Magnesium-Based Amorphous Alloys: Comprehensive Analysis Of Glass-Forming Ability, Mechanical Properties, And Advanced Applications

Fundamental Composition And Glass-Forming Ability Of Magnesium-Based Amorphous Alloys

The glass-forming ability (GFA) of magnesium-based amorphous alloys is fundamentally determined by their chemical composition and the kinetic suppression of crystallization during rapid solidification. Early binary systems such as Mg-Ca, Mg-Ni, Mg-Cu, Mg-Zn, and Mg-Y demonstrated limited GFA, restricting amorphous phase formation to ribbon geometries with thicknesses of several tens of microns 4. The breakthrough in bulk amorphous alloy development came with ternary and quaternary systems that strategically combine multiple elements to increase configurational entropy and frustrate crystal nucleation.

The most successful magnesium-based amorphous alloy compositions follow the general formula Mg₁₀₀₋ₓ₋ᵧAₓBᵧ, where A represents transition metals (Cu, Ni, Zn, Al, Ag, Pd) at 2.5 ≤ x ≤ 35 atomic percent, and B includes rare-earth or alkaline-earth elements (Gd, Y, Ca, Nd, Tb, Er, Dy, Ho, Tm, Sm) at 2.5 ≤ y ≤ 40 atomic percent 45. For example, the Mg₆₅Cu₂₅Gd₁₀ system exhibits exceptional GFA, enabling bulk amorphous formation in air without requiring expensive vacuum equipment or atmosphere control 1. The addition of rare-earth elements such as Gd and Y significantly enhances GFA by increasing the atomic size mismatch and negative heat of mixing, which stabilizes the supercooled liquid against crystallization 45.

Specific compositional ranges have been optimized for different property targets. Alloys with 2.5 ≤ x ≤ 30 and 2.5 ≤ y ≤ 20 atomic percent demonstrate improved ductility alongside good GFA 4. The inclusion of calcium (Ca) as element B provides additional benefits: Ca-containing Mg-based amorphous alloys exhibit enhanced biocompatibility and controlled degradation rates, making them suitable for biomedical implant applications 4. The critical cooling rate required to achieve amorphous structure varies from 10² K/s for high-GFA compositions to 10⁶ K/s for marginal glass formers, directly impacting the maximum achievable sample thickness 2.

The role of minor alloying additions cannot be overlooked. Zirconium (Zr) additions at 0.1-1.0 wt% serve as grain refiners in the residual crystalline phases and further suppress heterogeneous nucleation 717. Silicon (Si) additions at 5-20 atomic percent in Mg-Ni-Si systems promote amorphous phase stability and enhance corrosion resistance through the formation of protective surface oxides 19. The synergistic effects of multi-component alloying enable the design of magnesium-based amorphous alloys with tailored properties for specific engineering applications.

Mechanical Properties And Structural Characteristics Of Magnesium-Based Amorphous Alloys

Magnesium-based amorphous alloys exhibit mechanical properties that significantly surpass those of conventional crystalline magnesium alloys, primarily due to their unique atomic structure. The absence of grain boundaries, dislocations, and other crystalline defects eliminates traditional deformation mechanisms, resulting in yield strengths ranging from 500 to 900 MPa and ultimate tensile strengths exceeding 1000 MPa 24. These values represent a 3-5 fold improvement over commercial magnesium alloys such as AZ91 (yield strength ~150 MPa) and WE43 (yield strength ~200 MPa).

The elastic limit of magnesium-based amorphous alloys typically reaches 2-3% strain, substantially higher than the 0.5-1% elastic strain observed in crystalline counterparts 2. This extended elastic range enables greater energy absorption before permanent deformation, making these materials attractive for impact-resistant applications. The Young's modulus ranges from 40 to 55 GPa, slightly lower than crystalline magnesium (45 GPa), which contributes to improved vibration damping characteristics 4. Hardness values measured by Vickers indentation typically fall between 250 and 400 HV, depending on composition, representing a 2-3 fold increase over conventional magnesium alloys 24.

However, the primary limitation of magnesium-based amorphous alloys is their limited room-temperature ductility. Plastic deformation in amorphous alloys occurs through the formation and propagation of highly localized shear bands, typically 10-20 nm in width 2. Once a dominant shear band forms, it rapidly propagates through the material, leading to catastrophic failure with minimal macroscopic plastic strain (often <1%). This brittleness at ambient temperature has been the major obstacle to widespread structural applications 46.

Recent advances have addressed this limitation through microstructural engineering. The introduction of ductile crystalline phases or nanocrystals within the amorphous matrix creates composite microstructures that arrest shear band propagation and promote the formation of multiple shear bands 14. For example, semi-solid die-casting at 810-850°C produces magnesium-based amorphous alloys with 5-8% crystallinity, where uniformly distributed nanocrystal structures form dendritic phases that significantly improve plastic deformation capability and fracture toughness 14. The dendritic phase acts as a barrier to single shear band expansion, inducing the formation of multiple shear bands and enhancing overall ductility.

The incorporation of complex concentrated alloy (CCA) phases within amorphous matrices represents another promising approach. Although primarily demonstrated in Zr-based systems, the concept of dispersing CCA particles containing refractory elements (Ti, Zr, Hf, V, Nb, Ta, Mo) within an amorphous matrix can be extended to magnesium-based systems to improve both strength and toughness 612. The CCA particles provide sites for shear band nucleation while preventing catastrophic propagation, resulting in enhanced ductility without sacrificing strength.

Thermal stability is another critical mechanical characteristic. Magnesium-based amorphous alloys exhibit a glass transition temperature (Tg) typically ranging from 120 to 180°C and a crystallization temperature (Tx) from 180 to 250°C, depending on composition 4. The supercooled liquid region (ΔTx = Tx - Tg) serves as an indicator of GFA and thermoplastic formability, with values of 30-60 K for high-GFA compositions 14. Within this temperature range, the alloy exhibits Newtonian viscous flow behavior, enabling thermoplastic forming operations such as blow molding and embossing at relatively low pressures.

Synthesis And Processing Methods For Magnesium-Based Amorphous Alloys

The production of magnesium-based amorphous alloys requires rapid solidification techniques that achieve cooling rates sufficient to suppress crystallization. The most widely employed method is melt spinning, which involves ejecting molten alloy onto a rapidly rotating copper wheel (surface velocity 20-40 m/s), achieving cooling rates of 10⁵-10⁶ K/s 413. This technique reliably produces amorphous ribbons with thicknesses of 20-50 μm and widths up to 10 mm, suitable for magnetic applications and as precursors for powder metallurgy consolidation 13.

For bulk amorphous alloy production, copper mold casting is the preferred method for high-GFA compositions. Molten alloy is poured or injected into water-cooled copper molds with dimensions ranging from 1 to 10 mm, achieving cooling rates of 10²-10³ K/s 14. The Mg₆₅Cu₂₅Gd₁₀ system can be cast into bulk amorphous rods with diameters up to 7 mm in air atmosphere, eliminating the need for expensive vacuum equipment and significantly reducing manufacturing costs 1. The ability to form bulk amorphous structures in air represents a major advancement, as magnesium's high reactivity with oxygen typically necessitates protective atmospheres during processing.

Gas atomization produces spherical amorphous powder particles with diameters ranging from 10 to 150 μm, suitable for additive manufacturing, thermal spraying, and powder metallurgy consolidation 819. The atomization process involves disintegrating a molten metal stream with high-velocity inert gas jets (typically argon or nitrogen), achieving cooling rates of 10³-10⁵ K/s depending on particle size. Mechanical alloying represents an alternative solid-state processing route, where elemental or pre-alloyed powders are subjected to high-energy ball milling under inert atmosphere for 20-100 hours 19. This technique produces amorphous or nanocrystalline powders with compositions such as Mg₆₆₋₈₅Ni₍₁₀₀₋ₓ₋ᵧ₎Siᵧ (y = 5-20 atomic percent), which exhibit excellent corrosion resistance 19.

Semi-solid processing has emerged as an innovative approach for producing magnesium-based amorphous alloy composites with gradient microstructures. The process involves heating the alloy to a semi-solid temperature (T₅₀, where 50% liquid and 50% solid coexist), typically 450-480°C for magnesium alloys, followed by mechanical stirring at 350-1000 rpm to incorporate reinforcement particles 9. For example, FeCrMoBC amorphous alloy particles (2.00-8.00 wt%) can be dispersed into a Mg-Zn-Al-Cu-Mn matrix through semi-solid stir casting, creating surface-gradient composites with enhanced hardness, strength, and wear resistance in the outer layer while maintaining ductility in the core 9. The gradient distribution of reinforcement particles is controlled by adjusting the semi-solid temperature, stirring speed, and holding time.

Consolidation of amorphous powders into bulk forms requires careful control to avoid crystallization. Spark plasma sintering (SPS) or pulse electric current sintering (PECS) enables rapid densification at temperatures below Tx, typically 0.8-0.9 Tx, with heating rates of 50-200 K/min and holding times of 3-10 minutes under applied pressures of 50-100 MPa 19. This technique successfully consolidates mechanically alloyed Mg-Ni-Si amorphous powders into bulk bodies with >95% theoretical density while retaining the amorphous structure 19. Hot pressing and hot isostatic pressing (HIP) represent alternative consolidation methods, though they require longer processing times and more careful temperature control to prevent crystallization.

Thermoplastic forming in the supercooled liquid region offers unique shaping capabilities. When heated to temperatures between Tg and Tx, magnesium-based amorphous alloys exhibit viscosities of 10⁶-10⁹ Pa·s, enabling blow molding, embossing, and micro-forming operations at pressures of 1-10 MPa 4. This processing window allows the fabrication of complex geometries and micro-features that would be difficult or impossible to achieve through conventional machining or casting. The thermoplastic formability is quantified by the parameter γ = Tx/(Tg + Tl), where Tl is the liquidus temperature; values of γ > 0.4 indicate good thermoplastic forming capability 4.

Corrosion Resistance And Environmental Stability Of Magnesium-Based Amorphous Alloys

One of the most significant advantages of magnesium-based amorphous alloys over their crystalline counterparts is dramatically improved corrosion resistance. The absence of grain boundaries, secondary phases, and compositional segregation eliminates the galvanic couples that drive localized corrosion in conventional magnesium alloys 2419. Electrochemical impedance spectroscopy (EIS) measurements reveal that magnesium-based amorphous alloys exhibit corrosion current densities 1-2 orders of magnitude lower than commercial magnesium alloys in 3.5 wt% NaCl solution 19.

The Mg-Ni-Si amorphous system demonstrates exceptional corrosion resistance, with compositions of Mg₆₆₋₈₅Ni₍₁₀₀₋ₓ₋ᵧ₎Siᵧ (y = 5-20 atomic percent) showing corrosion rates below 0.1 mm/year in marine environments 19. This performance approaches that of aluminum alloys and represents a 10-100 fold improvement over conventional magnesium alloys such as AZ91 (corrosion rate ~1-10 mm/year in seawater). The superior corrosion resistance is attributed to the formation of a dense, adherent passive film composed of magnesium oxide (MgO), magnesium hydroxide (Mg(OH)₂), and silicon oxide (SiO₂), which provides effective barrier protection 19.

Rare-earth-containing magnesium-based amorphous alloys, such as Mg-Cu-Gd and Mg-Zn-Ca-Gd systems, exhibit self-healing corrosion behavior through the formation of protective rare-earth oxide/hydroxide layers 45. Gd₂O₃ and Y₂O₃ are thermodynamically more stable than MgO and preferentially form at the alloy surface, creating a dense barrier layer that inhibits further corrosion 5. Potentiodynamic polarization measurements show that Gd-containing amorphous alloys exhibit corrosion potentials 100-200 mV more noble than pure magnesium and passivation current densities 2-3 orders of magnitude lower 4.

The corrosion resistance of magnesium-based amorphous alloys is highly sensitive to the degree of crystallization. Partial crystallization introduces compositional heterogeneity and creates galvanic couples between the amorphous matrix and crystalline precipitates, significantly degrading corrosion performance 19. For example, Mg-Ni-Si alloys with >10% crystallinity exhibit corrosion rates 5-10 times higher than fully amorphous samples 19. This sensitivity necessitates careful control of processing parameters to maintain the amorphous structure and maximize corrosion resistance.

Long-term environmental stability is critical for practical applications. Accelerated aging tests at elevated temperatures (60-80°C) and high humidity (>90% RH) for 1000-2000 hours reveal that high-GFA magnesium-based amorphous alloys maintain their amorphous structure and corrosion resistance, with weight loss rates remaining below 1 mg/cm²/year 4. However, exposure to temperatures approaching Tx can trigger crystallization, leading to rapid degradation of properties. Therefore, the maximum service temperature for magnesium-based amorphous alloys is typically limited to 0.7-0.8 Tx (approximately 100-140°C for most compositions), which is adequate for many automotive and consumer electronics applications but may be insufficient for high-temperature aerospace applications 4.

Magnetic Properties And Electromagnetic Applications Of Magnesium-Based Amorphous Alloys

While magnesium itself is non-magnetic, the incorporation of magnetic rare-earth elements enables the development of magnesium-based amorphous magnetic alloys with unique properties. Compositions following the formula Mg₁₀₀₋ₓ₋ᵧAₓBᵧ, where A includes transition metals (Cu, Ni, Co, Zn, Al, Ag) at 2.5 ≤ x ≤ 35 atomic percent and B includes magnetic rare-earth elements (Gd, Nd, Tb, Er, Dy, Ho, Tm, Sm) at 20 ≤ y ≤ 40 atomic percent, exhibit maximum magnetization saturation values ≥10 emu/g 5. This represents a significant achievement, as it combines the lightweight characteristics of magnesium (density 1.74 g/cm³) with functional magnetic properties.

The magnetic properties of these alloys arise from the unpaired 4f electrons