Bulk Metallic Glass Ingot: Advanced Manufacturing Processes, Compositional Design, And Industrial Applications

Fundamental Principles Of Bulk Metallic Glass Ingot Formation And Glass-Forming Ability

Bulk metallic glass ingots are produced by solidifying multicomponent metallic melts at cooling rates sufficient to suppress crystallization, thereby preserving the disordered atomic arrangement characteristic of the supercooled liquid state 1. The glass-forming ability (GFA) of an alloy—quantified by parameters such as critical cooling rate (Rc), critical casting thickness (tmax), and reduced glass transition temperature (Trg = Tg/Tl)—determines the maximum ingot dimensions achievable without crystalline phase formation 2. High-GFA compositions typically exhibit deep eutectic points with asymmetric liquidus slopes, which expand the supercooled liquid region (ΔTx = Tx − Tg) and delay nucleation kinetics during solidification 3.

The production of robust bulk metallic glass ingots requires precise control over mold-melt interfacial heat transfer to prevent premature cooling below the glass transition temperature (Tg) at the contact surface 13. Patent literature describes casting protocols wherein the mold maintains temperatures above Tg for at least 5 seconds at the melt interface, allowing stress relaxation and minimizing thermal gradient-induced cracking 13. For Zr-based bulk metallic glasses such as Zr-Cu-Al-Nb alloys (e.g., Zr₆₃.₅Cu₂₄Al₄Nb₂.₅), this approach enables ingot production with critical diameters exceeding 30–40 mm while retaining full amorphicity 316.

Key compositional strategies for enhancing GFA in bulk metallic glass ingots include:

• Multicomponent alloying: Quaternary and higher-order systems (e.g., Ti-Ni-Cu-Sn, Fe-Cr-Mo-C-B) increase configurational entropy and frustrate crystal nucleation 89.
• Refractory metal additions: Elements such as Nb, Mo, W, and Ta raise liquidus temperatures selectively, creating asymmetric phase diagrams that favor glass formation 89.
• Metalloid incorporation: B, C, Si, and P occupy interstitial sites, increasing atomic packing density and stabilizing the amorphous phase 910.
• Base-metal optimization: High concentrations of Ti (47–72 at%) or Fe (59–70 at%) reduce raw material costs while maintaining Trg > 0.6 and ΔTx > 20 K 8910.

Theoretical phase diagram calculations using CALPHAD (Calculation of Phase Diagrams) methods enable rapid screening of alloy compositions by predicting liquidus temperatures and eutectic compositions, thereby accelerating the discovery of bulk metallic glass ingot-compatible chemistries 89.

Compositional Design And Alloy Systems For Bulk Metallic Glass Ingot Production

Zirconium-Based Bulk Metallic Glass Ingots

Zirconium-rich bulk metallic glasses represent the most commercially mature ingot systems, with quinary alloys such as Zr-Cu-Al-Ni-Ti achieving critical casting thicknesses of 5–15 mm and compressive strengths exceeding 1800 MPa 1318. The composition Zr₆₃.₅Cu₂₄Al₄Nb₂.₅ (at%) exhibits a supercooled liquid region ΔTx ≈ 50 K and a reduced glass transition temperature Trg ≈ 0.62, enabling ingot production via suction casting into copper molds 313. Substitution of Nb with Ti (up to 5 at%) or partial replacement of Cu with Ni, Fe, or Co modulates elastic modulus (90–100 GPa) and fracture toughness (20–80 MPa·m^(1/2)) without compromising GFA 1318.

Zirconium-based bulk metallic glass ingots are typically produced by arc melting elemental precursors under inert atmosphere (Ar or He), followed by re-melting 3–5 times to ensure chemical homogeneity 813. The molten alloy is then suction-cast into water-cooled copper sleeves with internal diameters of 5–20 mm, achieving cooling rates of 50–100 K/s 8. X-ray diffraction (XRD) and differential scanning calorimetry (DSC) confirm amorphicity, with characteristic broad diffraction halos and single glass transition events at Tg ≈ 350–420°C 1318.

Iron-Based Bulk Metallic Glass Ingots

Iron-based bulk metallic glass ingots offer cost advantages over Zr-based systems while providing ferromagnetic properties (saturation magnetization Ms ≈ 1.2–1.5 T) and superior corrosion resistance in marine environments 91015. The composition Fe₆₈C₁₂B₃Cr₅Mo₁₀W₂ (at%) exhibits ΔTx > 50 K and can be cast into amorphous plates with minimum dimensions of 0.5 mm via chill casting 910. Refractory metal additions (Mo, W, Cr) suppress the formation of brittle intermetallic phases (e.g., Fe₃B, Fe₂₃B₆) that compromise mechanical integrity 910.

Arc melting followed by rapid solidification on copper chill plates yields Fe-based bulk metallic glass ingots with tensile strengths of 3500–4000 MPa and elastic limits of 2% 910. The alloy Fe₅₈Cr₁₄Cu₆Si₆B₆ demonstrates exceptional resistance to seawater corrosion (corrosion rate < 0.01 mm/year in 3.5 wt% NaCl solution at 25°C), making it suitable for marine hardware and offshore structural components 15.

Titanium-Based Bulk Metallic Glass Ingots

Titanium-based bulk metallic glass ingots combine low density (ρ ≈ 4.5–5.0 g/cm³) with high specific strength (σy/ρ > 400 MPa·cm³/g), positioning them for aerospace and biomedical applications 8. The composition Ti₄₇Ni₃₃Cu₁₀Si₄Sn₆ (at%) achieves critical casting thicknesses of 3–5 mm and exhibits biocompatibility comparable to Ti-6Al-4V alloy 8. Metalloid elements (Si, Sn) stabilize the amorphous phase by increasing the enthalpy of mixing and reducing atomic mobility during solidification 8.

Suction casting into copper molds at cooling rates of 100–200 K/s produces Ti-based bulk metallic glass ingots with compressive yield strengths of 2000–2200 MPa and Young's moduli of 90–110 GPa 8. Thermogravimetric analysis (TGA) reveals oxidation onset temperatures above 400°C in air, indicating thermal stability suitable for elevated-temperature processing 8.

Gold-Based And Specialty Bulk Metallic Glass Ingots

Gold-based bulk metallic glass ingots (e.g., Au-Ag-Pd-Si-Ge quaternary systems with Au > 45 at%) offer high tarnish resistance and aesthetic appeal for luxury goods and electronic contacts 12. These alloys exhibit critical casting thicknesses of 1–3 mm and maintain metallic luster without surface oxidation after prolonged exposure to ambient conditions 12. The addition of Ge and Si enhances GFA by forming strong covalent-like bonds with noble metals, increasing viscosity in the supercooled liquid region 12.

Manufacturing Processes And Quality Control For Bulk Metallic Glass Ingot Production

Arc Melting And Homogenization

The production of bulk metallic glass ingots begins with arc melting of elemental precursors in a water-cooled copper hearth under high-purity inert atmosphere (O₂ < 1 ppm, H₂O < 0.5 ppm) 8913. Typical arc currents range from 200 to 400 A, generating localized temperatures of 2000–3000°C to ensure complete dissolution of refractory metals (Nb, Mo, W) 89. The molten button is flipped and re-melted 3–5 times to eliminate compositional gradients, with mass loss during melting kept below 0.5 wt% to maintain stoichiometry 813.

Induction melting in ceramic crucibles (Al₂O₃, ZrO₂) provides an alternative for reactive alloys (Ti-based, Zr-based) but introduces contamination risks from crucible dissolution 8. Levitation melting using electromagnetic fields eliminates crucible contact entirely, producing ultra-clean melts suitable for biomedical bulk metallic glass ingots, though throughput remains limited to laboratory scale 8.

Casting Techniques For Bulk Metallic Glass Ingots

Suction Casting: The homogeneous melt is drawn into a water-cooled copper mold via vacuum suction (ΔP ≈ 0.5–0.8 bar), achieving cooling rates of 50–200 K/s depending on mold geometry and wall thickness 813. Cylindrical ingots with diameters of 5–20 mm and lengths of 50–100 mm are routinely produced, with amorphicity confirmed by XRD (absence of Bragg peaks) and DSC (single Tg event) 813.

Chill Casting: Molten alloy is poured onto a water-cooled copper plate, producing thin ingots (0.5–2 mm thickness) with cooling rates exceeding 500 K/s 910. This method is preferred for Fe-based bulk metallic glasses with lower GFA, where rapid heat extraction is critical to avoid crystallization 910.

Tilt Casting With Pressure Cooling: A hybrid approach combines tilt-pour casting with upper punch compression, enabling production of large-diameter bulk metallic glass ingots (up to 30 mm) by accelerating heat removal from the melt surface 4. The upper punch, maintained at sub-ambient temperature, contacts the melt within 1–2 seconds of pouring, increasing effective cooling rate by 30–50% compared to conventional casting 4.

Controlled Mold Temperature Casting: To prevent thermal shock cracking, the mold is preheated to temperatures slightly below Tg (typically Tg − 50 K) and held at this temperature for 5–10 seconds after melt contact 13. This protocol allows stress relaxation in the supercooled liquid state before final quenching, reducing residual tensile stresses that cause ingot fracture 13. For Zr₆₃.₅Cu₂₄Al₄Nb₂.₅, mold temperatures of 300–320°C (Tg ≈ 370°C) yield crack-free ingots with diameters up to 15 mm 3.

Defect Mitigation And Microstructural Characterization

Common defects in bulk metallic glass ingots include:

• Crystalline inclusions: Detected by XRD and transmission electron microscopy (TEM), arising from insufficient cooling rates or compositional inhomogeneity 13. Mitigation strategies include increasing mold cooling capacity and optimizing re-melting cycles 13.
• Porosity: Gas entrapment during casting creates voids (10–100 μm diameter) that act as crack initiation sites 1. Vacuum degassing of the melt (P < 10⁻³ mbar) and slow mold filling rates (< 10 cm/s) reduce porosity to < 0.1 vol% 1.
• Surface oxidation: Exposure to residual oxygen forms oxide layers (1–5 μm thickness) that degrade mechanical properties 8. Casting under ultra-high-purity Ar (O₂ < 0.1 ppm) and post-casting surface machining eliminate oxide contamination 8.

Quality assurance protocols for bulk metallic glass ingots include:

• XRD analysis: Confirms amorphicity via absence of sharp Bragg peaks; acceptable ingots show only broad halos at 2θ ≈ 35–45° (Cu Kα radiation) 813.
• DSC measurements: Single glass transition (ΔCp ≈ 0.1–0.3 J/g·K) and crystallization exotherm (ΔHx > 50 J/g) verify thermal stability 913.
• Density measurements: Archimedes method confirms absence of porosity; deviations > 0.5% from theoretical density indicate defects 13.
• Ultrasonic testing: C-scan imaging detects internal voids and crystalline regions via acoustic impedance mismatch 13.

Thermoplastic Forming And Secondary Processing Of Bulk Metallic Glass Ingots

Bulk metallic glass ingots serve as feedstock for net-shape manufacturing via thermoplastic forming (TPF) in the supercooled liquid region (Tg < T < Tx) 21416. At temperatures 20–50 K above Tg, bulk metallic glasses exhibit Newtonian viscosity (η ≈ 10⁶–10⁹ Pa·s) and can be blow-molded, embossed, or forged into complex geometries with dimensional tolerances of ±10 μm 1416.

Thermoplastic Forming Process Parameters

Typical TPF cycles for bulk metallic glass ingots involve:

1. Heating: Ingot is heated to T = Tg + 30 K (e.g., 400°C for Zr-based alloys) in inert atmosphere at rates of 10–20 K/min to avoid crystallization 1416.
2. Forming: Uniaxial compression (σ = 10–50 MPa) or blow molding (ΔP = 0.5–2 MPa) shapes the viscous ingot into the desired geometry within 30–120 seconds 1416.
3. Quenching: Rapid cooling (> 50 K/s) to T < Tg freezes the amorphous structure, with water-cooled tooling or forced air convection providing sufficient heat extraction 1416.

For Zr₆₃.₅Cu₂₄Al₄Nb₂.₅ ingots, TPF at 410°C under 30 MPa uniaxial stress produces net-shape components with surface roughness Ra < 0.5 μm and no detectable crystallization (XRD-confirmed) 14. Cycle times of 60–90 seconds enable production rates of 20–40 parts per hour, competitive with injection molding of engineering polymers 14.

Composite Fabrication Via Co-Deformation

Bulk metallic glass ingots can be co-deformed with ductile metals (Cu, Al, Ti) in the supercooled liquid region to produce laminated composites with tailored mechanical properties 7. The process involves:

• Stacking alternating layers of bulk metallic glass and metal foils (thickness ratio 1:1 to 5:1) 7.
• Heating the stack to T = Tg + 20 K and applying uniaxial compression (σ = 50–100 MPa) for 10–30 minutes 7.
• Achieving interfacial bonding via viscous flow of the bulk metallic glass phase, which infiltrates surface asperities of the metal 7.

Co-deformed Zr-based bulk metallic glass/Cu composites exhibit tensile strengths of 1200–1500 MPa (intermediate between monolithic bulk metallic glass and Cu) and elongations of 5–10% (vs. < 2% for monolithic bulk metallic glass), combining high strength with improved ductility 7. Electrical conductivity remains high (> 40% IACS) due to continuous Cu pathways