high-entropy-alloys

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)

1. High entropy effect: stabilizes single phase (FCC, BCC, or HCP solid solution)
2. Lattice distortion: large atomic size mismatch → local lattice distortion → stronger diffuse barriers to dislocation
3. Sluggish diffusion: multiple elements on each sublattice → slow diffusion → stable microstructure → good creep resistance
4. 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.