Bulk Metallic Glass Copper-Based Alloy: Composition Design, Thermal Stability, And Advanced Manufacturing Applications

Fundamental Composition And Alloying Strategy Of Bulk Metallic Glass Copper-Based Alloy

Bulk metallic glass copper-based alloy systems are predominantly quaternary or higher-order compositions designed to maximize atomic packing efficiency and suppress crystallization kinetics during solidification 2,5. The archetypal Cu-based BMG composition follows the general formula Cu₄₇₋₍ₓ₊ᵧ₊ᵤ₎(Ti_a Zr_b)_c Ni₇₊ₓ Sn₁₊ᵧ Si_z, where c = 43–47 at%, a = 0.65–0.85, b = 0.15–0.35 (with a + b = 1.00), x = 0–7 at%, y = 0–3 at%, z = 0–3 at%, and y + z ≤ 4 at% 2,5. This composition window balances the need for deep eutectic behavior, negative heats of mixing among constituent pairs, and significant atomic size mismatch (δ > 12%) to stabilize the supercooled liquid against competing intermetallic phases 13.

The role of each element is highly specific: copper serves as the primary matrix former with intermediate electronegativity, zirconium and titanium provide large atomic radii (1.60 Å and 1.47 Å respectively) to frustrate crystalline packing, nickel enhances glass-forming ability through strong Cu–Ni interactions (ΔH_mix ≈ −4 kJ/mol), and aluminum reduces density while widening the supercooled liquid region (ΔT_x = T_x − T_g) 7,13. Minor additions of tin (Sn) and silicon (Si) further stabilize the amorphous phase by modifying short-range order and increasing the configurational entropy of the melt 2,5.

Advanced quaternary systems such as Cu–Zr–Hf–Al have demonstrated critical rod diameters up to 30 mm when cast via copper mold suction casting, with compositions near Cu₃₄Ni₂Zr₄₈Ag₈Al₈ exhibiting reduced glass transition temperature ratios (T_rg = T_g / T_l) of 0.60 and supercooled liquid stability parameters (λ = T_x / (T_g + T_l)) exceeding 0.41 3,7. The substitution of hafnium for a fraction of zirconium (typically 10–20 at%) increases the alloy's resistance to oxidation and raises the liquidus temperature, thereby expanding the processing window for thermoplastic forming operations 3.

Oxygen content, often considered an impurity, can be deliberately controlled up to 1.7 at% to fine-tune glass-forming ability without compromising mechanical integrity, as demonstrated in patent disclosures where oxygen-bearing precursors were used to reduce raw material costs while maintaining critical casting thickness above 5 mm 9,12. This approach leverages the formation of nanoscale oxide clusters that act as heterogeneous nucleation suppressors during rapid cooling.

Glass-Forming Ability And Critical Casting Thickness In Copper-Based Systems

The glass-forming ability (GFA) of bulk metallic glass copper-based alloy is quantitatively assessed through multiple criteria: the supercooled liquid region ΔT_x (typically 40–70 K for high-GFA Cu-based alloys), the reduced glass transition temperature T_rg (optimally 0.58–0.62), and the γ parameter (γ = T_x / (T_l − T_g), with values > 0.40 indicating excellent GFA) 7,13. Experimental studies on Cu₃₆₋ₓNiₓZr₄₈Ag₈Al₈ (where x = 0–2 at%) revealed that partial substitution of Cu by Ni increases ΔT_x from 55 K to 68 K and raises the critical rod diameter from 20 mm to 30 mm, attributed to enhanced atomic-level frustration and reduced driving force for crystallization 7.

Critical cooling rates for Cu-based BMGs range from 10² to 10³ K/s, significantly lower than the 10⁶ K/s required for melt-spun ribbons, enabling the production of bulk components via conventional casting methods such as copper mold casting, injection molding into metallic dies, and water quenching 2,5,13. The maximum achievable casting thickness is governed by the interplay between heat extraction rate and the time-temperature-transformation (TTT) curve nose, with compositions exhibiting deeper eutectics and higher viscosity in the supercooled liquid state demonstrating superior castability 13.

Comparative analysis of Cu–Zr–Ti–Ni–Al and Cu–Zr–Hf–Al systems shows that hafnium-containing alloys possess 15–20% higher critical casting thickness due to hafnium's larger atomic radius (1.59 Å vs. 1.60 Å for Zr) and stronger interaction with copper (ΔH_mix(Cu–Hf) ≈ −23 kJ/mol vs. −23 kJ/mol for Cu–Zr), which collectively suppress the nucleation rate of competing crystalline phases 3. Differential scanning calorimetry (DSC) measurements on Cu₄₇Ti₃₃Zr₁₁Ni₆Sn₂Si₁ revealed an onset crystallization temperature T_x of 738 K, glass transition temperature T_g of 670 K, and liquidus temperature T_l of 1123 K, yielding ΔT_x = 68 K and T_rg = 0.597, consistent with bulk glass formation in rods exceeding 10 mm diameter 2,5.

The influence of minor alloying elements on GFA has been systematically investigated: silicon additions up to 2 at% increase ΔT_x by 8–12 K through enhanced short-range ordering, while tin additions (1–3 at%) reduce the liquidus temperature by 15–25 K, thereby increasing T_rg and facilitating lower-temperature processing 2,5. However, excessive silicon (> 3 at%) or tin (> 4 at%) content triggers primary crystallization of Cu₅Zr or Cu₁₀Sn₃ intermetallics, sharply reducing GFA and critical thickness 2.

Thermal Stability And Supercooled Liquid Region Characteristics

The supercooled liquid region (SCLR) of bulk metallic glass copper-based alloy is a critical parameter for thermoplastic forming, defined as the temperature interval between the glass transition temperature T_g and the onset of crystallization T_x 2,5,8. High-performance Cu-based BMGs exhibit ΔT_x values ranging from 50 K to 75 K, with the widest regions observed in compositions optimized for low oxygen content and balanced Ti/Zr ratios 2,8. For instance, the alloy Cu₄₆Ti₃₄Zr₁₁Ni₇Sn₁Si₁ demonstrates T_g = 673 K, T_x = 743 K, and ΔT_x = 70 K, enabling blow molding, embossing, and micro-forming operations within a 70-second processing window at 700 K without crystallization 2,5.

Thermal stability is further characterized by the parameter λ = T_x / (T_g + T_l), which correlates with the alloy's resistance to devitrification during reheating cycles 7. Cu-based BMGs with λ > 0.41 exhibit negligible crystallization (< 2 vol%) after isothermal annealing at T_g + 20 K for 600 seconds, making them suitable for multi-step forming processes 7. Time-temperature-transformation (TTT) diagrams constructed via isothermal DSC reveal that the nose of the crystallization curve for Cu₄₇Ti₃₃Zr₁₁Ni₆Sn₂Si₁ occurs at approximately 720 K with an incubation time of 180 seconds, providing a quantitative basis for process parameter optimization 2,5.

The activation energy for crystallization (E_x) in Cu-based BMGs typically ranges from 280 to 350 kJ/mol, calculated via Kissinger analysis of DSC heating rate dependence 13. Higher E_x values correlate with improved thermal stability and reduced sensitivity to minor temperature excursions during processing. Alloys with higher Zr content (> 45 at%) exhibit E_x values approaching 340 kJ/mol, whereas Ti-rich compositions (Ti > 35 at%) show lower E_x (~ 290 kJ/mol) due to the higher diffusivity of titanium atoms in the supercooled liquid 13.

Oxygen contamination significantly impacts thermal stability: controlled oxygen additions up to 1.5 at% can increase ΔT_x by 5–8 K through the formation of stable Zr–O or Ti–O clusters that pin atomic mobility, but oxygen levels exceeding 2.0 at% induce heterogeneous nucleation of oxide phases, reducing ΔT_x by 15–20 K and compromising mechanical properties 9,12. Thermogravimetric analysis (TGA) coupled with DSC on oxygen-doped Cu₄₅Ti₃₅Zr₁₂Ni₆Sn₁.₅O₀.₅ showed a two-stage crystallization process with primary precipitation of Cu₁₀Zr₇ at 728 K followed by secondary formation of CuTi at 768 K, contrasting with the single-stage crystallization observed in oxygen-free compositions 9.

Mechanical Properties And Deformation Behavior Of Copper-Based Bulk Metallic Glass

Bulk metallic glass copper-based alloy exhibits a unique combination of high yield strength (1.5–2.1 GPa), large elastic strain limit (1.8–2.2%), and limited room-temperature plasticity (< 1% compressive strain) due to the absence of dislocation-mediated deformation mechanisms 7,13. Compressive testing of 3 mm diameter rods of Cu₃₄Ni₂Zr₄₈Ag₈Al₈ revealed a yield strength of 1.95 GPa, elastic modulus of 98 GPa, and fracture strength of 2.05 GPa, with failure occurring via catastrophic shear banding at 45° to the loading axis 7. The Vickers hardness of as-cast Cu-based BMGs ranges from 450 to 550 HV, comparable to hardened tool steels, making them attractive for wear-resistant applications 2,5,13.

The elastic properties are governed by the atomic packing density and the nature of interatomic bonding: Cu-based BMGs with higher Zr content exhibit lower shear modulus (G = 35–38 GPa) and Poisson's ratio (ν = 0.36–0.38), indicative of more "liquid-like" atomic arrangements that facilitate shear transformation zone (STZ) activation 13. The ratio G/B (shear modulus to bulk modulus) serves as a predictor of plasticity, with values < 0.41 correlating with enhanced ductility; however, most Cu-based BMGs exhibit G/B ≈ 0.43–0.45, limiting room-temperature plastic strain to < 0.5% in uniaxial compression 13.

Fracture toughness (K_IC) of monolithic Cu-based BMGs is modest, typically 15–25 MPa·m^(1/2), due to the propensity for rapid shear band propagation without crack-tip blunting 13. Strategies to enhance toughness include in-situ formation of ductile crystalline phases (e.g., Cu solid solution dendrites) via controlled partial crystallization, or ex-situ incorporation of second-phase particles such as tungsten fibers or graphite flakes 14. For example, Cu₄₇Ti₃₃Zr₁₁Ni₇Sn₂ reinforced with 10 vol% graphite particles (50–100 μm diameter) exhibited K_IC = 35 MPa·m^(1/2) and compressive plasticity of 3.5%, attributed to crack deflection and energy dissipation at the BMG/graphite interface 14.

Thermoplastic forming in the supercooled liquid region enables net-shape manufacturing with dimensional tolerances < 10 μm, as the viscosity decreases from 10^12 Pa·s at T_g to 10^6 Pa·s at T_x, facilitating flow into micro-featured molds 2,5. Blow molding experiments on Cu₄₆Ti₃₄Zr₁₁Ni₇Sn₁Si₁ at 700 K (T_g + 27 K) under 2 MPa argon pressure produced hemispherical shells with wall thickness uniformity ± 5% and surface roughness Ra < 0.2 μm, demonstrating the alloy's suitability for precision component fabrication 2,5.

Synthesis Routes And Processing Techniques For Copper-Based Bulk Metallic Glass

The synthesis of bulk metallic glass copper-based alloy begins with high-purity elemental feedstocks (> 99.9% purity for Cu, Zr, Ti, Ni; > 99.5% for Al, Sn, Si) to minimize heterogeneous nucleation sites 2,5,9,12. Master alloys are prepared via arc melting under inert atmosphere (argon or helium, < 10 ppm O₂) on a water-cooled copper hearth, with multiple remelting cycles (typically 4–6) to ensure compositional homogeneity within ± 0.5 at% 13. The use of titanium or zirconium getters during melting reduces residual oxygen to < 500 ppm, critical for achieving maximum GFA 9,12.

Bulk casting is performed using copper mold suction casting, where the molten alloy (superheated 50–100 K above liquidus) is drawn into a cylindrical copper mold (inner diameter 3–30 mm, length 50–100 mm) under vacuum (< 10^-2 mbar), achieving cooling rates of 10²–10³ K/s depending on mold diameter 2,5,7,13. Alternative methods include injection molding into pre-heated steel or copper dies (die temperature 400–500 K) for complex geometries, and water quenching of small ingots (< 5 g) to produce rods with diameters up to 10 mm 13. The critical parameter is the Biot number (Bi = hL/k, where h is heat transfer coefficient, L is characteristic length, k is thermal conductivity), which must remain < 1 to ensure uniform cooling and avoid surface crystallization 13.

Additive manufacturing of Cu-based BMG has been explored using selective laser melting (SLM) and binder jetting followed by infiltration 1. SLM of Cu₄₇Ti₃₃Zr₁₁Ni₇Sn₂ powder (particle size 15–45 μm) at laser power 200 W, scan speed 800 mm/s, and layer thickness 30 μm yielded parts with 95% amorphous content and relative density > 98%, though localized crystallization (< 5 vol% Cu₁₀Zr₇) occurred in heat-affected zones due to thermal cycling 1. Post-processing via hot isostatic pressing (HIP) at 0.9T_g under 100 MPa argon pressure for 2 hours eliminated porosity and homogenized the microstructure, restoring full amorphous character 1.

Thermoplastic forming (TPF) in the supercooled liquid region is the preferred