Amorphous Alloy Oxidation Resistant Alloy: Comprehensive Analysis Of Composition, Properties, And Industrial Applications

Fundamental Composition And Structural Characteristics Of Amorphous Alloy Oxidation Resistant Alloys

Amorphous alloy oxidation resistant alloys are distinguished by their non-crystalline atomic arrangement, which eliminates grain boundaries—the primary sites for corrosion initiation and oxidation penetration in conventional crystalline alloys 34. The absence of long-range atomic order provides homogeneous chemical distribution and suppresses preferential attack pathways.

Core Alloying Systems And Compositional Design Principles

The development of oxidation-resistant amorphous alloys relies on strategic selection of base metals and alloying elements that simultaneously promote glass-forming ability (GFA) and establish protective oxide layers:

• Fe-Based Systems: Iron-based amorphous alloys for oxidation resistance typically contain 18.5–22.5 wt% Cr, 16–20 wt% Mo, 3–6 wt% B, and 0.5–4 wt% C, with the balance being Fe 13. The high chromium content (≥18.5 wt%) enables formation of dense Cr₂O₃ passive films, while molybdenum (16–20 wt%) enhances pitting resistance and high-temperature stability 513. Phosphorus (8–15 atom%) and carbon (5–9 atom%) serve as glass-forming elements that suppress crystallization during rapid solidification 67.

• Ni-Based Systems: Nickel-based amorphous alloys achieve oxidation resistance through compositions containing ≥66 atom% Ni combined with 5–25 atom% B as metalloid glass-former 34. Advanced Ni-based formulations incorporate 7–11 atom% Ta, 6–23 atom% Nb, 6–23 atom% Zr, and 1–5 atom% Mo 20. Tantalum and niobium act as refractory elements that increase viscosity of the supercooled liquid and form stable oxide barriers (Ta₂O₅, Nb₂O₅) at elevated temperatures 20.

• Zr-Based Systems: Zirconium-based oxidation-resistant amorphous alloys are formulated as Zr_a Cu_b Ni_c M_d N_e Re_f, where 50≤a≤70, 5≤b≤20, 5≤c≤20, with M representing one or more of Hf, Ti, Nb, V, Mo, W, Ta, Ag (0≤d≤15), N representing Al, B, Ca, Mg, Be (0≤e≤15), and Re representing rare earth elements (0≤f≤2) 2. The high zirconium content (50–70 atom%) provides excellent GFA, while copper and nickel optimize the supercooled liquid region (ΔTₓ = Tₓ - Tg, where Tₓ is crystallization temperature and Tg is glass transition temperature) to exceed 60 K 212.

• Cu-Based Antibacterial Systems: Copper-based amorphous alloys combine oxidation resistance with antimicrobial properties through compositions containing 15–85 atom% Ta and/or Nb with balance Cu, or formulations with ≥1 atom% Ta, Ti, and/or Zr totaling 15–85 atom% 16. These alloys resist oxidation and discoloration while maintaining antibacterial efficacy through controlled copper ion release 16.

Glass-Forming Ability And Critical Cooling Rate Requirements

The formation of amorphous phases requires cooling rates sufficient to bypass crystallization. Critical cooling rates (Rc) for oxidation-resistant amorphous alloys vary by system:

• Fe-based alloys with optimized Cr-Mo-P-C compositions achieve amorphous formation at cooling rates of 10⁵–10⁶ K/s, enabling production of ribbons up to 50 μm thickness via melt-spinning 513.
• Ni-based alloys containing Ta-Nb-Zr require cooling rates of 10⁴–10⁵ K/s, allowing thicker sections (100–200 μm) through copper mold casting 20.
• Zr-based systems with optimized Cu-Ni ratios exhibit exceptional GFA with critical cooling rates as low as 10²–10³ K/s, permitting bulk amorphous alloy formation in sections exceeding 5 mm diameter 212.

The supercooled liquid region (ΔTₓ) serves as a quantitative indicator of GFA and thermal stability. High-performance oxidation-resistant amorphous alloys exhibit ΔTₓ ≥ 30 K, with advanced Zr-based compositions achieving ΔTₓ > 60 K 2612.

Oxidation Resistance Mechanisms And High-Temperature Performance

Passive Film Formation And Composition

The superior oxidation resistance of amorphous alloys derives from rapid formation of dense, adherent oxide layers that inhibit further oxidative attack. The chemical homogeneity of the amorphous structure ensures uniform oxide nucleation and growth, eliminating the compositional heterogeneities present at grain boundaries in crystalline alloys 18.

Chromium-Rich Passive Films: In Fe-based and Ni-based systems containing ≥10 atom% Cr, exposure to oxidizing environments at 400–800°C produces continuous Cr₂O₃ layers with thickness 5–20 nm within the first hour of exposure 1613. The amorphous substrate provides rapid chromium diffusion (diffusion coefficient D ≈ 10⁻¹⁴ cm²/s at 600°C, approximately 10× faster than in crystalline stainless steels) enabling self-healing of oxide defects 6.

Molybdenum-Enhanced Passivity: Molybdenum additions (5–20 wt%) in Fe-based amorphous alloys create mixed Cr-Mo oxide films with enhanced resistance to chloride-induced pitting 1513. Electrochemical impedance spectroscopy (EIS) measurements on Fe₆₅Cr₁₈Mo₁₆B₁ amorphous alloy in 3.5% NaCl solution at 80°C reveal passive film resistance Rₚ > 10⁶ Ω·cm², compared to Rₚ ≈ 10⁵ Ω·cm² for crystalline 316L stainless steel under identical conditions 13.

Tantalum And Niobium Oxide Barriers: Ni-based amorphous alloys containing Ta (7–11 atom%) and Nb (6–23 atom%) develop stratified oxide structures with outer NiO layer (10–30 nm) and inner Ta₂O₅/Nb₂O₅ barrier layer (5–15 nm) upon oxidation at 700–900°C 20. The refractory oxide barrier exhibits oxygen diffusion coefficient D_O ≈ 10⁻¹⁶ cm²/s at 800°C, providing long-term oxidation resistance with parabolic rate constant kₚ < 10⁻¹² g²·cm⁻⁴·s⁻¹ 20.

High-Temperature Oxidation Kinetics And Weight Gain Analysis

Quantitative oxidation resistance is assessed through isothermal oxidation testing with continuous weight gain measurement. Representative performance data include:

• Fe-Based Amorphous Alloys: Fe₆₈Cr₁₉Mo₁₆P₁₀C₃B₄ amorphous ribbon exhibits weight gain of 0.15 mg/cm² after 100 hours exposure at 600°C in air, following parabolic kinetics with kₚ = 2.3 × 10⁻¹³ g²·cm⁻⁴·s⁻¹ 13. Comparative crystalline Fe-Cr-Mo alloy shows weight gain of 0.85 mg/cm² under identical conditions (kₚ = 7.2 × 10⁻¹² g²·cm⁻⁴·s⁻¹), demonstrating 30-fold improvement in oxidation resistance 13.

• Ni-Based Amorphous Alloys: Ni₇₀Ta₉Nb₁₅Zr₄Mo₂ amorphous alloy maintains weight gain below 0.08 mg/cm² after 200 hours at 800°C in air, with no evidence of breakaway oxidation 20. The protective oxide scale remains adherent through thermal cycling (25°C ↔ 800°C, 50 cycles), indicating excellent thermal expansion compatibility 20.

• Zr-Based Amorphous Alloys: Zr₆₅Cu₁₅Ni₁₀Al₇Co₃ bulk amorphous alloy exhibits oxidation resistance superior to crystalline Zr alloys at temperatures below 400°C, with weight gain of 0.05 mg/cm² after 500 hours at 350°C 212. Above 450°C, crystallization of the amorphous phase occurs, necessitating protective coatings for higher temperature applications 12.

Corrosion Resistance In Aggressive Chemical Environments

Beyond oxidation resistance, amorphous alloys demonstrate exceptional corrosion resistance in acidic, alkaline, and chloride-containing media due to their chemical homogeneity and rapid passive film formation 1346.

Phosphoric Acid Resistance: Ta-containing amorphous alloys (10–40 atom% Ta with Mo, Cr, W, P, B, Si) exhibit corrosion rates below 0.01 mm/year in concentrated phosphoric acid (85 wt% H₃PO₄) at 150°C, making them suitable for phosphoric acid fuel cell (PAFC) bipolar plates and chemical processing equipment 1. The tantalum-rich passive film (Ta₂O₅) provides exceptional stability in acidic environments with pH < 1 1.

Chloride Pitting Resistance: Ni-based amorphous alloys containing ≥66 atom% Ni with Mo and Nb additions demonstrate pitting potential Eₚᵢₜ > +800 mV vs. saturated calomel electrode (SCE) in 3.5% NaCl solution at 25°C, compared to Eₚᵢₜ ≈ +200 mV for conventional Ni-Cr-Mo crystalline alloys 34. The absence of grain boundaries eliminates preferential pitting sites, while molybdenum enrichment in the passive film enhances chloride resistance 3.

Alkaline Stability: Fe-based amorphous alloys with high chromium content (18–22 wt%) maintain passive behavior in 1 M NaOH solution at 80°C with corrosion current density iₒᵣᵣ < 1 μA/cm², indicating excellent alkaline resistance for applications in chemical processing and waste treatment 13.

Manufacturing Processes And Powder Production Techniques For Oxidation Resistant Amorphous Alloys

Rapid Solidification Methods For Ribbon And Powder Production

The production of oxidation-resistant amorphous alloys requires rapid solidification techniques that achieve cooling rates exceeding the critical value for glass formation:

Melt-Spinning For Ribbon Production: Continuous amorphous ribbons (20–50 μm thickness, 1–10 mm width) are produced by ejecting molten alloy onto a rapidly rotating copper wheel (surface velocity 20–40 m/s) 513. For Fe-based oxidation-resistant alloys, melt temperature is maintained at 1350–1450°C, with ejection pressure 0.3–0.5 bar and wheel surface temperature controlled at 50–100°C to achieve cooling rates of 10⁵–10⁶ K/s 13. The resulting ribbons exhibit >90 vol% amorphous phase as confirmed by X-ray diffraction (XRD) showing broad diffuse maxima at 2θ ≈ 42–45° (Cu Kα radiation) without sharp crystalline peaks 613.

Gas Atomization For Spherical Powder: Oxidation-resistant amorphous alloy powders are produced via gas atomization, where molten alloy is disintegrated by high-velocity inert gas jets (N₂ or Ar at 3–6 MPa) 517. For Fe-based compositions (Fe-Cr-Mo-B-C-P), melt superheat of 100–200°C above liquidus temperature (Tₗ ≈ 1150°C) is employed, with atomization gas flow rate 0.8–1.2 kg/s per kg/min of melt flow 5. Particle size distribution is controlled through gas pressure and nozzle geometry, yielding powders with d₅₀ = 15–45 μm and >85% amorphous content in particles <50 μm diameter 5.

Centrifugal Atomization For Flake Powder: Flake-shaped amorphous alloy powders for corrosion-resistant coatings are produced by centrifugal atomization combined with rapid cooling 17. Molten alloy is atomized by gas jets to form droplets, which are then impacted onto a rotating umbrella-shaped cooling body (rotation speed 3000–8000 rpm, surface temperature 100–200°C) 17. The droplets flatten upon impact and solidify rapidly, producing flake powders with thickness 0.5–5 μm, short diameter 5–500 μm, and aspect ratio (length/thickness) of 5–100 1017. Ni-based (Ni-Cr-Mo-P-C) and Fe-based (Fe-Cr-Mo-P-C) flake powders produced by this method exhibit >95% amorphous content and are suitable for incorporation into corrosion-resistant paints and thermal spray coatings 1017.

Thermal Spray Coating Deposition Techniques

Oxidation-resistant amorphous alloy coatings are applied to metallic substrates through thermal spray processes that maintain the amorphous structure during deposition:

High-Velocity Oxygen Fuel (HVOF) Spraying: HVOF spraying of Fe-based amorphous alloy powders (Fe-Cr-Mo-P-C-B) is conducted with oxygen flow rate 900–1100 L/min, fuel (propylene or propane) flow rate 120–150 L/min, and powder feed rate 40–80 g/min 13. Substrate temperature is maintained below 150°C through compressed air cooling to prevent crystallization. The resulting coatings exhibit thickness 200–500 μm, porosity <2%, and amorphous content >80% as confirmed by XRD and differential scanning calorimetry (DSC) 13. Coating hardness reaches 850–1050 HV₀.₃, with adhesion strength >50 MPa measured by ASTM C633 tensile test 13.

Plasma Spraying With Controlled Parameters: Atmospheric plasma spraying (APS) of Ni-based amorphous alloy powders (Ni-Ta-Nb-Zr-Mo) requires careful parameter optimization to balance particle melting and minimize substrate heating 20. Typical parameters include plasma power 35–45 kW, primary gas (Ar) flow 40–50 L/min, secondary gas (H₂) flow 8–12 L/min, spray distance 100–120 mm, and powder feed rate 30–50 g/min 20. Substrate preheating to 200–250°C improves coating adhesion, while post-spray cooling rate >100 K/s preserves amorphous structure 20.

Bulk Amorphous Alloy Casting For Structural Components

Zr-based oxidation-resistant amorphous alloys with exceptional GFA can be cast into bulk forms for structural applications:

Copper Mold Casting: Bulk amorphous alloy rods and plates are produced by tilt-casting molten Zr-Cu-Ni-Al alloy