Bulk Metallic Glass Billet: Advanced Manufacturing, Structural Properties, And Industrial Applications

Fundamental Characteristics And Glass-Forming Ability Of Bulk Metallic Glass Billets

Bulk metallic glass billets are distinguished by their ability to maintain an amorphous atomic structure throughout cross-sections significantly larger than conventional metallic glasses. The critical thickness—defined as the maximum dimension achievable while avoiding crystallization during cooling—has historically limited BMG production to sub-millimeter scales, requiring cooling rates up to 1000°C/sec 1. However, advances in alloy design and processing have enabled billet production with diameters exceeding 5 mm and, in optimized systems, reaching several centimeters 11.

The glass-forming ability (GFA) of billet-scale BMGs depends on three interrelated factors: (1) the alloy's critical cooling rate, (2) the thermal conductivity of the mold system, and (3) the suppression of heterogeneous nucleation sites. Zirconium-rich bulk metallic glass alloys, such as Zr₅₈.₄₇Nb₂.₇₆Cu₁₅.₄Ni₁₂.₆Al₁₀.₃₇, demonstrate exceptional GFA by stabilizing the supercooled liquid region through fractional compositional tuning 10. These alloys exhibit glass transition temperatures (Tg) in the range of 350–450°C and supercooled liquid regions (ΔTx = Tx - Tg) exceeding 50°C, providing a robust processing window for billet consolidation 11,16.

Beryllium-containing BMG billets, such as those in the Zr-Ti-Cu-Ni-Be family, historically achieved superior GFA but faced regulatory and toxicity concerns. Recent innovations have reduced beryllium content to 0.06–0.08 wt%, maintaining liquidus temperature suppression while improving environmental compliance 3. Alternative beryllium-free systems, including Ti-Zr-Cu-Ni-Al-Si-Hf alloys, now achieve comparable billet dimensions through microalloying strategies that neutralize oxygen and other impurities 17,18.

The mechanical properties of BMG billets are exceptional: fracture strengths reach 1.5–2.0 GPa (double that of crystalline counterparts), elastic strain limits approach 2%, and Young's moduli range from 80–120 GPa depending on composition 4,11. However, billets exhibit brittle fracture behavior under tensile loading due to highly localized shear banding, a challenge addressed through composite reinforcement strategies discussed in subsequent sections 4,6.

Manufacturing Processes For Large-Scale Bulk Metallic Glass Billets

Tilt-Casting And Pressure-Assisted Cooling (CAP Method)

The CAP (Cooling-Accelerated Pressure) casting method represents a breakthrough in producing large-diameter BMG billets 2. This technique involves melting the alloy in an open-top furnace, tilting the furnace floor to inject molten metal into a water-cooled copper mold, and simultaneously applying pressure via an upper punch covering the melt surface within the mold cavity 2. The combined effects of forced convection cooling and pressure suppression of void formation enable billet diameters exceeding the critical thickness predicted by conventional quenching models 2.

Key process parameters for CAP casting include: (1) melt superheat of 50–100°C above liquidus to ensure fluidity, (2) tilt rates of 10–30°/sec to minimize turbulence-induced oxide entrapment, (3) mold pre-cooling to 10–50°C to maximize heat extraction, and (4) applied pressures of 10–50 MPa during solidification 2. For Zr-based billets, this method achieves cooling rates of 50–200°C/sec in sections up to 30 mm diameter, sufficient to bypass the nose of the time-temperature-transformation (TTT) curve 2,11.

Powder Consolidation And Hot-Pressing Routes

An alternative approach to billet production involves consolidating rapidly solidified BMG powders or ribbons through hot-pressing in the supercooled liquid region 8,13. This method begins with gas atomization or melt-spinning to produce amorphous precursor particles (10–100 μm diameter) or ribbons (20–50 μm thickness) 8,13. The precursors are packed into a graphite die, evacuated to <10⁻³ Pa to prevent oxidation, and heated to Tg + 20–50°C under uniaxial pressures of 100–500 MPa 8,13.

At these conditions, the BMG exhibits viscosities of 10⁶–10⁹ Pa·s, enabling viscous flow consolidation without crystallization 13. Holding times of 5–30 minutes allow complete densification and interfacial bonding, yielding billets with >99.5% theoretical density and mechanical properties equivalent to monolithic castings 8,13. The Fe₅₈Cr₁₄Cu₆Si₆B₆ alloy, consolidated via this route, demonstrates excellent seawater corrosion resistance and has been scaled to billet dimensions of 50 mm diameter × 100 mm length 8.

Near-Net-Shape Casting With Thermoplastic Forming Integration

For applications requiring complex geometries, near-net-shape casting of BMG billets followed by thermoplastic forming offers design flexibility 9. This hybrid process involves: (1) casting a billet with approximate final dimensions using multi-axis molds with varying cross-sectional profiles, (2) cooling at rates sufficient to maintain amorphicity (typically 10–100°C/sec for optimized alloys), and (3) reheating the billet to Tg + 30–80°C for blow-molding, forging, or extrusion into final shapes 9.

The thermoplastic forming step exploits the supercooled liquid's Newtonian flow behavior, with strain rates of 10⁻³–10⁻¹ sec⁻¹ achievable under pressures of 1–10 MPa 9. Materion Corporation's Zr-based BMG billets, processed via this route, achieve dimensional tolerances of ±0.05 mm and surface roughness <0.2 μm Ra without post-machining 9. Critical process controls include: (1) heating rates <10°C/sec to avoid crystallization during ramp-up, (2) isothermal hold times <5 minutes to minimize devitrification, and (3) quenching rates >20°C/sec post-forming to lock in the amorphous structure 9.

Composite Billet Architectures For Enhanced Ductility And Functionality

BMG/Metal Co-Deformation Composites

To overcome the brittleness limitation of monolithic BMG billets, co-deformation with ductile crystalline metals produces composite billets with combined high strength and ductility 6. This process involves stacking alternating layers of BMG foil (50–200 μm thick) and crystalline metal foil (e.g., stainless steel, titanium, or copper; 100–500 μm thick), heating the stack to Tg + 20–50°C, and applying compressive deformation of 30–70% strain at rates of 10⁻⁴–10⁻² sec⁻¹ 6.

The resulting composite billets exhibit: (1) tensile ductility of 5–15% (versus <2% for monolithic BMG), (2) fracture toughness (KIC) of 50–100 MPa·m^(1/2) (versus 20–40 MPa·m^(1/2) for monolithic BMG), and (3) electrical conductivity 10–50% of pure copper when using copper interlayers 6. The ductile phase arrests shear band propagation through crack bridging and deflection mechanisms, while the BMG phase provides load-bearing capacity 6. Composite billets with 30–50 vol% BMG content achieve optimal property balance for structural applications 6.

BMG/Graphite And BMG/Ceramic Particulate Composites

Embedding graphite flakes or ceramic particles (SiC, Al₂O₃, ZrO₂) within BMG billets enhances wear resistance and thermal stability 4. Graphite-reinforced BMG composites are produced by: (1) dispersing 5–20 vol% graphite flakes (10–50 μm diameter, 1–5 μm thickness) in molten BMG alloy using mechanical stirring or ultrasonic agitation, (2) casting the suspension into copper molds at cooling rates of 50–200°C/sec, and (3) annealing at Tg - 50°C for 1–2 hours to relieve residual stresses 4.

The graphite phase acts as a solid lubricant, reducing the coefficient of friction from 0.6–0.8 (monolithic BMG) to 0.2–0.4 (composite) under dry sliding conditions 4. Additionally, graphite flakes deflect and blunt propagating shear bands, increasing compressive ductility from <2% to 5–10% 4. Ceramic particulate composites, produced via similar routes, exhibit hardness increases of 10–30% and wear rates reduced by 50–80% compared to monolithic BMG billets 4.

Alloy Systems And Compositional Design For Billet Production

Zirconium-Based BMG Billet Alloys

Zirconium-based alloys dominate commercial BMG billet production due to their combination of high GFA, mechanical performance, and corrosion resistance 11,16. The Zr-Al-Ti-Cu-Ni quinary system, exemplified by compositions such as Zr₅₂.₅Al₁₀Ti₅Cu₁₇.₉Ni₁₄.₆, achieves critical casting thicknesses of 10–15 mm and exhibits: (1) tensile strength of 1.7–1.9 GPa, (2) elastic limit of 2.0–2.2%, (3) density of 6.5–6.8 g/cm³, and (4) glass transition temperature of 400–420°C 11.

Fractional compositional tuning—adjusting element ratios by 0.1–1.0 at%—significantly impacts GFA by altering the competition between amorphous and crystalline phase formation 10. For example, increasing niobium content from 0 to 2.76 at% in Zr-Cu-Ni-Al alloys suppresses formation of the Zr₂Cu intermetallic phase, expanding the supercooled liquid region from 40°C to 65°C and enabling billet diameters to increase from 8 mm to 20 mm 10.

Zirconium-based BMG billets demonstrate exceptional corrosion resistance in chloride environments, with pitting potentials exceeding +600 mV (vs. saturated calomel electrode) in 3.5 wt% NaCl solution—superior to 316L stainless steel (+200 mV) 11. This performance stems from the absence of grain boundaries and the formation of a passive ZrO₂ surface film 11.

Titanium-Based BMG Billet Alloys For Biomedical Applications

Titanium-based BMG billets, such as Ti₄₀Zr₁₀Cu₃₆Pd₁₄ and Ti₄₅Zr₁₀Cu₃₁Ni₈Si₆, offer biocompatibility and low elastic modulus (80–95 GPa) matching cortical bone (10–30 GPa), reducing stress-shielding effects in orthopedic implants 18. These alloys achieve critical casting thicknesses of 5–10 mm and exhibit: (1) tensile strength of 1.8–2.1 GPa, (2) corrosion current density <10 nA/cm² in simulated body fluid, and (3) cytotoxicity levels meeting ISO 10993 standards 18.

Welding of Ti-based BMG billets for implant fabrication requires pre-cooling substrates to -50°C to -100°C before laser or electron-beam welding, preventing heat-affected zone crystallization 7. Optimized welding parameters include: (1) beam power of 200–500 W, (2) scan speeds of 5–20 mm/sec, (3) defocused beam diameters of 0.5–1.5 mm, and (4) argon shielding gas flow rates of 10–20 L/min 7. Post-weld annealing at Tg - 30°C for 30 minutes relieves residual stresses while maintaining amorphicity 7.

Gold-Based BMG Billets For Luxury Applications

Gold-based BMG billets, such as Au₄₉Ag₅.₅Pd₂.₃Cu₂₆.₉Si₁₆.₃, combine the aesthetic appeal of high gold content (≥45 at%) with superior hardness (300–400 HV) and tarnish resistance 15. These alloys achieve critical casting thicknesses of 3–8 mm and exhibit: (1) density of 13–15 g/cm³, (2) elastic limit of 1.5–2.0%, (3) low casting temperatures (700–850°C), and (4) minimal shrinkage (<0.5%) enabling precision investment casting 15.

The inclusion of silicon and germanium in Au-based BMG billets suppresses surface oxidation and sulfidation, maintaining mirror-finish surfaces after 1000 hours of exposure to H₂S-containing atmospheres (10 ppm H₂S, 50% RH, 25°C)—conditions that severely tarnish conventional 18K gold alloys 15. This tarnish resistance, combined with twice the scratch resistance of crystalline gold alloys, positions Au-based BMG billets as premium materials for watch cases, jewelry, and decorative hardware 15.

Applications Of Bulk Metallic Glass Billets Across Industrial Sectors

Aerospace And Defense: High-Strength Structural Components

BMG billets serve as feedstock for aerospace fasteners, landing gear components, and missile casings where high strength-to-weight ratios and corrosion resistance are critical 11,14. Zr-based BMG billets are thermoplastically formed into high-aspect-ratio parts (length-to-diameter ratios >10:1) such as turbine blades and structural brackets 14. The net-shape forming capability eliminates 60–80% of machining operations compared to titanium alloys, reducing production costs by 30–50% 14.

For defense applications, BMG billet-derived components exhibit: (1) ballistic penetration resistance 20–40% higher than hardened steel at equivalent areal density, (2) fragment velocities upon impact 15–25% lower due to superior energy absorption, and (3) radar cross-sections reduced by 10–20 dB in certain frequency bands due to unique electromagnetic properties 14. The U.S. Department of Defense has qualified Zr₅₂.₅Al₁₀Ti₅Cu₁₇.₉Ni₁₄.₆ BMG billets for use in kinetic energy penetrators and shaped charge liners 14.

Precision Instrumentation: Coriolis Mass Flowmeter Tubes

Ti-based BMG billets are extruded into thin-walled tubes (outer diameter 3–10 mm, wall thickness 0.2–0.5 mm) for Coriolis mass flowmeters, achieving measurement sensitivities 2–3 times higher than stainless steel tubes 18. The low Young's modulus (85–95 GPa) and high elastic limit (2.0–2.2%) of Ti₄₀Zr₁₀Cu₃₆Pd₁₄ enable larger tube deflections under equivalent flow-induced forces, improving signal-to-noise ratios from 40 dB (steel) to 55 dB (BMG) 18.

Manufacturing involves: (1) casting Ti-based BMG billets (diameter 15–25