloy refractory modified systems demonstrate cost-effective design by replacing expensive Co, Ni, and Cr with abundant Fe while maintaining mechanical performance. The Cr-Fe-Co-Ni-Mo system (3–15 at% Cr, 40–60 at% Fe, 5–20 at% Co, 5–20 at% Ni, 3–15 at% Mo) forms FCC matrix with coherent Mo-rich precipitates, achieving yield strengths of 800–1200 MPa and elongations of 25–40% through precipitation strengthening and transformation-induced plasticity110. The Al-Cr-Fe-Mn quaternary system satisfies the ratio 3 ≤ ([Fe]+[Cr])/([Mn]+[Al]) ≤ 16, producing dual-phase microstructures (FCC + BCC) with room-temperature yield strengths exceeding 530 MPa and ultimate tensile strengths above 970 MPa2.

Critical alloying element effects include:

• Chromium (6–15 at%): Stabilizes BCC phase, enhances corrosion resistance, and controls stacking fault energy (SFE) to promote deformation-induced martensitic transformation from FCC to BCC/HCP phases at cryogenic temperatures571619
• Molybdenum (3–15 at%): Forms coherent L1₂-ordered precipitates within FCC matrix, providing Orowan strengthening with precipitate sizes of 5–20 nm and number densities exceeding 10²³ m⁻³110
• Aluminum (3–15 at%): Reduces density (6.5–7.2 g·cm⁻³ vs. 8.0–8.5 g·cm⁻³ for Al-free alloys), promotes B2/BCC phase formation, and enables spinodal decomposition in Cu-Fe-Mn-Al systems3814
• Titanium/Zirconium (45–80 at% combined): Elevates melting points above 1600°C, improves oxidation resistance through formation of protective TiO₂/ZrO₂ scales, and provides solid-solution strengthening in refractory systems4615

Phase selection criteria for medium entropy alloy refractory modified alloy follow empirical parameters including valence electron concentration (VEC), atomic size difference (δ), and enthalpy of mixing (ΔH_mix). FCC phase stability correlates with VEC ≥ 8.0, while BCC phases dominate at VEC < 6.872913. The atomic size difference δ = 100√[Σc_i(1 - r_i/r̄)²] should remain below 6.6% to avoid amorphous phase formation, where c_i represents atomic fraction and r_i denotes atomic radius15. Negative ΔH_mix values (-15 to -5 kJ·mol⁻¹) promote ordered intermetallic formation, while near-zero values favor solid solutions814.

Microstructural Evolution And Phase Transformation Mechanisms In Medium Entropy Alloy Refractory Modified Alloy

Medium entropy alloy refractory modified alloy systems exhibit complex microstructural evolution pathways governed by thermodynamic stability, kinetic barriers, and deformation-induced phase transformations. The metastable FCC phase in Cr-Fe-Co-Ni systems undergoes strain-induced martensitic transformation to BCC/ε-HCP phases during plastic deformation, particularly at cryogenic temperatures (77 K to 198 K), where the Gibbs free energy difference between FCC and BCC phases decreases from ΔG_FCC→BCC ≈ +500 J·mol⁻¹ at 298 K to ΔG_FCC→BCC ≈ -200 J·mol⁻¹ at 77 K571619.

The TRIP effect mechanism operates through the following sequence: (1) dislocation glide in FCC matrix generates localized stress concentrations exceeding 1.5 GPa; (2) stacking faults nucleate on {111}_FCC planes with stacking fault energy (SFE) values of 15–25 mJ·m⁻² for Cr₁₀Fe₅₇Co₁₆.₅Ni₁₆.₅ composition7; (3) overlapping stacking faults form ε-HCP embryos via Shockley partial dislocation motion (a/6⟨112⟩_FCC); (4) ε-HCP plates transform to α'-BCC martensite through Kurdjumov-Sachs orientation relationship: {111}_FCC ∥ {110}_BCC and ⟨110⟩_FCC ∥ ⟨111⟩_BCC519. This transformation sequence absorbs plastic strain energy, delays necking instability, and produces ultimate tensile strengths of 1.2–1.5 GPa with elongations of 50–70% at 77 K716.

Precipitation strengthening in Mo-containing medium entropy alloys proceeds through coherent L1₂-ordered precipitate formation during aging treatments at 600–800°C for 1–100 hours. The Cr₁₀Fe₄₀Co₂₅Ni₁₅Mo₁₀ alloy develops spherical (Ni,Co)₃(Mo,Fe) precipitates with lattice parameter a_L1₂ = 0.357 nm, exhibiting lattice misfit δ = 2(a_L1₂ - a_FCC)/(a_L1₂ + a_FCC) ≈ 0.2% that maintains coherency up to precipitate diameters of 50 nm110. The critical resolved shear stress for Orowan looping around precipitates follows τ_Orowan = 0.4Gb/λ, where G represents shear modulus (80 GPa), b denotes Burgers vector magnitude (0.25 nm), and λ indicates inter-precipitate spacing (30–80 nm), yielding strengthening increments of 400–800 MPa10.

Spinodal decomposition in Al-Cu-Fe-Mn medium entropy alloys produces nanoscale compositional modulations with wavelengths of 5–15 nm, forming alternating Cu-rich (FCC, a = 0.361 nm) and Fe-Mn-rich (FCC, a = 0.358 nm) domains8. The decomposition kinetics follow the Cahn-Hilliard equation, with amplification factor R(β) = -2Mβ²[∂²f/∂c² + 2κβ²] reaching maximum at critical wavenumber β_c = (1/2κ)(∂²f/∂c²)^(1/2), where M represents atomic mobility, κ denotes gradient energy coefficient, and f indicates free energy density8. This modulated structure provides yield strengths of 470–650 MPa and elongations exceeding 36% at 298 K through coherency strain hardening8.

Refractory high-entropy amorphous alloy formation occurs in Ti-Zr-Hf-Nb-Ta systems with critical cooling rates exceeding 10⁶ K·s⁻¹, achieved through melt-spinning onto copper rollers rotating at 40–60 m·s⁻¹15. The glass-forming ability (GFA) correlates with parameter γ = T_x/(T_g + T_l), where T_x represents crystallization temperature, T_g denotes glass transition temperature, and T_l indicates liquidus temperature; γ values exceeding 0.40 indicate excellent GFA15. Amorphous ribbons exhibit yield strengths of 2.0–2.5 GPa, elastic limits of 2.0–2.5%, and fracture toughness K_IC values of 20–40 MPa·m^(1/2), suitable for corrosion-resistant applications in nuclear reactor piping15.

Grain refinement through thermomechanical processing follows the Hall-Petch relationship: σ_y = σ_0 + k_y·d^(-1/2), where σ_y represents yield strength, σ_0 denotes friction stress (200–300 MPa), k_y indicates Hall-Petch coefficient (400–600 MPa·μm^(1/2)), and d represents average grain size1112. Cold-rolling to 50–80% thickness reduction followed by annealing at 800–1250°C for 1–5 minutes produces recrystallized grain sizes of 1–10 μm, achieving yield strengths of 600–900 MPa and elongations of 30–50%11.

Thermomechanical Processing Routes And Property Optimization For Medium Entropy Alloy Refractory Modified Alloy

Manufacturing medium entropy alloy refractory modified alloy components requires integrated thermomechanical processing strategies that control phase constitution, grain size, texture, and defect density. The baseline processing route comprises: (1) vacuum arc melting or induction melting under inert atmosphere (Ar or He, purity ≥99.999%) at temperatures 100–200°C above liquidus to ensure complete dissolution; (2) casting into copper molds with cooling rates of 10²–10³ K·s⁻¹ to minimize segregation; (3) homogenization heat treatment at 1000–1200°C for 12–48 hours to eliminate dendritic microsegregation; (4) hot-working (forging, rolling, or extrusion) at 900–1100°C with strain rates of 10⁻³–10⁻¹ s⁻¹ to refine grain structure; (5) cold-working at ambient temperature to 30–80% reduction to introduce dislocation density of 10¹⁴–10¹⁵ m⁻²; (6) recrystallization annealing at 800–1250°C for 1–300 minutes to control final grain size and texture121011.

For TiZrCr refractory medium-entropy alloy spherical powder production, plasma rotating electrode process (PREP) atomization provides superior sphericity and purity compared to gas atomization6. The process parameters include: electrode rotation speed 15,000–25,000 rpm, plasma arc current 200–400 A, plasma gas (Ar or He) flow rate 30–60 L·min⁻¹, and chamber pressure 50–200 Pa6. The resulting powder exhibits: particle size distribution d₁₀ = 20 μm, d₅₀ = 80 μm, d₉₀ = 150 μm; sphericity index (4πA/P²) > 0.95 where A represents particle projected area and P denotes perimeter; oxygen content < 500 ppm; and absence of satellite particles and internal porosity, making it suitable for laser powder bed fusion (L-PBF) additive manufacturing6.

Additive manufacturing of medium entropy alloy refractory modified alloy via L-PBF requires optimized process windows: laser power 200–400 W, scanning speed 800–1400 mm·s⁻¹, hatch spacing 80–120 μm, layer thickness 30–50 μm, and volumetric energy density E_v = P/(v·h·t) = 40–80 J·mm⁻³, where P represents laser power, v denotes scanning speed, h indicates hatch spacing, and t represents layer thickness6. These parameters produce relative densities exceeding 99.5%, with residual porosity < 0.5 vol% consisting primarily of spherical gas pores (diameter 5–20 μm) rather than lack-of-fusion defects6.

Cryogenic treatment protocols enhance mechanical properties through increased martensite transformation and dislocation multiplication. The optimized cryogenic processing route involves: (1) solution treatment at 1000–1100°C for 1 hour followed by water quenching to retain metastable FCC phase; (2) immersion in liquid nitrogen (77 K) for 1–24 hours to induce partial FCC→BCC/HCP transformation (10–30 vol% martensite); (3) tempering at 200–400°C for 1–4 hours to relieve residual stresses and precipitate nanoscale carbides/nitrides571619. This treatment sequence increases yield strength by 200–400 MPa and ultimate tensile strength by 150–300 MPa while maintaining elongation above 40% at 77 K719.

Surface modification techniques improve wear resistance and corrosion resistance of medium entropy alloy refractory modified alloy components:

• Nitriding treatment: Gas nitriding at 500–600°C for 10–50 hours in NH₃/H₂ atmosphere produces nitrogen-enriched surface layers (depth 50–200 μm) with hardness 800–1200 HV, compared to 300–500 HV for substrate20
• Laser surface melting: Pulsed laser treatment (wavelength 1064 nm, pulse duration 5–20 ms, energy density 50–150 J·cm⁻²) refines surface grain size to 0.5–2 μm and increases surface hardness by 150–300 HV11
• Physical vapor deposition (PVD): Magnetron sputtering deposits 1–5 μm thick coatings with hardness 1500–2500 HV and friction coefficients 0.15–0.30 against steel counter