Bulk Metallic Glass Injection Molded Alloy: Comprehensive Analysis Of Composition, Processing, And Industrial Applications

Fundamental Composition And Glass-Forming Ability Of Bulk Metallic Glass Injection Molded Alloy

The compositional design of bulk metallic glass injection molded alloy systems is governed by empirical rules that promote glass formation through kinetic suppression of crystallization. Iron-based BMG alloys typically comprise at least one primary element selected from nickel (Ni), zirconium (Zr), cerium (Ce), molybdenum (Mo), aluminum (Al), tantalum (Ta), cobalt (Co), yttrium (Y), chromium (Cr), copper (Cu), and manganese (Mn), combined with no more than three glass-forming elements from phosphorous (P), carbon (C), boron (B), and silicon (Si), with the balance being iron (Fe) 1. These alloys exhibit critical defect sizes (ac) ranging from 100 μm to 300 μm, which directly influence their fracture toughness and processability during injection molding operations 1.

Zirconium-based bulk metallic glass injection molded alloy systems represent the most extensively studied family for thermoplastic forming applications. A commercially significant composition is Zr52.5Ti5Cu17.9Ni14.6Al10 (atomic percentages), which demonstrates a critical casting thickness of at least 10 mm and an extended supercooled liquid region (SCLR) that facilitates injection molding without crystallization 15. The Zr—Cu—Al—Nb quaternary system, particularly the composition containing 23.5–24.5 wt% copper, 3.5–4.0 wt% aluminum, and 1.5–2.0 wt% niobium (balance zirconium), is commercially available as AMZ4® and exhibits exceptional thermal stability with a glass transition temperature (Tg) of approximately 410°C and a crystallization temperature (Tx) exceeding 480°C 15. This wide processing window of ~70 K enables reliable injection molding with process viscosities in the range of 10^6–10^8 Pa·s 16.

The glass-forming ability (GFA) of bulk metallic glass injection molded alloy can be quantitatively assessed through the critical cooling rate (Rc) and the reduced glass transition temperature (Trg = Tg/Tl, where Tl is the liquidus temperature). Alloys suitable for injection molding typically exhibit Rc values below 100 K/s and Trg values exceeding 0.60 7,13. Fractional compositional variations can dramatically alter GFA; for instance, in the Zr—Nb—Cu—Ni—Al system, maintaining the ratio b/a (where b and a represent atomic percentages of Nb and Zr, respectively) below 0.040 and the ratio c/d (Cu to Ni) below 1.15 significantly enhances thermal stability and expands the supercooled liquid region from approximately 40 K to over 70 K 13. This compositional optimization enables thermoplastic forming operations with reduced risk of crystallization during the injection molding cycle.

Copper-based bulk metallic glass injection molded alloy systems offer advantages in terms of lower material cost and higher thermal conductivity compared to zirconium-based alloys. A representative composition is Cu47-(x+y+z)(TiaZrb)cNi7+xSn1+ySiz, 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% 20. These alloys exhibit critical casting thicknesses of 3–5 mm and glass transition temperatures in the range of 420–450°C, making them suitable for injection molding of smaller components where high strength (compressive yield strength ~2000 MPa) and wear resistance are required 20.

Advanced Injection Molding Processes For Bulk Metallic Glass Injection Molded Alloy

Injection Compression Molding Technology

Injection compression molding represents the most widely adopted manufacturing route for bulk metallic glass injection molded alloy components, combining the advantages of injection molding (complex geometry capability) with compression molding (uniform filling and reduced defects) 9. The process involves melting the BMG alloy feedstock in a controlled atmosphere furnace (typically under inert gas or vacuum to prevent oxidation), injecting the molten alloy into a mold cavity at temperatures 50–100 K above Tg, and simultaneously applying compressive force through a movable mold part to maintain thermal contact and ensure complete cavity filling 9. This technique enables the production of BMG articles with aspect ratios (first dimension/second dimension) exceeding 10:1 or below 0.1:1, which are difficult to achieve through conventional casting methods 9.

The thermal management during injection compression molding is critical to prevent premature crystallization. The mold is typically maintained at temperatures 20–50 K below Tg to achieve cooling rates of 10–100 K/s, which are sufficient to retain the amorphous structure in alloys with critical cooling rates below 100 K/s 9,16. For the Zr52.5Ti5Cu17.9Ni14.6Al10 alloy, optimal injection temperatures range from 460°C to 480°C (Tg + 50 to 70 K), with mold temperatures maintained at 360–380°C and injection pressures of 50–150 MPa applied for 5–30 seconds 16. These parameters result in BMG components with fully amorphous microstructures (>95 vol% amorphous phase) and minimal surface defects such as cold shuts or incomplete filling 9.

Rapid Capacitor Discharge Forming For Bulk Metallic Glass Injection Molded Alloy

Rapid capacitor discharge forming (RCDF) represents an innovative approach for processing bulk metallic glass injection molded alloy from powder or foil feedstocks, which are more cost-effective than monolithic ingots 8. The RCDF technique involves packing metallic glass-forming alloy powder (particle size 10–100 μm) or stacking amorphous foils (thickness 20–200 μm) to form a green body, followed by rapid heating to temperatures between Tg and the melting point (Tm) using electrical discharge from high-capacity capacitors 8. Heating rates of 10^3–10^5 K/s are achieved, which minimize the time spent in the crystallization temperature range and enable consolidation of marginal glass-formers (alloys with critical casting thicknesses <1 mm) into bulk forms 8.

The RCDF process for bulk metallic glass injection molded alloy typically involves the following steps: (1) powder packing or foil stacking in a graphite or ceramic die with applied pressure of 10–50 MPa to achieve green densities of 60–80% of theoretical density; (2) rapid heating to a process temperature of Tg + 30 to 100 K for 0.1–5 seconds using capacitor discharge (energy densities of 1–10 kJ/cm³); (3) simultaneous application of compressive stress (50–200 MPa) to promote viscous flow and eliminate porosity; and (4) rapid cooling at rates exceeding the critical cooling rate to retain the amorphous structure 8. This technique has been successfully applied to produce BMG components from Zr-based, Ti-based, and Fe-based alloy powders with final densities exceeding 99% and amorphous volume fractions above 90% 8.

Inductive Heating And Thermoplastic Forming Of Bulk Metallic Glass Injection Molded Alloy

Inductive heating offers a non-contact method for rapidly heating bulk metallic glass injection molded alloy shells and thin-walled structures to the supercooled liquid region, enabling subsequent thermoplastic forming operations such as blow molding, stamp forging, or dynamic forging 16. The technique utilizes electromagnetic induction to generate eddy currents within the electrically conductive BMG material, resulting in volumetric heating rates of 10^3–10^4 K/s without requiring direct electrical connections 16. For closed-loop, thin-walled BMG samples (wall thickness 0.5–3 mm), inductive coupling can achieve uniform heating to process temperatures (Tg to Tx) in time scales of several milliseconds to a few seconds, significantly faster than conventional resistance heating or furnace heating methods 16.

The inductive heating process for bulk metallic glass injection molded alloy involves placing the BMG preform within an induction coil (typically copper tubing with water cooling) and applying alternating current at frequencies of 10–500 kHz with power densities of 10–100 kW/cm² 16. The skin depth of electromagnetic penetration (δ = √(2ρ/ωμ), where ρ is electrical resistivity, ω is angular frequency, and μ is magnetic permeability) for typical BMG alloys at these frequencies ranges from 0.1 to 1 mm, enabling efficient heating of thin-walled structures 16. Once heated to a process viscosity of 10^6–10^9 Pa·s (corresponding to temperatures of Tg + 20 to 80 K), the BMG can be shaped via injection molding, blow molding (gas pressures of 1–10 MPa), or forging (applied stresses of 10–100 MPa) in time frames of less than 1 second, followed by rapid cooling to below Tg to freeze the amorphous structure 16.

Near Net Shape Casting And Thermoplastic Forming Integration For Bulk Metallic Glass Injection Molded Alloy

The combination of near net shape casting and subsequent thermoplastic forming represents a hybrid manufacturing strategy that leverages the advantages of both processes to produce bulk metallic glass injection molded alloy components with complex geometries and minimal post-processing requirements 4. Near net shape casting involves feeding molten BMG alloy into a mold with a three-dimensional cavity geometry, where multiple two-dimensional cross-sections differ from one another in planes displaced along the normal direction 4. This approach enables the production of BMG castings with intricate features such as undercuts, internal channels, and variable wall thicknesses that approximate the final component geometry 4.

The near net shape casting process for bulk metallic glass injection molded alloy typically employs copper molds with forced water cooling to achieve cooling rates of 10–100 K/s, which are sufficient to retain amorphous structure in bulk glass-forming alloys with critical casting thicknesses exceeding 5 mm 4. For the Zr—Cu—Al—Nb system, casting temperatures of 900–1000°C (approximately 100–200 K above Tl) and mold temperatures of 200–300°C result in BMG castings with amorphous volume fractions exceeding 95% and minimal casting defects such as porosity or shrinkage cavities 4. The as-cast BMG components can then be subjected to thermoplastic forming operations at temperatures above Tg to impart additional geometric features or improve surface finish 4.

Thermoplastic forming of near net shape BMG castings involves heating the as-cast component to the supercooled liquid region (Tg to Tx) and applying mechanical force through dies, punches, or gas pressure to induce viscous flow and shape change 4. For the Zr52.5Ti5Cu17.9Ni14.6Al10 alloy, thermoplastic forming temperatures of 430–460°C, applied stresses of 10–50 MPa, and forming times of 10–300 seconds enable strain accumulations of 10–100% without crystallization 4. This hybrid approach of near net shape casting followed by thermoplastic forming allows for the production of bulk metallic glass injection molded alloy components with geometric complexities and dimensional tolerances (±0.05–0.1 mm) that are difficult to achieve through casting or forming alone 4.

Multistage Casting Methods For Diameter Increase Of Bulk Metallic Glass Injection Molded Alloy

Multistage casting represents a specialized technique for overcoming the critical casting thickness limitation of bulk metallic glass injection molded alloy systems, enabling the production of larger-diameter components through sequential casting operations 2,6,12. The process involves casting an initial BMG bulk material with a diameter at or near the critical casting thickness, inserting this bulk material into a larger mold cavity, and injecting molten BMG alloy of the same or compatible composition into the void space between the bulk material and the mold wall 2,6,12. Upon solidification, the newly cast material bonds metallurgically with the existing bulk material, resulting in a composite structure with increased overall diameter while maintaining amorphous character throughout 2,6,12.

The multistage casting process for bulk metallic glass injection molded alloy requires careful control of thermal conditions to ensure bonding between the existing bulk material and the newly cast layer without inducing crystallization in either region 2,12. The existing bulk material is typically preheated to temperatures of 0.5–0.8 Tg (200–350°C for Zr-based alloys) to reduce thermal shock and promote interfacial bonding, while the molten alloy is injected at temperatures of 1.1–1.3 Tl (950–1100°C for Zr-based alloys) 2,12. The mold is maintained at temperatures of 0.3–0.5 Tg (150–250°C) to achieve cooling rates of 10–50 K/s in the newly cast layer, which are sufficient to retain amorphous structure in bulk glass-forming alloys 2,12. This technique has been successfully applied to increase the diameter of Co-based BMG alloys from an initial 3 mm to final diameters exceeding 10 mm through three sequential casting stages 6.

The CAP (Casting And Pressure cooling) method represents an advanced variant of multistage casting that incorporates simultaneous pressure cooling during the casting operation to enhance the critical diameter of bulk metallic glass injection molded alloy 2. The CAP method involves melting the BMG alloy in a furnace with an open upper surface, tilting the furnace floor to inject the molten alloy into a forcibly cooled mold (inclined angle casting), and simultaneously applying pressure through an upper punch that covers nearly the entire top surface of the melt within the mold cavity 2. The applied pressure (10–100 MPa) enhances thermal contact between the molten alloy and the mold surfaces, increasing heat extraction rates and enabling the retention of amorphous structure in larger cross-sections 2. For Zr-based BMG alloys, the CAP method has achieved critical diameters exceeding 25 mm, compared to 10–15 mm for conventional casting methods 2.

Mechanical Properties And Structure-Property Relationships Of Bulk Metallic Glass Injection Molded Alloy

Strength And Elastic Properties

Bulk metallic glass injection molded alloy exhibits exceptional mechanical strength due to the absence of crystalline defects such as grain boundaries and dislocations that serve as stress concentrators in conventional crystalline alloys 1,15. Iron-based BMG alloys with compositions optimized for additive manufacturing demonstrate compressive yield strengths of 2500–3500 MPa, tensile yield strengths of 1500–2500 MPa, and elastic strain limits of 2.0–2.5%, which are 2–3 times higher than high-strength steels of comparable density (7.2–7.8 g/cm³) 1. The Young's modulus of Fe-based BMG alloys ranges from 150 to 200 GPa, while Zr-based alloys exhibit lower moduli of 80–100 GPa, reflecting the lower atomic packing density and weaker interatomic bonding in the latter system 15.

The mechanical properties of bulk metallic glass injection molded alloy are strongly influenced by the volume fraction of crystalline phases that may form during processing. Alloys with 1–50 vol% crystalline metal phases (such as Cu, Al, V, Cr, Fe, Co, Ni, or Mo) dispersed within the amorphous matrix exhibit enhanced ductility and fracture toughness compared to fully amorphous structures, while maintaining yield strengths above 2000 MPa 1. For the Fe-based BMG system with 10–30 vol% crystalline α-Fe or Fe-Co phases (particle size