Amorphous Alloy And Glassy Alloy: Comprehensive Analysis Of Composition, Processing, And Engineering Applications

Historical Evolution And Glass-Forming Ability Of Amorphous Alloys

The development trajectory of amorphous alloys has been marked by continuous improvements in glass-forming ability (GFA), which directly correlates with the critical cooling rate required to suppress crystallization during solidification. Early amorphous alloys, first produced in the 1960s with Fe-P-C-based compositions, necessitated extremely high cooling rates on the order of 10⁵ °C/s to achieve an amorphous structure 1,2. This requirement imposed severe geometric constraints, limiting these materials to thin ribbons, foils, or powders with dimensions typically below 100 micrometers in at least one direction 1,2. The rapid quenching was accomplished through techniques such as melt-spinning onto cooled substrates, thin-layer casting, or splat quenching, all designed to extract heat at rates sufficient to bypass the crystallization nose in time-temperature-transformation diagrams 3.

A paradigm shift occurred in the early 1990s with the discovery of bulk-solidifying amorphous alloys, predominantly based on Zr-Ti and related multicomponent systems 1,2. These alloys exhibited dramatically reduced critical cooling rates—often below 10³ °C/s and in some cases as low as 10 °C/s—enabling the formation of fully amorphous articles with critical casting thicknesses ranging from approximately 1.0 mm to 20 mm 1,2. Representative alloy families demonstrating this enhanced GFA include Zr-Ti-Ni-Cu-Be, Zr-Ti-Ni-Cu-Al, Mg-Y-Ni-Cu, La-Ni-Cu-Al, and various Fe-based compositions 1,2. The improved processability of these bulk metallic glasses (BMGs) has facilitated their shaping into three-dimensional objects using conventional manufacturing techniques such as metal mold casting, die casting, and injection molding 1,2.

The glass-forming ability of an amorphous alloy can be quantitatively assessed through several criteria, including the reduced glass transition temperature (Trg = Tg/Tl, where Tg is the glass transition temperature and Tl is the liquidus temperature), the supercooled liquid region (ΔTx = Tx - Tg, where Tx is the crystallization temperature), and the critical casting thickness 8. For instance, Cu-based amorphous alloys with optimized compositions exhibit Trg values of 0.56 or higher, indicating excellent stability of the supercooled liquid state and facilitating thermoplastic forming operations 8. The wide supercooled liquid region observed in these alloys—often exceeding 50 K—provides a processing window for near-net-shape fabrication through viscous flow forming at temperatures between Tg and Tx 8.

Compositional Design Principles And Alloy Systems

Zr-Based And Ti-Based Bulk Amorphous Alloys

Zr-based amorphous alloys constitute one of the most extensively studied BMG families due to their exceptional combination of glass-forming ability, mechanical strength, and corrosion resistance 4,12. A typical composition range for Zr-based BMGs includes 66-68 mass% zirconium (Zr), 24.5-26.5 mass% copper (Cu), 2.6-4.6 mass% aluminum (Al), and 2.9-4.9 mass% nickel (Ni) as main component metals 11. The control of inevitable impurities is critical for achieving high amorphous ratios: oxygen (O) content should be maintained between >10 and <2,000 ppm, hydrogen (H) between >1 and <150 ppm, nitrogen (N) between >10 and <500 ppm, carbon (C) between >10 and <500 ppm, and iron (Fe) between >1 and <500 ppm 11. These stringent impurity limits are necessary because even minor deviations can act as heterogeneous nucleation sites, promoting premature crystallization during cooling and reducing the achievable amorphous fraction 11.

Zr-based amorphous alloys designed for biomedical applications require additional compositional constraints to ensure biocompatibility 4. Specifically, nickel content must be restricted to less than 1 atomic percent to minimize cytotoxic and allergenic responses 4. Such Ni-free or Ni-lean compositions maintain good glass-forming ability while meeting regulatory requirements for implantable devices, with critical cooling rates remaining below 10⁶ K/s 4. The alloys should exhibit miscibility over a wide composition and temperature range to facilitate processing flexibility and enable tailoring of mechanical properties for specific clinical applications 4.

Ni-Based Refractory Amorphous Alloys

Ni-based bulk amorphous alloys, particularly those based on the Ni-Nb-Sn ternary system and Ni-Cu-Ti-Zr-Al quaternary/quinary systems, represent an important class of refractory metallic glasses with elevated glass transition temperatures and enhanced thermal stability 1,2,5. The Ni-Nb-Sn ternary system demonstrates robust glass-forming ability across specific composition ranges, enabling the production of bulk amorphous samples through conventional casting methods 5. These alloys typically require cooling rates on the order of 10³ to 10⁵ °C/s, which is significantly lower than the rates needed for early-generation amorphous alloys but somewhat higher than those for optimized Zr-based BMGs 5.

The Ni-Cu-Ti-Zr-Al system offers additional compositional flexibility and can be tailored to achieve critical casting thicknesses exceeding several millimeters 1,2. The inclusion of aluminum in these alloys serves multiple functions: it reduces the overall density, enhances oxidation resistance, and modifies the electronic structure to stabilize the amorphous phase 1,2. Copper additions improve the castability by lowering the liquidus temperature and increasing the supercooled liquid region, while titanium and zirconium contribute to the formation of a dense, topologically disordered atomic network that resists crystallization 1,2. The resulting alloys exhibit high strength (typically 1.5-2.0 GPa in compression), large elastic strain limits (approximately 2%), and good fracture toughness, making them suitable for structural applications in demanding environments 1,2.

Fe-Based Soft Magnetic Amorphous Alloys

Fe-based amorphous alloys have garnered significant attention for soft magnetic applications due to their combination of high saturation magnetization, low coercivity, and excellent magnetic permeability 6,10,15,16. A representative Fe-based soft magnetic glassy alloy composition includes iron as the primary constituent, with additions of phosphorus, boron, silicon, and various transition metals to stabilize the amorphous phase and optimize magnetic properties 6,16. For example, Fe-Al-Ga-P-C-B-Si-based alloys can be produced as ribbons with thicknesses up to approximately 200 μm using single-roll melt-spinning techniques 16. These alloys exhibit a wide supercooled liquid region, facilitating thermoplastic forming operations and enabling the production of bulk components for magnetic cores in transformers, choke coils, and electromagnetic interference shielding applications 16.

The glass transition temperature (Tg) and crystallization temperature (Tx) are critical parameters for Fe-based soft magnetic glassy alloys, as they define the processing window for thermoplastic forming and the upper service temperature limit 6. Typical Tg values range from 400 to 500 °C, with Tx occurring 30-80 K above Tg, depending on the specific composition 6. The term "amorphous" in this context refers to the absence of long-range atomic order, as evidenced by the lack of Bragg peaks in X-ray diffraction spectra, although the presence of nanoscale crystalline precipitates within the amorphous matrix is not excluded 6. Such nanocrystalline phases, when present in controlled volume fractions, can enhance soft magnetic properties by reducing magnetocrystalline anisotropy and increasing saturation magnetization 6.

Co-based metallic glass alloys represent another important category of soft magnetic materials, characterized by low coercive force and high glass-forming ability 15. These alloys, which include compositions such as Co-Fe-Si-B and Co-Zr-Nb-B, can be produced as bulk samples with thicknesses significantly exceeding the 200 μm limit of conventional melt-spun ribbons 15. The development of Co-based BMGs has progressed from early rapid-solidification alloys (requiring cooling rates of 10⁴ K/s or more) to modern compositions exhibiting high GFA and enabling casting of larger-section components 15. The soft magnetic characteristics of Co-based metallic glasses, including high saturation magnetization and low coercivity, make them attractive for high-frequency transformer cores, magnetic sensors, and actuator applications 15.

Cu-Based And Mg-Based Amorphous Alloys

Cu-based amorphous alloys have emerged as promising candidates for structural and functional applications due to their excellent glass-forming ability, high strength, and relatively low material cost compared to Zr-based BMGs 8,13. A typical Cu-based BMG composition includes copper as the primary element, with additions of zirconium, beryllium, and one or more elements selected from aluminum, tin, silicon, and transition metals (excluding Cu and Zr) 13. For example, Cu-Zr-Be-M alloys (where M represents Al, Sn, Si, or transition metals) exhibit reduced glass transition temperatures (Trg) of 0.56 or higher, indicating excellent stability of the supercooled liquid and facilitating thermoplastic forming into complex shapes 8,13. The wide supercooled liquid region (ΔTx) observed in these alloys—often exceeding 60 K—enables near-net-shape processing through blow molding, compression molding, or extrusion at temperatures between Tg and Tx 8.

An alternative Cu-based composition replaces beryllium with rare-earth elements (RE) to form Cu-Zr-RE-M alloys, where RE is selected from the lanthanide series 13. This substitution eliminates the toxicity concerns associated with beryllium while maintaining good glass-forming ability and mechanical properties 13. The preparation method for Cu-based amorphous alloys typically involves melting the raw materials (Cu, Zr, Be or RE, and M) in an inert atmosphere or vacuum, followed by rapid solidification through techniques such as copper mold casting, injection casting, or melt-spinning 13. The resulting amorphous alloys exhibit compressive strengths in the range of 1.8-2.2 GPa, elastic strain limits of approximately 2%, and fracture toughness values comparable to or exceeding those of high-strength aluminum alloys 8,13.

Mg-based amorphous alloys offer the advantage of low density (approximately 2.0-2.5 g/cm³) combined with good glass-forming ability and ductility, making them attractive for lightweight structural applications 9. A representative composition range for Mg-based BMGs is Mg₁₀₀₋ₓ₋ᵧAₓBᵧ, where x and y satisfy 2.5 ≤ x ≤ 30 and 2.5 ≤ y ≤ 20 in atomic percent 9. Element A includes at least one selection from Cu, Ni, Zn, Al, Ag, and Pd, while element B includes at least one selection from Gd, Y, Ca, and Nd 9. The inclusion of rare-earth elements (Gd, Y, Nd) and alkaline-earth elements (Ca) enhances the glass-forming ability by increasing the atomic size mismatch and the negative heat of mixing, both of which stabilize the amorphous phase against crystallization 9. Mg-based BMGs exhibit compressive strengths of 500-800 MPa, elastic strain limits of 1.5-2.5%, and improved ductility compared to Zr-based and Cu-based BMGs, with plastic strain values reaching 1-3% in compression 9.

Specialty Amorphous Alloys For Functional Applications

Beyond structural applications, certain amorphous alloy compositions have been developed for specialized functional purposes, including information recording, corrosion resistance, and catalysis 10,14. For instance, Fe-Co-P-W amorphous alloys produced by electrolytic deposition exhibit high saturation magnetization, excellent corrosion resistance, and crystallization temperatures exceeding 450 °C 10. The composition is represented by the general formula (Fe₁₋ₐCoₐ)₁₋ₓ₋ᵧ₋ᵧPₓWᵧMᵧ, where 0 ≤ a ≤ 0.9, 0.04 ≤ x ≤ 0.16, 0.005 ≤ y ≤ 0.05, 0 ≤ z ≤ 0.2, and M represents at least one transition metal element other than Fe, Co, and W 10. The electrolytic deposition process is conducted in an acidic electrolytic bath using phosphorous acid or its salts as the phosphorus source and sodium tungstate as the tungsten source, or alternatively using sodium phosphotungstate as a combined P and W source 10. This synthesis route enables the production of thin amorphous coatings with controlled composition and thickness, suitable for magnetic recording media, corrosion-resistant coatings, and electromagnetic shielding applications 10.

Fe-Te amorphous alloys, comprising iron and tellurium with Te content ranging from 14 to 90 atomic percent, represent another specialty composition with applications in optical recording materials and corrosion-resistant coatings 14. These alloys exhibit excellent corrosion resistance and thermal stability, with the amorphous phase remaining stable up to temperatures approaching 300 °C 14. The production process typically involves rapid solidification techniques such as melt-spinning or vapor deposition, followed by controlled annealing to optimize the microstructure and functional properties 14. The unique combination of optical, magnetic, and chemical properties in Fe-Te amorphous alloys makes them suitable for phase-change memory devices, magneto-optical recording media, and protective coatings in harsh chemical environments 14.

Processing And Manufacturing Techniques For Amorphous Alloys

Rapid Solidification And Casting Methods

The production of amorphous alloys fundamentally relies on achieving cooling rates sufficient to suppress crystallization during solidification from the liquid state. For early-generation amorphous alloys, this necessitated cooling rates on the order of 10⁵ to 10⁶ °C/s, which could only be achieved through techniques such as melt-spinning, planar flow casting, or atomization 1,2,3. In melt-spinning, a stream of molten alloy is ejected onto the surface of a rapidly rotating copper wheel maintained at near-ambient temperature, resulting in the formation of continuous ribbons with thicknesses typically ranging from 20 to 100 μm 3,16. The high thermal conductivity of the copper substrate and the thin cross-section of the ribbon enable the required heat extraction rate, but the resulting product geometry is limited to thin strips or foils 3.

For bulk-solidifying amorphous alloys with enhanced glass-forming ability, conventional casting methods such as copper mold casting, die casting, and injection casting become viable 1,2,8. In copper mold casting, the molten alloy is poured into a water-cooled copper mold with the desired cavity geometry, and the high thermal conductivity of the mold walls provides sufficient cooling to suppress crystallization in sections up to 20 mm in thickness 1,2. Die casting and injection casting offer additional process control and enable the production of complex three-dimensional shapes with near-net-shape accuracy 1,2. The critical casting thickness achievable for a given alloy composition depends on the critical cooling rate, the thermal diffusivity of the alloy, and the heat transfer coefficient at the mold-metal interface 1,2.

Thermoplastic Forming And Secondary Processing

One of the unique advantages of bulk metallic glasses is their ability to undergo thermoplastic forming in the supercooled liquid region between Tg and