Bulk Metallic Glass Oxidation Resistant Alloy: Composition Design, Corrosion Mechanisms, And Advanced Applications

Fundamental Composition Design And Glass-Forming Ability Of Oxidation Resistant Bulk Metallic Glasses

The development of oxidation resistant bulk metallic glasses (BMGs) requires careful balance between glass-forming ability (GFA) and corrosion resistance through strategic elemental selection 3811. Zr-Ti-based systems have emerged as primary candidates due to their intrinsic passivation behavior and wide supercooled liquid region (ΔTx), typically 30-60°C, enabling thermoplastic forming operations 613. The compositional formula for high-performance oxidation resistant BMGs commonly follows Zr₍ₐ₎Ti₍ᵦ₎Cu₍ᵧ₎Ni₍ᵨ₎Al₍ₑ₎ structures, where precise control of atomic percentages determines both amorphous phase stability and surface passivation kinetics 315.

Critical to oxidation resistance is the elimination or minimization of highly electronegative elements such as Cu and Ni, which create galvanic couples promoting localized corrosion 3. Research demonstrates that Zr-Ti BMGs with reduced Cu content (below 15 at%) and controlled Ni levels exhibit corrosion rates 2-5 times lower than conventional crystalline alloys in oxidizing environments 312. The addition of refractory metals (Nb, Ta, Mo) at 2-20 at% further enhances oxidation resistance by forming stable oxide barriers and increasing glass transition temperature (Tg) above 400°C 8918.

Zr-Based Bulk Metallic Glass Systems With Enhanced Oxidation Resistance

Zr-based BMGs represent the most extensively studied oxidation resistant amorphous alloys, with compositions optimized for both GFA and corrosion performance 3614. The alloy system Zr₅₈.₄₇Nb₂.₇₆Cu₁₅.₄Ni₁₂.₆Al₁₀.₃₇ demonstrates critical casting thickness (dc) exceeding 10 mm while maintaining superior oxidation resistance through Nb-enriched passive films 56. Thermal stability analysis reveals ΔTx values of 45-55°C and Tg ranging from 380-420°C, providing substantial processing windows for thermoplastic forming without crystallization 613.

Compositional modifications targeting oxidation resistance focus on three strategies: (1) increasing Zr/Ti ratio to enhance passive film stability, (2) incorporating oxygen-scavenging elements (Hf, Y) at 0.5-3 at%, and (3) optimizing Cu/Ni ratios below 1.15 to minimize electrochemical heterogeneity 368. The Zr₃₆₋ₙ₋ₘNi₆₄₊ₙAlₘ system (where -5≤n≤5, 0≤m≤5) exhibits exceptional corrosion resistance when n=0 and m=0, attributed to homogeneous amorphous structure without compositional fluctuations that could initiate localized attack 14. Electrochemical impedance spectroscopy (EIS) measurements show polarization resistance values exceeding 10⁶ Ω·cm² in 3.5 wt% NaCl solution, compared to 10⁴-10⁵ Ω·cm² for crystalline Zr alloys 14.

Ni-Based And Cu-Based Bulk Metallic Glass Alloys For Extreme Oxidation Environments

Ni-based BMGs containing high refractory metal content (15-20 at% Nb, Ta, or Mo) provide superior oxidation resistance in highly aggressive environments, including boiling nitric acid and molten metal contact 91218. The Ni-Nb-Zr-Ti-Ta system with composition Ni₅₅₋₆₀Nb₁₅₋₂₀Zr₅₋₂₀Ti₀₋₁₅Ta₀₋₁₅ demonstrates corrosion rates below 0.1 mm/year in 5 mol/L boiling HNO₃ containing oxidizing ions (Pu⁴⁺, Cr₂O₇²⁻), significantly outperforming NAR-310Nb stainless steel (0.5-1.2 mm/year) 12. The exceptional resistance derives from rapid formation of dense Nb₂O₅-enriched passive layers (5-15 nm thickness) that inhibit further oxidation 12.

Cu-based BMG systems, particularly Cu₁₀₀₋ₐ₋ᵦZrₐAlᵦ (30≤a≤60, 0≤b≤15), offer cost-effective oxidation resistance combined with excellent electrical conductivity (2-4 × 10⁶ S/m) for electrochemical applications 17. Partial substitution of Zr or Al with Nb/Ta (up to 10 at% each) enhances corrosion resistance by stabilizing the amorphous phase and forming mixed oxide barriers 17. These alloys exhibit no crystalline grain boundaries—the primary oxidation initiation sites—resulting in uniform passive film formation and corrosion current densities 10-50 times lower than crystalline Cu alloys in acidic environments 17.

Fe-based BMGs with composition (Fe,Cr)₁₀₀₋₍ₐ₊ᵦ₊ᵧ₎WₐTMᵦ(C,B,P)ᵧ (TM=Mo, Ta, V, Nb; a=2-20 at%, b=0-15 at%, c=20-30 at%, Fe=35-55 at%) demonstrate exceptional resistance to molten zinc corrosion when ΔTx≥30°C and Tg exceeds zinc melting point by ≥20°C 18. The W and TM additions promote formation of stable tungsten and transition metal oxides that act as diffusion barriers against zinc penetration 18.

Oxidation Mechanisms And Passive Film Formation In Bulk Metallic Glass Oxidation Resistant Alloys

The superior oxidation resistance of BMGs fundamentally derives from their amorphous structure, which eliminates fast diffusion paths (grain boundaries, dislocations) present in crystalline alloys 31214. Oxidation kinetics in BMG systems typically follow parabolic or logarithmic rate laws, indicating protective oxide scale formation, whereas crystalline counterparts often exhibit linear or breakaway oxidation 318. Thermogravimetric analysis (TGA) of Zr-Ti-Cu-Ni-Al BMGs in air at 400-600°C reveals weight gain rates of 0.05-0.2 mg/cm²·h, compared to 0.5-2.0 mg/cm²·h for crystalline Zr alloys under identical conditions 36.

Passive Oxide Layer Composition And Structure In Bulk Metallic Glass Oxidation Resistant Alloys

X-ray photoelectron spectroscopy (XPS) analysis of oxidized Zr-Ti BMG surfaces reveals stratified passive films consisting of outer ZrO₂/TiO₂ mixed oxide layer (10-30 nm) and inner oxygen-enriched amorphous zone (20-50 nm) 314. The oxide composition ratio ZrO₂:TiO₂ typically ranges from 2:1 to 4:1, with higher Zr content promoting tetragonal ZrO₂ phase formation that provides superior mechanical stability and lower oxygen diffusivity (D_O ≈ 10⁻¹⁸ cm²/s at 500°C) compared to monoclinic ZrO₂ 3. Incorporation of Nb at 2-5 at% results in Nb₂O₅ enrichment at the oxide/metal interface, creating a secondary diffusion barrier that reduces oxidation rates by 40-60% 6812.

For Ni-based BMGs in highly oxidizing nitric acid environments, the passive film consists primarily of NiO (outer layer, 5-10 nm) and Nb₂O₅/Ta₂O₅ (inner layer, 10-20 nm), with the refractory metal oxides providing chemical stability against acid dissolution 12. Auger electron spectroscopy (AES) depth profiling confirms oxygen concentration gradients from 60-65 at% at the surface to 5-10 at% at 50 nm depth, indicating controlled oxygen ingress rather than catastrophic oxidation 12. The absence of grain boundaries prevents preferential oxidation paths, resulting in uniform film thickness variation within ±2 nm across macroscopic surfaces 1214.

Cu-based BMGs develop Cu₂O/CuO duplex oxide structures with Zr-enriched sublayers that stabilize the oxide against reduction in electrochemical environments 17. The amorphous substrate provides continuous supply of Zr to the oxide/electrolyte interface, maintaining passive film integrity even under anodic polarization up to +0.8 V vs. saturated calomel electrode (SCE) 17.

Role Of Active Elements And Oxygen Scavengers In Bulk Metallic Glass Oxidation Resistant Alloys

Strategic incorporation of active metals (Hf, Y, La, Ce) at 0.5-3 at% significantly enhances long-term oxidation resistance through reactive element effect (REE) mechanisms 4. These elements preferentially oxidize to form stable oxide pegs (HfO₂, Y₂O₃) at the oxide/metal interface, improving oxide scale adhesion and reducing growth stresses that cause spallation 4. Coating studies on Ni-based substrates demonstrate that Hf-containing γ′-Ni₃Al phases act as Hf reservoirs, continuously supplying Hf to the growing oxide scale and maintaining protective Al₂O₃ layer adhesion for >1000 hours at 1100°C 4.

The oxidation resistant alloy coating process involves embedding metal substrates in diffusion agents containing metal oxides (Al₂O₃, Cr₂O₃, SiO₂), active metals (Hf, Zr, Y, Ti, La, Ce, Mg, Ca), and catalytic compounds (NH₄Cl, NH₄F, HCl, NaCl, NaF), followed by heat treatment at 700-1340°C for 1 minute to 25 hours in inert or hydrogen atmospheres 4. This process creates multi-layered oxide structures with active metal concentrations varying from 2-8 at% in different phases, providing both immediate oxidation protection and long-term oxide scale stability 4.

For BMG systems, in-situ formation of active element oxides during initial oxidation creates nucleation sites for protective oxide phases, refining oxide grain size and reducing oxygen diffusion coefficients by 1-2 orders of magnitude 48. The presence of 1-2 at% Hf in Zr-based BMGs results in HfO₂ particle dispersion (5-20 nm diameter) within the ZrO₂ matrix, acting as oxygen getters and stress accommodation sites that prevent oxide cracking under thermal cycling 8.

Processing Technologies And Microstructural Control For Bulk Metallic Glass Oxidation Resistant Alloys

Manufacturing oxidation resistant BMGs requires precise control of cooling rates, oxygen contamination, and compositional homogeneity to achieve fully amorphous structures with optimal corrosion performance 811. Critical cooling rates for Zr-Ti-based systems range from 1-100 K/s depending on composition, with lower Cu content alloys requiring faster cooling to suppress crystallization 61315. Maximum achievable casting thickness varies from 3-15 mm for different alloy systems, with Zr₅₈.₄₇Nb₂.₇₆Cu₁₅.₄Ni₁₂.₆Al₁₀.₃₇ demonstrating dc ≈ 12 mm under copper mold casting conditions 56.

Melt Processing And Oxygen Control In Bulk Metallic Glass Oxidation Resistant Alloy Production

High-purity raw materials (≥99.9% purity) are essential for achieving optimal oxidation resistance, as trace impurities (Fe, Si, C) can act as heterogeneous nucleation sites promoting crystallization and creating compositional inhomogeneities that compromise corrosion performance 811. Arc melting under high-purity argon atmosphere (O₂ < 1 ppm, H₂O < 0.5 ppm) followed by suction casting into copper molds represents the standard laboratory-scale production method 81315. Industrial-scale production employs induction melting in ceramic crucibles (Y₂O₃-stabilized ZrO₂ or graphite) with continuous oxygen monitoring and gettering systems (Ti sponge) to maintain melt oxygen content below 100 ppm 8.

Interestingly, controlled oxygen addition (0.1-0.5 at%) can enhance GFA in certain Zr-based systems by modifying liquid structure and increasing viscosity, as demonstrated in the alloy system x(aZr_bHf_cM_dNb_eO)yCu_zAl where intentional oxygen incorporation (e=0.1-0.5 at%) increases dc by 15-25% without compromising oxidation resistance 811. This counterintuitive effect results from oxygen-induced short-range ordering that stabilizes the supercooled liquid against crystallization 8. However, oxygen levels exceeding 0.8 at% lead to oxide particle formation that degrades mechanical properties and creates preferential corrosion sites 8.

Thermal spray techniques, particularly high-velocity oxygen fuel (HVOF) spraying, enable deposition of oxidation resistant BMG coatings (50-500 μm thickness) onto conventional alloy substrates 212. Ni-Nb-Zr-Ti-Ta BMG powders (15-45 μm particle size) sprayed at velocities of 600-800 m/s and substrate temperatures of 150-250°C produce coatings with >95% amorphous content and porosity <2%, exhibiting corrosion resistance equivalent to bulk BMG materials 12. Post-spray heat treatment at 0.85-0.95 Tg for 10-30 minutes can further densify coatings and relieve residual stresses without inducing crystallization 12.

Additive Manufacturing And Functionally Graded Bulk Metallic Glass Oxidation Resistant Structures

Additive manufacturing (AM) technologies, including selective laser melting (SLM) and directed energy deposition (DED), enable fabrication of complex-geometry BMG components with tailored oxidation resistance 2. Laser processing parameters critically influence amorphous phase retention: laser power 150-400 W, scan speed 200-1200 mm/s, layer thickness 20-50 μm, and hatch spacing 50-120 μm produce cooling rates of 10³-10⁶ K/s, sufficient for vitrification of most BMG compositions 2. However, thermal accumulation during multi-layer deposition can cause partial crystallization, requiring interlayer cooling intervals (5-15 seconds) or substrate cooling (maintained at 50-150°C) 2.

Functionally graded BMG coatings with thickness ≥0.05 mm demonstrate superior wear and oxidation resistance through controlled microstructural gradients from fully amorphous surface layers to nanocrystalline/amorphous composite subsurface regions 2. The graded structure, achieved through systematic variation of laser energy density (50-200 J/mm³) during deposition, provides both surface hardness (800-1200 HV) for wear resistance and subsurface toughness (fracture toughness K_IC = 20-50 MPa√m) to prevent coating delamination 2. Base metal-transition metal-boron-silicon BMG systems (e.g., Fe₄₀