Bulk Metallic Glass High Toughness Alloys: Composition Design, Mechanical Properties, And Engineering Applications

Fundamental Composition Strategies For Bulk Metallic Glass High Toughness Alloys

The design of bulk metallic glass high toughness alloys relies on multi-component systems where atomic size mismatch, negative heat of mixing, and optimal glass-forming ability converge to suppress crystallization during cooling. Iron-based systems such as Fe-Ni-Mo-P-C-B demonstrate that tightly controlling the metalloid moiety—specifically the ratios of phosphorus, carbon, and boron—yields alloys with surprisingly low shear modulus (typically 30–50 GPa) and high toughness 357. The shear modulus directly correlates with fracture toughness through the relationship K_IC ∝ √(G·γ), where G is the shear modulus and γ is the fracture energy; thus, reducing G while maintaining high γ enhances toughness 7.

Nickel-based bulk metallic glass high toughness alloys, exemplified by Ni-Cr-Nb-P-B compositions, achieve critical rod diameters of at least 5 mm and notch toughness values between 96 and 120 MPa m^1/2^ 118. The addition of chromium in controlled amounts (3–13 atomic %) stabilizes the supercooled liquid region and refines the atomic packing density, while niobium (3.8–4.2 atomic % following the relation x-y*a, where x=3.8–4.2 and y=0.11–0.14) acts as a strong glass former by increasing the viscosity of the undercooled melt 18. Phosphorus content in the range of 16.25–17 atomic % and boron at 2.75–3.5 atomic % provide the necessary metalloid contribution to achieve deep eutectic compositions with liquidus temperatures below 1000°C 118.

Zirconium-rich bulk metallic glass alloys, particularly Zr-Cu-Ni-Al-Ti quinary systems, offer high fracture toughness (up to 50 MPa m^1/2^) combined with excellent castability and corrosion resistance 14. The composition range of 28–45 atomic % copper, 1–12 atomic % nickel, 1–15 atomic % aluminum, and balance zirconium with titanium additions enables formation of fully amorphous rods with diameters exceeding 5 mm at cooling rates below 100 K/s 14. These alloys exhibit compressive strengths near 2 GPa and Young's modulus around 90 GPa, making them suitable for load-bearing applications 14.

Key compositional principles for bulk metallic glass high toughness alloys include:

• Multi-component complexity: At least five principal elements to maximize configurational entropy and suppress competing crystalline phases 3514.
• Atomic size ratio optimization: Radius ratios between 1.1 and 1.4 among constituent elements to promote dense random packing and inhibit long-range ordering 714.
• Negative enthalpy of mixing: Preferably between -20 and -50 kJ/mol among major constituents to stabilize the liquid phase and increase viscosity upon undercooling 57.
• Controlled metalloid content: Phosphorus, boron, carbon, and silicon in precise ratios (typically 15–25 atomic % total) to lower liquidus temperature and enhance glass-forming ability without embrittling the matrix 357.

Processing Routes And Glass-Forming Ability Enhancement In High Toughness Bulk Metallic Glass Alloys

The glass-forming ability (GFA) of bulk metallic glass high toughness alloys is quantified by the critical cooling rate (R_c) required to avoid crystallization, the critical casting thickness (t_c), and the reduced glass transition temperature (T_rg = T_g/T_l, where T_g is glass transition temperature and T_l is liquidus temperature). Alloys with T_rg ≥ 0.57 and supercooled liquid region ΔT_x = T_x - T_g ≥ 40 K (where T_x is crystallization onset temperature) exhibit superior GFA and are amenable to thermoplastic forming operations 612.

Melt Overheating And Thermal History Control

A critical processing innovation for improving toughness and GFA involves melt overheating to temperatures significantly above the liquidus temperature (T_liquidus) before quenching 10. Experimental studies demonstrate that overheating the alloy melt to a threshold temperature T_tough—typically 100–200 K above T_liquidus—results in substantial improvement in notch toughness compared to conventionally processed metallic glass 10. This effect arises from the dissolution of high-melting-point clusters and chemical short-range order fluctuations that otherwise act as heterogeneous nucleation sites during solidification 10.

The melt overheating method involves:

1. Initial melting: Heating the alloy constituents in an inert atmosphere (argon or helium) to fully liquefy all components, typically 50–100 K above T_liquidus 10.
2. Overheating stage: Raising the melt temperature to T_tough (often 1400–1600 K for Zr-based alloys, 1300–1500 K for Ni-based alloys) and holding for 5–15 minutes to homogenize the liquid structure 10.
3. Equilibration: Optionally cooling the melt to an intermediate temperature T_int (T_liquidus < T_int < T_tough) and holding for 2–5 minutes to allow controlled cluster formation 10.
4. Rapid quenching: Injecting the melt into a copper mold or using suction casting at cooling rates of 10^2^–10^4^ K/s to bypass the nose of the time-temperature-transformation (TTT) curve 10.

This thermal protocol increases the critical rod diameter by 20–50% and enhances notch toughness by 15–30% relative to standard processing, as demonstrated in Zr-Cu-Ni-Al and Ni-Cr-Nb-P-B systems 10.

Additive Manufacturing Of Bulk Metallic Glass High Toughness Composites

Additive manufacturing (AM) techniques, particularly powder-based methods such as selective laser melting (SLM) and laser powder bed fusion (LPBF), enable fabrication of bulk metallic glass matrix composites (BMGMCs) with tailored microstructures for enhanced toughness 49. The approach involves mixing a bulk metallic glass composition powder with a secondary metallic powder that forms a ductile crystalline phase upon solidification 49.

For example, a Zr-based BMG powder (Zr_52.5_Cu_17.9_Ni_14.6_Al_10_Ti_5, atomic %) is blended with 10–30 volume % of tantalum, niobium, or tungsten powder (particle size 10–50 μm) 9. During laser processing, the BMG matrix melts and rapidly solidifies into an amorphous phase, while the refractory metal particles partially dissolve and precipitate as dendritic inclusions (1–10 μm diameter) dispersed throughout the matrix 9. These soft, ductile dendrites arrest crack propagation by blunting crack tips and inducing localized plastic deformation, increasing fracture toughness from ~50 MPa m^1/2^ (monolithic BMG) to 80–120 MPa m^1/2^ (BMGMC) 49.

Critical processing parameters for AM of bulk metallic glass high toughness alloys include:

• Laser power: 150–400 W, adjusted to achieve melt pool temperatures 100–200 K above T_liquidus without excessive vaporization 9.
• Scan speed: 200–800 mm/s, optimized to maintain cooling rates above the critical value (typically >10^3^ K/s for Zr-based alloys) 9.
• Layer thickness: 20–50 μm, ensuring sufficient overlap and minimizing porosity (<2% by volume) 9.
• Powder size distribution: D_50 = 20–40 μm with narrow distribution (D_90/D_10 < 2.5) to promote uniform melting and reduce defects 9.
• Inert atmosphere: Oxygen content <50 ppm to prevent oxidation and embrittlement of the amorphous matrix 9.

Cold Gas Spray Deposition For Large-Scale Bulk Metallic Glass Components

Cold gas spray (CGS) technology offers an alternative route for producing bulk metallic glass high toughness alloy components with critical dimensions exceeding 50 mm while maintaining >95% amorphous content 13. In CGS, metallic glass powder particles (typically 10–50 μm diameter) are accelerated to supersonic velocities (500–1200 m/s) using a converging-diverging nozzle with heated nitrogen or helium carrier gas (200–600°C, below T_g) 13. Upon impact with the substrate, particles undergo severe plastic deformation and bonding through adiabatic shear instability, forming a dense coating or freestanding part without melting 13.

FeSiB-based bulk metallic glass alloys processed by CGS achieve:

• Critical dimensions: >120 mm in diameter or thickness, far exceeding conventional casting limits 13.
• Amorphous fraction: >95% by volume, confirmed by X-ray diffraction (XRD) showing broad halos without crystalline peaks 13.
• Porosity: <1% by volume, measured via optical microscopy and helium pycnometry 13.
• Deposition rate: 5–20 kg/h, enabling economical production of large components 13.

The absence of melting during CGS prevents crystallization and oxidation, preserving the high toughness and corrosion resistance inherent to the amorphous structure 13.

Mechanical Properties And Toughness Mechanisms In Bulk Metallic Glass High Toughness Alloys

Fracture Toughness And Notch Sensitivity

Fracture toughness, quantified by the stress intensity factor at crack initiation (K_IC), is the critical parameter distinguishing bulk metallic glass high toughness alloys from conventional brittle metallic glasses. Iron-based alloys such as Fe_48_Ni_18_Mo_14_P_10_C_7.5_B_2.5 (atomic %) exhibit K_IC values of 80–100 MPa m^1/2^ when the metalloid composition is optimized, compared to 3–10 MPa m^1/2^ for early Fe-based glasses with higher carbon and boron content 357. This improvement correlates with a reduction in shear modulus from 60–70 GPa to 30–40 GPa, achieved by increasing the phosphorus-to-boron ratio and incorporating molybdenum 7.

Notch toughness measurements on 3 mm diameter rods with circumferential notches (notch depth 1–2 mm, root radius 0.1–0.15 mm) provide a standardized metric for comparing bulk metallic glass high toughness alloys 118. Ni-Cr-Nb-P-B alloys with chromium content of 4.5–5 atomic % and molybdenum additions of 0.5–1 atomic % achieve notch toughness between 96 and 120 MPa m^1/2^, with critical rod diameters of 5–11 mm 118. The notch toughness increases with chromium content up to ~8 atomic %, beyond which the glass-forming ability deteriorates and crystalline phases precipitate during casting 18.

Zirconium-rich bulk metallic glass alloys demonstrate fracture toughness up to 50 MPa m^1/2^ in the as-cast condition, with further enhancement to 70–90 MPa m^1/2^ achievable through controlled partial crystallization (devitrification) to form nanocrystalline precipitates (5–20 nm diameter) that impede crack propagation 14. The composition Zr_41.2_Ti_13.8_Cu_12.5_Ni_10_Be_22.5 (Vitreloy 1) exhibits K_IC ≈ 55 MPa m^1/2^, compressive strength of 1.9 GPa, and tensile strength of 1.5 GPa, making it suitable for structural applications despite the presence of beryllium 14.

Shear Band Formation And Plastic Deformation

The toughness of bulk metallic glass high toughness alloys arises from their ability to accommodate plastic deformation through the formation and multiplication of shear bands—narrow regions (10–20 nm thick) of localized shear strain that operate as the primary deformation mechanism in amorphous metals 357. Unlike crystalline metals where dislocations glide on specific slip planes, metallic glasses deform via cooperative shear transformations involving clusters of 10–100 atoms 7.

In high-toughness alloys, the density of shear bands increases due to:

• Low shear modulus: Reduces the energy barrier for shear transformation zone (STZ) activation, promoting multiple shear band nucleation rather than catastrophic propagation of a single band 7.
• Compositional heterogeneity: Nanoscale fluctuations in atomic packing density and chemical composition create preferential sites for STZ formation, distributing plastic strain more uniformly 57.
• Ductile phase inclusions: In BMGMCs, crystalline dendrites (e.g., Ta, Nb, β-Ti) with yield strengths of 200–500 MPa deform plastically and arrest shear bands, preventing runaway crack growth 49.

Compression tests on 3 mm diameter, 6 mm length cylinders of Fe-Ni-Mo-P-C-B bulk metallic glass high toughness alloys reveal compressive strengths of 3.5–4.2 GPa, elastic strain limits of 2–2.5%, and plastic strain before fracture of 0.5–2% (compared to <0.1% for brittle Fe-based glasses) 35. The stress-strain curve exhibits serrations corresponding to individual shear band events, with serration amplitudes of 10–50 MPa indicating stable shear band propagation 5.

Temperature Dependence And Cryogenic Performance

The fracture toughness of bulk metallic glass high toughness alloys exhibits moderate temperature dependence, with K_IC increasing by 10–20% upon cooling from room temperature (298 K) to liquid nitrogen temperature (77 K) due to suppression of thermally activated shear band propagation 37. This behavior contrasts with body-centered cubic (BCC) steels, which suffer ductile-to-brittle transitions below ~200 K, making bulk metallic glass high toughness alloys attractive for cryogenic applications such as liquefied natural gas (LNG) storage and aerospace components 3.

Conversely, heating bulk metallic glass high toughness alloys above the glass transition temperature (T_g, typically 550–650 K for Fe-based alloys, 620–680 K for Ni-based alloys, 620–670 K for Zr-based alloys) induces a transition to the supercooled liquid state, where viscosity drops from 10^12^ Pa·s to 10^6^–10^9^ Pa·s, enabling thermoplastic forming operations such as blow molding, embossing, and extrusion 612. The supercooled liquid region (ΔT_x = T_x - T_g) for high-toughness alloys ranges from 40 to 80 K, providing a processing window of several minutes before crystallization occurs 612.

Applications Of Bulk Metallic Glass High Toughness Alloys In Engineering Systems

Structural Components In Aerospace And Defense

Bulk metallic glass high toughness alloys offer a compelling combination of high specific strength (strength-to-density ratio), corros