Copper-Based Amorphous Alloy: Composition Design, Glass-Forming Ability, And Advanced Applications In Engineering Systems

Fundamental Composition And Glass-Forming Ability Of Copper-Based Amorphous Alloys

The design of copper-based amorphous alloys hinges on achieving sufficient glass-forming ability (GFA) to suppress crystallization during cooling from the melt. Early Cu-based systems were limited to thin ribbons or powders due to high critical cooling rates (>10⁵ K/s). Modern formulations leverage multi-component alloying to reduce the critical cooling rate and enable bulk casting. A representative Cu-base amorphous alloy composition exhibits a reduced glass transition temperature (Tg/Tl) ≥0.56 and a wide supercooled liquid region (ΔTx = Tx - Tg), facilitating thermoplastic forming into rods or plates with diameters exceeding 1 mm via metal mold casting 1. The alloy's high GFA stems from atomic size mismatch, negative heats of mixing, and the formation of dense random packing structures that kinetically hinder nucleation.

Key compositional strategies include:

• Ternary Cu-Zr-Ti systems: Zirconium (40–70 at.%) and titanium (5–15 at.%) additions create deep eutectic points and suppress competing intermetallic phases. For example, Zr₄₀Al₁₀Cu₃₀Ni₁₀Be₅Sn₂ compositions achieve critical casting diameters of 10 mm with compressive strengths up to 4295 MPa 14.
• Quaternary and quinary additions: Elements such as Al (5–30 at.%), Ni (5–15 at.%), and minor additions of Sn (0.2–4 at.%) or rare-earth elements (e.g., Tm ≤6 at.%) further stabilize the amorphous phase. The addition of Tm modifies atomic pair distributions and enhances thermal stability, enabling centimeter-scale bulk amorphous alloys via novel copper mold casting under natural water cooling (cooling rate ~10² K/s) 9.
• Hf-Cu-based systems: Hafnium-copper alloys with compositions (Hf-Cu)₁₀₀₋ₐ₋ᵦ₋꜀₋ᵈZrₐAgᵦAl꜀Beᵈ (where a = 5–25 at.%, b = 5–20 at.%, c = 2–20 at.%, d < 10 at.%) satisfy empirical bulk glass-forming criteria and exhibit enhanced oxidation resistance 6.
• Noble-metal-doped alloys: Pt-Cu-P (50 ≤ Pt ≤ 75 at.%, 5 ≤ Cu ≤ 35 at.%, 15 ≤ P ≤ 25 at.%) and Pt-Pd-Cu-P quaternary systems form amorphous phases at moderate cooling rates (10⁻¹ to 10² K/s), offering high corrosion resistance and catalytic activity 3.

The atomic-scale origin of GFA in Cu-based amorphous alloys involves:

1. Topological frustration: Large atomic size ratios (e.g., Zr/Cu ≈ 1.3) prevent efficient crystal packing.
2. Chemical short-range order: Preferential Cu-Zr and Cu-Ti bonding (negative enthalpy of mixing: ΔHmix ≈ -23 kJ/mol for Cu-Zr) stabilizes icosahedral clusters.
3. Kinetic barriers: High viscosity in the supercooled liquid (η > 10⁶ Pa·s near Tg) retards atomic diffusion required for crystallization.

Quantitative GFA metrics include the γ parameter (γ = Tx/(Tg + Tl)), with values >0.4 indicating excellent bulk glass-forming ability 11. For Cu₆₀Zr₃₀Ti₁₀, γ ≈ 0.42 and ΔTx ≈ 60 K, enabling casting into 5 mm diameter rods 1.

Mechanical Properties And Deformation Mechanisms Of Copper-Based Amorphous Alloys

Copper-based amorphous alloys exhibit mechanical properties that significantly exceed those of conventional crystalline copper alloys, driven by the absence of dislocations and grain boundaries. Key performance metrics include:

• Compressive strength: 1800–4295 MPa, depending on composition. Fe₄₄₋ₓCo₆Cr₁₅Mo₁₄C₁₅B₆Tmₓ (x ≤ 6) achieves 4295 MPa at x = 3, attributed to solid-solution strengthening by rare-earth atoms 9.
• Vickers hardness: 600–1220 HV. High hardness arises from dense atomic packing (atomic packing fraction ~0.72) and the lack of soft grain boundaries 9.
• Elastic limit: 2–2.5%, approximately twice that of crystalline Cu alloys (εelastic ≈ 1%). The extended elastic regime enables energy absorption in impact applications 1.
• Fracture toughness: 10–50 MPa·m^(1/2), lower than ductile crystalline alloys but improvable via composite strategies (e.g., dendrite phase dispersion) 15.

Deformation in amorphous alloys occurs via shear band formation rather than dislocation glide. Under uniaxial loading, localized shear bands (thickness ~10–20 nm) nucleate at stress concentrations and propagate catastrophically, leading to brittle fracture. Strategies to enhance ductility include:

1. Nanocrystal dispersion: Semi-solid die-casting at 810–850°C introduces 5–8 vol.% nanocrystals (grain size ~50 nm) into the amorphous matrix. Dendrite phases arrest shear band propagation and induce multiple shear bands, improving plastic strain to ~3% 15.
2. Compositional tuning: Adding Sn (0.2–4 at.%) and Ti (1–5 at.%) promotes free volume and enhances plasticity by facilitating shear transformation zones 14.
3. Thermal treatments: Controlled annealing below Tg (e.g., 0.9Tg for 1 h) relaxes residual stresses and homogenizes the structure, reducing brittleness 11.

The yield strength (σy) of Cu-based amorphous alloys scales with the shear modulus (G) and Poisson's ratio (ν) according to σy ≈ 0.0267G/(1 - ν). For Cu₅₀Zr₅₀, G ≈ 30 GPa and ν ≈ 0.37, predicting σy ≈ 1.3 GPa, consistent with experimental values 1.

Synthesis And Processing Routes For Copper-Based Amorphous Alloys

Rapid Solidification Techniques

Traditional methods for producing Cu-based amorphous alloys rely on rapid quenching to bypass crystallization:

• Melt spinning: Molten alloy is ejected onto a rotating copper wheel (tangential velocity 20–40 m/s), achieving cooling rates of 10⁵–10⁶ K/s. This produces ribbons (thickness 20–50 μm, width 1–10 mm) suitable for magnetic cores and coatings 2.
• Gas atomization: Inert gas (Ar or N₂) disintegrates a molten stream into droplets (diameter 10–100 μm), which solidify in flight. Powder yields are high (>80%), but amorphous fraction depends on particle size (smaller particles cool faster) 2.
• Suction casting: Molten alloy is drawn into a water-cooled copper mold under vacuum, producing rods (diameter 1–10 mm). Critical cooling rates are ~10²–10³ K/s, sufficient for high-GFA compositions 1.

Novel Copper Mold Casting Method

A breakthrough approach eliminates the need for suction molds by directly cooling a copper mold with circulating water under negative pressure during arc melting 9. The process involves:

1. Alloy preparation: High-purity elemental ingots (Cu, Zr, Ti, rare earths) are arc-melted under Ar atmosphere (pressure ~0.05 MPa) to ensure homogeneity.
2. Mold design: A cylindrical copper mold (inner diameter 5–20 mm, wall thickness 5 mm) is connected to a water reservoir. Negative pressure (−0.02 MPa) enhances water flow and heat extraction.
3. Casting: The molten alloy (temperature ~1200°C) is poured into the mold. Water cooling achieves effective cooling rates of 50–200 K/s, sufficient for Fe-based and Cu-Zr-based systems with γ > 0.4.
4. Post-processing: Surface impurities are removed via mechanical polishing or chemical etching (e.g., 10% HNO₃ solution for 30 s).

This method produces bulk amorphous ingots (maximum diameter 16.52 mm for Fe₄₄Co₆Cr₁₅Mo₁₄C₁₅B₆Tm₃) at reduced cost (~30% lower than suction casting) and without diameter limitations imposed by mold availability 9. The technique is scalable for industrial production of structural components.

Semi-Solid Die-Casting For Composite Amorphous Alloys

To overcome brittleness, semi-solid die-casting introduces controlled crystallization 15:

1. Smelting: Master alloy is melted in a vacuum die-casting machine at 950°C.
2. Cooling to semi-solid state: Temperature is reduced to 810–850°C, where 20–30 vol.% of the melt solidifies as primary crystals (dendrites).
3. Die-casting: The semi-solid slurry is injected into a steel mold (pressure 50–100 MPa), forming a composite with 5–8 vol.% nanocrystals (size 30–80 nm) embedded in an amorphous matrix.
4. Characterization: X-ray diffraction confirms amorphous halo with weak crystalline peaks; transmission electron microscopy reveals dendrite morphology.

The resulting composite exhibits fracture toughness of 35 MPa·m^(1/2) (vs. 15 MPa·m^(1/2) for fully amorphous alloy) and compressive plastic strain of 3.2% 15.

Corrosion Resistance And Antibacterial Properties Of Copper-Based Amorphous Alloys

The absence of grain boundaries and compositional segregation in amorphous alloys confers superior corrosion resistance compared to crystalline counterparts. Cu-based amorphous alloys exhibit passive film formation in aggressive environments:

• Seawater corrosion: Cu₆₀Ti₂₀Ni₁₀Sn₅Si₃Al₂ amorphous coatings (thickness 50–200 μm) deposited via thermal spraying on nickel-aluminum bronze substrates reduce corrosion current density from 8.5 μA/cm² (uncoated) to 0.3 μA/cm² in 3.5 wt.% NaCl solution (pH 8.2, 25°C) after 720 h immersion 5. The passive film (thickness ~5 nm) consists of Cu₂O and TiO₂, confirmed by X-ray photoelectron spectroscopy.
• Acidic environments: Cu-Ta-Ti-Zr amorphous alloys (15 ≤ Ta+Ti+Zr ≤ 85 at.%) maintain corrosion rates <0.01 mm/year in 1 M H₂SO₄ at 60°C, attributed to Ta₂O₅ enrichment in the passive layer 7.
• Oxidation resistance: Hf-Cu-Zr-Ag-Al-Be alloys form a protective HfO₂-rich scale (thickness ~2 μm) at 400°C in air, preventing further oxidation for >1000 h 6.

Antibacterial functionality arises from sustained Cu²⁺ ion release. Cu-Ta-Nb-Ti amorphous coatings (30 ≤ Ta+Nb+Ti ≤ 62.5 at.%) on non-woven fabrics exhibit >99.9% bacterial reduction (Staphylococcus aureus, Escherichia coli) after 24 h contact, meeting JIS Z 2801 standards 7. The mechanism involves:

1. Ion release: Cu²⁺ concentration in saline extract reaches 2–5 ppm after 24 h (measured by inductively coupled plasma optical emission spectrometry).
2. Membrane disruption: Cu²⁺ binds to bacterial cell membranes, increasing permeability and causing cytoplasmic leakage.
3. Reactive oxygen species (ROS) generation: Cu²⁺ catalyzes Fenton-like reactions, producing hydroxyl radicals that damage DNA and proteins.

Applications include antimicrobial insoles (Cu-Ta-Nb-Ti coating on polyvinylidene chloride layers) and hospital touch surfaces 7.

Applications Of Copper-Based Amorphous Alloys In Marine Engineering

Marine environments impose severe erosion-corrosion challenges on copper-based components (pumps, propellers, propulsion shafts, ball valves). Conventional nickel-aluminum bronze (NAB) alloys suffer from dealloying and cavitation damage at high flow velocities (>10 m/s). Copper-based amorphous alloy coatings address these issues:

Erosion-Corrosion Resistance

Cu₆₀Ti₂₀Ni₁₀Sn₅Si₃Al₂ amorphous coatings (deposited via high-velocity oxy-fuel spraying at 2800°C, particle velocity 600 m/s) on NAB substrates demonstrate:

• Mass loss reduction: 0.8 mg/cm² (amorphous-coated) vs. 12.5 mg/cm² (uncoated NAB) after 72 h in sand-laden seawater (sand concentration 5 g/L, flow velocity 15 m/s, 25°C) 5.
• Hardness retention: Coating hardness remains 680 HV after erosion testing, compared to 420 HV for NAB (initial hardness 180 HV).
• Mechanism: The amorphous structure lacks preferential attack sites (grain boundaries, second phases), and the high hardness resists particle impingement. Sn and Si additions enhance passivation kinetics 5.

Coating Process Optimization

Key parameters for thermal spray deposition include:

1. Powder preparation: Gas-atomized Cu-Ti-Ni-Sn-Si-Al powder (particle size 15–45 μm, amorphous fraction >95% confirmed by differential scanning calorimetry).
2. Substrate preparation: Grit blasting with Al₂O₃ (mesh 24) to roughness Ra = 8–12 μm, followed by acetone cleaning.
3. Spray conditions: Oxygen flow 900 L/min, fuel (kerosene) flow 25 L/min, spray distance 300 mm, traverse speed 500 mm/s.
4. Post-treatment: Sealing with epoxy resin (thickness 20 μm) to eliminate interconnected porosity (<2