Bulk Metallic Glass Magnesium-Based Alloy: Composition, Properties, And Advanced Applications

Molecular Composition And Structural Characteristics Of Bulk Metallic Glass Magnesium-Based Alloy

The fundamental composition of bulk metallic glass magnesium-based alloys follows a carefully balanced multi-component system designed to maximize glass-forming ability (GFA) while maintaining desirable mechanical properties. The most extensively studied compositions conform to the general formula Mg₁₀₀₋ₓ₋ᵧAₓBᵧ, where component A typically includes transition metals (Cu, Ni, Zn, Al, Ag, Pd) and component B comprises rare earth elements (Gd, Y, Ca, Nd) 6. The atomic percentages are constrained within specific ranges: 2.5 ≤ x ≤ 30 and 2.5 ≤ y ≤ 20, with the combined ratio 18 ≤ (x+y) ≤ 30 atomic percent to ensure optimal amorphous phase stability 6.

Research has demonstrated that the addition of rare earth elements significantly enhances the glass-forming ability by increasing the supercooled liquid region (ΔTₓ = Tₓ - Tg, where Tₓ is the crystallization temperature and Tg is the glass transition temperature) 3. For instance, Mg-Cu-Y-based systems exhibit ΔTₓ values exceeding 40 K, enabling bulk glass formation at cooling rates below 10³ K/s 4. The atomic size mismatch between magnesium (atomic radius ~160 pm), copper (~128 pm), and yttrium (~180 pm) creates dense random packing that frustrates crystallization and stabilizes the amorphous structure 6.

The structural characteristics of Mg-BMGs are defined by short-range order with icosahedral-like atomic clusters and long-range disorder that eliminates grain boundaries. X-ray diffraction patterns of these alloys display broad diffuse halos characteristic of amorphous materials, with the first sharp diffraction peak corresponding to the average nearest-neighbor distance of approximately 3.2–3.4 Å 3. Differential scanning calorimetry (DSC) analysis reveals distinct glass transition events at temperatures typically ranging from 130°C to 180°C, followed by crystallization exotherms at 200°C to 250°C, depending on composition 4. The reduced glass transition temperature (Tg/Tm, where Tm is the liquidus temperature) for optimized Mg-based BMGs reaches values of 0.55–0.62, indicating excellent glass-forming ability according to the Turnbull criterion 6.

Glass-Forming Ability And Critical Casting Thickness Of Magnesium-Based Bulk Metallic Glass

The glass-forming ability of magnesium-based alloys represents a critical parameter determining the maximum achievable bulk dimensions. The GFA is quantitatively assessed through multiple criteria, including the supercooled liquid region (ΔTₓ), the reduced glass transition temperature (Tg/Tm), and the γ parameter defined as Tₓ/(Tg + Tm) 4. For Mg-Cu-Y-Gd quaternary systems, the critical casting thickness reaches 4–7 mm in copper mold casting, representing a significant advancement over earlier Mg-based amorphous alloys that were limited to ribbon or powder forms 36.

The composition Mg₆₅Cu₂₅Y₁₀ has been identified as exhibiting exceptional GFA, with a supercooled liquid region of 48 K and critical diameter of 5 mm when cast in a copper mold 3. Substitution of yttrium with gadolinium (Gd) or neodymium (Nd) further enhances GFA, with Mg₆₅Cu₂₅Gd₁₀ achieving critical thicknesses up to 7 mm 6. The enhanced GFA in these systems is attributed to:

• Deep eutectic composition: The alloy compositions are positioned near deep eutectic points in the phase diagram, resulting in low liquidus temperatures (typically 450–500°C) and large undercooling capacity 4.
• Large negative heat of mixing: The enthalpy of mixing between Mg and rare earth elements (Y, Gd) is approximately -19 to -27 kJ/mol, promoting strong chemical interactions that stabilize the liquid phase against crystallization 6.
• Significant atomic size difference: The atomic radius ratio between constituent elements exceeds 12%, satisfying the Inoue empirical rules for bulk glass formation 3.
• Multi-component complexity: The presence of three or more elements with different atomic sizes and chemical affinities creates topological and chemical frustration that inhibits nucleation of crystalline phases 6.

Recent advances have demonstrated that controlled atmosphere processing, particularly casting under inert gas or vacuum conditions, can extend the critical casting thickness to 10–12 mm for optimized Mg-Cu-Y-Zn-Ca quinary systems 4. However, magnesium's high reactivity with oxygen necessitates stringent processing controls, as oxygen contamination above 0.5 wt% significantly degrades GFA by promoting heterogeneous nucleation sites 4.

Mechanical Properties And Deformation Behavior Of Bulk Metallic Glass Magnesium-Based Alloy

Bulk metallic glass magnesium-based alloys exhibit a unique combination of mechanical properties that distinguish them from both crystalline magnesium alloys and other BMG systems. The compressive yield strength of Mg-BMGs typically ranges from 600 to 850 MPa, representing a 3–4 fold increase over conventional cast magnesium alloys (AZ91: ~160 MPa) 3. The elastic modulus is relatively low at 45–55 GPa, providing a favorable strength-to-modulus ratio for applications requiring elastic energy storage 13.

A critical challenge in BMG systems is the limited room-temperature plasticity, as deformation typically occurs through highly localized shear bands that rapidly propagate to catastrophic failure. However, magnesium-based BMGs demonstrate enhanced ductility compared to other BMG systems, with plastic strains reaching 1.5–3.0% in compression before fracture 3. This improved ductility is attributed to:

• Lower glass transition temperature: The Tg of Mg-BMGs (130–180°C) is closer to room temperature compared to Zr-based or Fe-based BMGs, facilitating thermally activated shear band multiplication 3.
• Composite microstructure formation: In situ formation of ductile crystalline phases (α-Mg or Mg-rich solid solutions) during controlled devitrification creates a composite structure that arrests shear band propagation 3.
• Lower elastic modulus: The reduced stiffness allows for greater elastic strain accommodation (typically 1.8–2.2%) before yielding, providing a larger elastic energy reservoir 1.

The fracture toughness (KIC) of monolithic Mg-BMGs ranges from 8 to 15 MPa·m^(1/2), which is moderate compared to Zr-based BMGs (50–80 MPa·m^(1/2)) but substantially higher than brittle ceramics 3. To further enhance toughness, researchers have developed Mg-BMG matrix composites reinforced with ductile phases. For example, the Mg-based BMG composite containing TiZr alloy particles exhibits compressive strength of 720 MPa with plastic strain exceeding 5%, and fracture toughness values approaching 25 MPa·m^(1/2) 12. The TiZr reinforcement (typically 10–30 vol%) provides crack deflection and bridging mechanisms that dissipate fracture energy 1.

Tensile properties of Mg-BMGs are generally inferior to compressive properties due to the material's sensitivity to surface flaws and the asymmetric nature of shear band formation. Tensile yield strengths range from 400 to 600 MPa with limited ductility (typically <1% plastic strain) 3. Surface finishing and the introduction of compressive residual stresses through shot peening or laser shock processing can improve tensile performance by suppressing surface crack initiation 3.

Synthesis Routes And Processing Methods For Bulk Metallic Glass Magnesium-Based Alloy

The production of bulk metallic glass magnesium-based alloys requires precise control of melting, alloying, and solidification processes to achieve the critical cooling rates necessary for glass formation while managing magnesium's high reactivity. The primary synthesis routes include:

Copper Mold Casting

Copper mold casting represents the most widely employed method for producing Mg-BMG samples with dimensions of 1–7 mm 346. The process involves:

1. Alloy preparation: High-purity elemental constituents (Mg ≥99.9%, Cu ≥99.99%, Y/Gd ≥99.9%) are weighed according to the target composition with 2–3 wt% excess magnesium to compensate for evaporation losses 4.
2. Induction melting: The mixture is melted in a graphite or boron nitride crucible under protective atmosphere (high-purity argon with <10 ppm O₂ and H₂O, or SF₆/CO₂ gas mixture at 0.5–1.0 vol% SF₆) at temperatures of 750–850°C 46.
3. Melt homogenization: The melt is held at temperature for 5–10 minutes with electromagnetic stirring to ensure compositional uniformity 4.
4. Rapid solidification: The molten alloy is injected or poured into a water-cooled copper mold with dimensions corresponding to the desired sample geometry, achieving cooling rates of 10²–10³ K/s 36.

The copper mold casting method is limited by the maximum achievable cooling rate, which constrains the critical casting thickness. For compositions with marginal GFA, the maximum thickness may be only 2–3 mm 6.

Suction Casting And Injection Molding

Suction casting employs differential pressure to draw molten alloy into a copper mold, enabling the production of complex geometries including rods, tubes, and near-net-shape components 12. The process achieves cooling rates similar to conventional copper mold casting but offers superior mold filling and reduced porosity. Injection molding into heated molds (maintained at temperatures slightly below Tg) allows for thermoplastic forming of Mg-BMGs in the supercooled liquid region, enabling replication of intricate features with dimensional tolerances of ±50 μm 3.

Rapid Solidification Processing

For compositions with lower GFA or for producing precursor materials, rapid solidification techniques such as melt spinning and gas atomization are employed 5. Melt spinning produces continuous ribbons with thicknesses of 20–50 μm at cooling rates exceeding 10⁶ K/s, ensuring complete vitrification even for compositions with limited bulk glass-forming ability 5. Gas atomization generates spherical powders with diameters of 10–150 μm, which can be consolidated through spark plasma sintering (SPS) or hot pressing to produce bulk components 5. The consolidation process must be carefully controlled to avoid crystallization, typically employing temperatures within the supercooled liquid region (Tg + 10 to Tg + 30°C) and short processing times (<10 minutes) 5.

Additive Manufacturing

Emerging research has explored selective laser melting (SLM) and laser powder bed fusion (LPBF) for producing Mg-BMG components with complex geometries 3. The extremely high cooling rates achievable in laser processing (10⁴–10⁶ K/s in the melt pool) can induce glass formation even in compositions with moderate GFA. However, challenges include managing the high reflectivity of magnesium, controlling oxidation in the build chamber, and minimizing residual stresses that can trigger devitrification during subsequent thermal cycles 3.

Corrosion Resistance And Biocompatibility Of Bulk Metallic Glass Magnesium-Based Alloy

The corrosion behavior of bulk metallic glass magnesium-based alloys represents a critical consideration for applications, particularly in biomedical implants where controlled degradation is desired. The amorphous structure of Mg-BMGs provides inherent advantages over crystalline magnesium alloys by eliminating grain boundaries, secondary phases, and compositional segregation that serve as preferential corrosion sites 12.

In physiological environments (simulated body fluid at 37°C, pH 7.4), Mg-BMG alloys exhibit corrosion rates of 0.5–2.5 mm/year, which is significantly lower than pure magnesium (5–15 mm/year) but higher than conventional biomedical alloys such as 316L stainless steel or Ti-6Al-4V 1. The corrosion mechanism proceeds through the anodic dissolution of magnesium according to the reaction:

Mg → Mg²⁺ + 2e⁻

accompanied by cathodic hydrogen evolution:

2H₂O + 2e⁻ → H₂ + 2OH⁻

The formation of a surface hydroxide layer (Mg(OH)₂) provides limited protection due to its poor adhesion and high solubility in chloride-containing environments 1. However, the homogeneous composition of Mg-BMGs results in uniform corrosion rather than the localized pitting observed in crystalline alloys, which is advantageous for maintaining mechanical integrity during degradation 2.

The Mg-BMG/TiZr composite developed for biomedical applications demonstrates enhanced corrosion resistance compared to monolithic Mg-BMG, with corrosion rates of 0.3–0.8 mm/year in simulated body fluid 12. The TiZr phase (typically Ti₅₀Zr₅₀ composition) forms a stable passive oxide layer (TiO₂/ZrO₂) that provides galvanic protection to the surrounding Mg-BMG matrix 1. The composite exhibits a corrosion potential of approximately -1.55 V vs. saturated calomel electrode (SCE), which is 150–200 mV more noble than pure magnesium 2.

Biocompatibility studies of Mg-BMG composites have demonstrated excellent cytocompatibility with osteoblast cells (MC3T3-E1), with cell viability exceeding 85% after 72 hours of culture in extracts 1. The degradation products (Mg²⁺, Ca²⁺, and trace amounts of Cu²⁺ and Y³⁺) remain within physiologically acceptable concentrations, with no evidence of cytotoxicity at the release rates observed 12. In vivo studies in animal models have shown that Mg-BMG suture anchors promote bone integration while gradually degrading over 6–12 months, eliminating the need for secondary removal surgery 12. The hydrogen gas evolution during corrosion is managed through the porous structure of surrounding tissue, with no evidence of gas pocket formation or adverse inflammatory response 1.

Applications Of Bulk Metallic Glass Magnesium-Based Alloy In Biomedical Engineering

Biodegradable Orthopedic Implants And Suture Anchors

Bulk metallic glass magnesium-based alloys have emerged as promising materials for biodegradable orthopedic implants, particularly suture anchors used in rotator cuff repair and ligament reconstruction surgeries 12. The Mg-BMG/TiZr composite suture anchor exhibits compressive strength of 720 MPa and shear strength exceeding 450 MPa, which surpasses the mechanical requirements for secure fixation in cancellous bone (typical pullout strength requirement: 200–300 N) 1. The anchor design incorporates a threaded geometry with pitch of 1.5 mm and major diameter of 5.5 mm, enabling insertion torque of 1.2–1.5 N·m and pullout force exceeding 400 N in synthetic bone models 12.

The controlled degradation profile of the Mg-BMG composite allows for gradual load transfer to healing tissue over a period of 6–12 months, matching the typical timeline for