| name | high-entropy-alloys |
| description | High-entropy alloys (HEA) — definition, CALPHAD design, mechanical properties (Cantor alloy, refractory HEA), phase stability, strengthening mechanisms, applications, processing. |
| metadata | {"priority":7,"promptSignals":{"phrases":["high entropy alloy","HEA","multi-principal element alloy","Cantor alloy","CrMnFeCoNi","refractory high entropy"],"minScore":3}} |
High-Entropy Alloys — Complete Skill
Definition and Core Concept
High-entropy alloys (HEA): alloys with ≥ 5 principal elements in equimolar or near-equimolar ratios (5–35 at.%)
Alternative name: multi-principal element alloys (MPEA), compositionally complex alloys (CCA)
Entropy driving force:
ΔS_mix = -R Σ x_i ln(x_i) [per mole; R = 8.314 J/mol·K]
For n equimolar elements: ΔS_mix = R ln(n)
n=5 → ΔS_mix = 1.61 R = 13.4 J/mol·K (high entropy ≥ 1.5R)
High configurational entropy suppresses formation of intermetallic compounds → single-phase solid solution possible
Four Core Effects (Traditional Framework)
- High entropy effect: stabilizes single phase (FCC, BCC, or HCP solid solution)
- Lattice distortion: large atomic size mismatch → local lattice distortion → stronger diffuse barriers to dislocation
- Sluggish diffusion: multiple elements on each sublattice → slow diffusion → stable microstructure → good creep resistance
- Cocktail effect: non-linear combination of properties; unexpected synergies
Note: Recent research shows these effects are not universal; must be evaluated per system
Cantor Alloy (CrMnFeCoNi) — Benchmark HEA
Composition: equimolar CrMnFeCoNi; FCC structure; single phase at high T
Mechanical properties:
RT: S_y = 125–145 MPa, S_u = 335–400 MPa, El = 50–60%
77 K: S_y = 200–260 MPa, S_u = 600–750 MPa, El = 60–70% (dramatic improvement at low T)
K_IC: 200 MPa√m at RT; > 300 MPa√m at 77 K (exceptional cryogenic toughness)
Strengthening mechanisms: lattice distortion (solid solution); Hall-Petch (grain refinement)
Applications: structural at cryogenic temperature; cryogenic pressure vessels, LNG applications
Al₀.₁CoCrFeNi — Common FCC HEA
Al addition promotes BCC → dual phase or single FCC with strengthening
S_y = 200–350 MPa (Al increases with more Al)
At high Al (Al₀.₅+): BCC phase appears → higher strength, lower ductility
Refractory HEAs (RHEA)
BCC structure from refractory elements: Mo, Nb, Ta, W, Cr, V, Hf, Zr
MoNbTaW:
Density: 13.8 g/cm³ (high; contains W and Mo)
S_y at RT: > 400 MPa; S_y at 1000°C: > 400 MPa (still strong!)
Melting point: > 2500°C
Brittleness at RT: challenge; limited ductility
Alloy development goal: balance strength at high T + ductility at RT
NbMoTaW / VNbMoTaW:
RT compression: σ ≈ 1000–1500 MPa (brittle); target: alloys with both RT ductility + HT strength
CALPHAD-Guided Design
CALPHAD (CALculation of PHAse Diagrams) predicts phase stability from thermodynamic databases
Key parameters for single-phase prediction:
ΔH_mix: -15 to +5 kJ/mol (moderate mixing enthalpy → not too strong compound tendency)
δ (atomic size mismatch): δ = 100 × √(Σ x_i(1 - r_i/r̄)²) < 6.6% (small lattice distortion)
VEC (Valence Electron Concentration): VEC < 6.87 → BCC; VEC > 8.0 → FCC; 6.87–8.0 → mixed
Ω = T_m × ΔS_mix / |ΔH_mix| ≥ 1.1 (thermodynamic stability criterion)
Software: Thermo-Calc with TCHEA (HEA database); FactSage; Pandat; JMatPro
Strengthening in HEAs
Solid solution hardening: strong in HEAs due to lattice distortion; ΔTau ≈ f(misfit parameter δ)
Precipitation hardening: add L1₂ precipitates (Ni₃Al type) in FCC HEA → much higher strength
Grain refinement (Hall-Petch): S_y = S_0 + k_y × d^(-1/2); effective in HEAs
Work hardening: TRIP (transformation-induced plasticity) in some HEAs → excellent work hardening
Processing
Arc melting: laboratory scale; drop cast; prone to segregation
Powder metallurgy: ball mill + SPS (spark plasma sintering) → fine grain; uniform composition
Casting + hot rolling: industrial scale; grain refinement by deformation
Additive manufacturing (DED, SLM): emerging; non-equilibrium microstructures possible; gradient HEAs
Microstructure considerations:
Homogenization anneal: 1200°C, 24 hr required to eliminate segregation
Single-phase field confirmed by XRD (single FCC or BCC peaks); TEM/APT for composition uniformity
Applications
Cryogenic (Cantor family): LN₂, LH₂, fusion reactor components
Nuclear (radiation hardening): HEAs show reduced radiation damage; potential fusion plasma facing
High temperature (RHEAs): next-gen turbine blades (if ductility solved)
Wear-resistant coatings: HEA coatings by PVD/HVOF for cutting tools, die casting
Output
Provide: alloy system and composition (at.%), predicted structure (FCC/BCC/HCP), ΔS_mix, δ, VEC, CALPHAD-predicted phases, S_y and S_u at operating temperature [MPa], K_IC [MPa√m], density [g/cm³], processing route, comparison to conventional alternatives.
