Amorphous Metals and Metallic Glasses: Production, Structure, and Properties

Amorphous metals — more commonly called metallic glasses — are solidified from the melt at rates fast enough to prevent crystallisation, trapping atoms in a disordered, liquid-like configuration. The absence of a periodic lattice eliminates grain boundaries, dislocations, and crystallographic anisotropy, producing a class of materials with exceptional strength-to-weight ratios, near-perfect elasticity, superior soft-magnetic behaviour, and outstanding corrosion resistance. This article examines the thermodynamic and kinetic foundations of glass formation, the alloy design strategies that enable centimetre-thick bulk castings, and the spectrum of engineering properties that distinguish metallic glasses from conventional crystalline alloys.

1. What Are Amorphous Metals?

Conventional crystalline metals possess a periodic, three-dimensional arrangement of atoms — the crystal lattice. When a crystalline metal solidifies from the melt, atoms diffuse into regular positions on the lattice planes characteristic of their crystal structure (BCC, FCC, or HCP for most elemental metals and simple alloys; see the BCC/FCC/HCP crystal structures guide for a detailed treatment). This ordering releases the latent heat of fusion and establishes the grain structure that dictates mechanical and physical behaviour.

Amorphous metals circumvent this ordering entirely. When a melt is quenched at or above the critical cooling rate R_c, atoms do not have sufficient thermal energy and time to rearrange into crystallographic positions. The result is a disordered, isotropic solid that retains the short-range atomic arrangement of the liquid (nearest-neighbour distances, coordination numbers, and local bonding chemistry) but lacks any long-range periodic order. X-ray diffraction (XRD) patterns of amorphous metals show only broad, diffuse humps characteristic of liquids — no sharp Bragg peaks corresponding to crystallographic planes.

1.1 Historical Development

The first metallic glass was produced in 1960 by Pol Duwez at Caltech: an Au₇₅Si₂₅ alloy quenched from the melt at approximately 10⁶ K/s by a splat-quenching technique ("gun method"). The discovery that metals could form glasses was unexpected — prevailing theory held that the high atomic mobility and non-directional metallic bonding in liquid metals would inevitably produce crystallisation upon cooling.

By the 1970s, Pond and Maddin at Allied Chemical developed melt spinning: a process that produces continuous amorphous ribbon by ejecting molten alloy onto a rapidly rotating copper drum. This enabled the first commercial metallic glass products, notably Fe-B and Fe-Si-B amorphous ribbon for transformer cores. The critical breakthrough to bulk glass formation came in the late 1980s and 1990s through work by Inoue (Tohoku University) and Johnson (Caltech), who identified multicomponent alloy systems — particularly Zr-based BMGs — with critical cooling rates below 1 K/s, enabling centimetre-scale castings.

2. Thermodynamics and Kinetics of Glass Formation

2.1 The Glass Transition

When a metallic liquid is cooled below its equilibrium melting point T_m without crystallising, it enters a metastable supercooled liquid state. As temperature falls, viscosity rises exponentially. At the glass transition temperature T_g, atomic diffusion becomes so sluggish (structural relaxation times exceed the experimental time scale) that the liquid structure is kinetically frozen in. T_g is not a thermodynamic phase transition — it is a kinetic arrest. It depends on cooling rate: faster cooling produces a slightly higher T_g and a higher-energy amorphous structure with greater free volume.

The temperature interval between T_g and the crystallisation onset temperature T_x defines the supercooled liquid region (SLR), denoted ΔT_x:

Wide ΔT_x (up to 127 K in Pd₄₃Cu₂₇Ni₁₀P₂₀) indicates strong resistance to crystallisation, which both improves glass-forming ability and enables thermoplastic forming operations on the glassy solid.

2.2 Glass-Forming Ability Criteria

Three quantitative parameters are widely used to assess and compare GFA across alloy systems:

Reduced Glass Transition Temperature (T_rg): Proposed by Turnbull, this is the ratio of glass transition temperature to melting temperature:

For T_rg ≥ 2/3, the viscosity of the supercooled liquid at T_m is sufficiently high that critical cooling rates become accessible by conventional casting. Alloys approaching T_rg = 2/3 include Vitreloy-class BMGs. The Kauzmann temperature T_K — where the supercooled liquid entropy equals the crystal entropy — lies below T_g and sets an absolute lower bound for T_g.

Gamma Parameter (γ): Proposed by Lu and Liu (2002), γ correlates better with maximum section thickness than T_rg alone:

Values of γ > 0.40 are associated with robust BMG-forming systems. The best Zr-based BMGs achieve γ ≈ 0.43–0.46.

Supercooled Liquid Region Width (ΔT_x): Wide ΔT_x indicates sluggish crystallisation kinetics in the frozen glass. Pd-Cu-Ni-P alloys show ΔT_x > 90 K, enabling extensive thermoplastic forming windows.

2.3 Inoue Empirical Rules for BMG Design

Akihisa Inoue identified three empirical design rules that correlate with high GFA in multicomponent metallic systems:

• Rule 1: Three or more components. Multicomponent systems increase configurational entropy of the liquid, impeding the cooperative atomic rearrangements needed for crystalline nucleation.
• Rule 2: Significant atomic size ratios (>12% difference). Large and small atoms together fill space more efficiently in the amorphous state, frustrating crystalline packing.
• Rule 3: Negative heats of mixing among constituent elements. Attractive interactions between unlike atoms favour the disordered liquid-like structure over ordered intermetallic or crystalline phases.

3. Critical Cooling Rate and Production Routes

3.1 Critical Cooling Rate

The critical cooling rate R_c is the minimum cooling rate required to suppress formation of any crystalline phase detectable by XRD. It is related to the time-temperature-transformation (TTT) diagram (Figure 1): R_c is determined by the slope of the tangent line from T_m just bypassing the nose of the crystallisation C-curve. R_c spans approximately 14 orders of magnitude across known amorphous alloy systems:

3.2 Melt Spinning (Thin Ribbon Production)

Melt spinning remains the dominant industrial process for amorphous ribbon. A stream of molten alloy (~1–2 mm diameter) is ejected through a nozzle under inert gas pressure onto the outer surface of a polished copper drum rotating at 15–40 m/s. Heat extraction through the drum achieves cooling rates of 10⁵–10⁶ K/s, producing continuous ribbon 20–50 µm thick and 2–25 mm wide. Ribbon geometry limits this process to applications such as transformer core laminations and brazing filler foils — section thickness cannot exceed ~0.1 mm for alloys requiring R_c > 10⁴ K/s.

3.3 Copper Mould Casting (Bulk Metallic Glasses)

For low-R_c BMG alloys, arc melting followed by injection or suction casting into water-cooled copper moulds achieves the required cooling rates at section thicknesses of millimetres to centimetres. The process requires:

• High-purity raw materials (<200 ppm oxygen, <50 ppm nitrogen) to avoid oxide and nitride heterogeneous nucleation sites
• Inert atmosphere throughout (high-purity argon, <0.5 ppm O₂ or high vacuum)
• Mould temperature control to manage interfacial heat transfer coefficients
• Rapid injection (pneumatic or electromagnetic) to fill thin-section moulds before crystallisation onset

3.4 Thermoplastic Forming (TPF)

Within the supercooled liquid region, metallic glasses undergo viscous Newtonian flow at stresses that are practical for metal forming operations. At temperatures T_g < T < T_x, viscosity η follows a Vogel-Fulcher-Tammann (VFT) relationship:

log(η) = log(η∞) + A / (T − T₀)

η∞: high-temperature limiting viscosity (~10⁻³ Pa·s)
A:  fragility-related constant
T₀: Vogel temperature (T₀ < T𝑔)

Viscosities of 10⁷–10⁹ Pa·s at T_g + 20 K enable isothermal pressing, blow moulding, embossing, and die forging of near-net-shape parts. Surface replication fidelity exceeds optical lithographic quality, enabling micro/nanoscale surface features directly from mould tooling. The process must be completed within the available time window before T_x is crossed and crystallisation commences.

4. Alloy Systems and Compositions

4.1 Zr-Based Bulk Metallic Glasses (Vitreloy Family)

The Zr-Ti-Cu-Ni-Be system discovered by Johnson and Peker at Caltech in 1993 remains the archetype for engineering BMGs. Vitreloy 1 (Zr_41.2Ti_13.8Cu_12.5Ni₁₀Be_22.5, at.%) has a T_g of 625 K, T_x of 705 K (ΔT_x = 80 K), T_m ≈ 993 K, and T_rg ≈ 0.63. It can be copper mould cast as rods up to 50 mm diameter. The commercial designation is Liquidmetal® 1 (Liquidmetal Technologies). Be addition is critical: Be has a very negative heat of mixing with Zr and a small atomic radius, strongly frustrating crystalline nucleation. However, beryllium toxicity restricts use in biomedical and food-contact applications.

Be-free Zr-Al-Ni-Cu alloys (e.g., Zr₆₅Al_7.5Ni₁₀Cu_17.5) achieve maximum cast thickness of 15 mm and are preferred for biomedical implants and consumer goods where beryllium exposure must be avoided.

4.2 Pd-Based BMGs

Pd₄₃Cu₂₇Ni₁₀P₂₀ (Inoue, 1997) achieves R_c ≈ 0.1 K/s and has been cast as rods 72 mm in diameter — the largest BMG cross-section reliably produced. Its ΔT_x ≈ 90 K provides an exceptional thermoplastic forming window. These alloys demonstrate near-perfect glass forming behaviour but their cost (Pd > $50/g) limits industrial application to precision instruments and jewellery.

4.3 Fe-Based BMGs and Amorphous Ribbons

Iron-based amorphous alloys bifurcate into two application classes:

• Soft magnetic ribbons (melt-spun): Fe₈₀B₂₀, Fe₇₈Si₉B₁₃, Fe₄₀Ni₃₈Mo₄B₁₈ (Metglas® alloys). Used in distribution transformer cores, magnetic sensors, and anti-theft tags. Saturation induction 1.4–1.8 T, coercivity <5 A/m.
• High-strength structural BMGs: Fe₄₈Cr₁₅Mo₁₄Er₂C₁₅B₆, Fe₄₁Co₇Cr₁₅Mo₁₄C₁₅B₆Y₂. These systems achieve yield strengths of 3.5–4.5 GPa and maximum section thicknesses of 5–12 mm. Their low cost relative to Zr- and Pd-based BMGs makes them attractive for structural and wear-resistant coatings applied by thermal spray.

4.4 Ti- and Cu-Based Systems

Ti₄₀Zr₂₅Cu₁₂Ni₃Be₂₀ (Liquidmetal® Ti) offers specific strength competitive with Ti-6Al-4V at lower density. Cu₄₇Ti₃₄Zr₁₁Ni₈ achieves ~12 mm maximum section and is notable for its room-temperature compressive plasticity arising from shear band multiplication. La- and Ce-based BMGs (e.g., La₅₅Al₂₅Ni₂₀) have T_g near room temperature, enabling room-temperature thermoplastic forming, but their mechanical properties are modest.

5. Mechanical Properties

5.1 Deformation Mechanisms: Shear Bands

In crystalline metals, plastic deformation proceeds by dislocation nucleation, multiplication, and glide on crystallographic slip planes (see the grain boundaries and dislocation guide for context). Because amorphous metals lack a periodic lattice, this mechanism is unavailable. Instead, plastic deformation in metallic glasses concentrates in extremely thin shear bands — planar regions 10–20 nm thick — where local free volume accumulates under applied shear stress, reducing viscosity and enabling localised flow. The shear band velocity approaches the speed of sound in the material once nucleated, leading to catastrophic failure in tension with near-zero macroscopic ductility in most BMGs.

The critical shear stress to nucleate a shear band follows an approximate relationship with the shear modulus G and activation volume Ω:

τ_c ≈ G/50    (empirical approximation for metallic glasses)

Yield stress: σ_y ≈ 2τ_c / (1 + μ_f)

where μ_f is the friction coefficient of the shear band
(typically 0.10–0.15 for BMGs)

This yields theoretical strengths of G/25 to G/10 — far above those achievable by crystalline alloys limited by Frank-Read source operation at G/1000 to G/100.

5.2 Mechanical Properties Summary

* UTS ≈ yield strength in metallic glasses due to near-zero work hardening in tension; failure occurs by propagation of a single dominant shear band.

5.3 Improving Ductility: BMG Composites

Monolithic BMGs fail in a brittle manner in tension. Two strategies produce measurable macroscopic ductility:

In-situ composite BMGs: Alloy composition is tuned to partially crystallise, leaving dendritic or spherical crystalline phases within the amorphous matrix. The dendritic phase (e.g., β-Ti dendrites in DH1–DH3 Zr-Ti-Nb-Cu-Be alloys from the Johnson group) transforms via stress-induced martensitic transformation under load, shielding advancing shear bands and nucleating new bands. Tensile elongations of 5–8% have been achieved with yield strengths exceeding 1.5 GPa.

Ex-situ composites: Reinforcing particles (W, Ta, SiC) or fibres are incorporated into the BMG melt before casting. These act as shear band deflectors and arrest propagation. Plasticity is modest (<3%) but repeatable.

6. Magnetic Properties of Amorphous Metals

6.1 Soft Magnetic Behaviour

Fe-Si-B and Fe-Ni-Mo-B amorphous ribbons exhibit outstanding soft magnetic properties that arise directly from the amorphous structure:

• No magnetocrystalline anisotropy: With no crystal lattice, there are no preferred crystallographic magnetisation directions. Domain walls move freely at very low applied fields.
• No grain boundaries: Boundaries in crystalline materials pin domain walls, requiring additional energy (coercive field) to move them.
• Low magnetostriction compositions: Alloy chemistry can be tuned to minimise magnetoelastic coupling (e.g., Fe₄₀Ni₃₈Mo₄B₁₈, Metglas 2826MB).

Metglas® 2605SA1 (Fe₇₈Si₉B₁₃) properties: saturation induction B_s = 1.56 T; coercivity H_c < 3 A/m; core loss at 60 Hz and 1.4 T of approximately 0.125 W/kg — compared to 1.5 W/kg for conventional cold-rolled silicon steel. Deployed in distribution transformers, amorphous core designs (e.g., ABB's AMDT technology) reduce no-load losses by up to 70% versus silicon steel cores.

6.2 Nanocrystalline Derivatives (FINEMET, NANOPERM)

Controlled annealing of amorphous Fe-Si-B-Cu-Nb ribbon through T_x produces nanocrystalline grains (10–15 nm diameter) embedded in a residual amorphous matrix. This FINEMET structure (Yoshizawa, 1988) achieves even lower coercivity (<0.5 A/m) and higher permeability than the fully amorphous precursor by exploiting random anisotropy averaging across the fine-grained structure (exchange correlation length ξ_ex > grain size). FINEMET Fe_73.5Cu₁Nb₃Si_13.5B₉ exhibits μ_i > 10⁵ at 1 kHz. This devitrification-by-design represents a case where controlled partial crystallisation of a metallic glass yields superior functional properties — see martensite formation for analogous transformation-controlled microstructure engineering in steels.

7. Corrosion Resistance

The corrosion behaviour of metallic glasses differs fundamentally from crystalline alloys. The absence of grain boundaries, second-phase particles, and compositional heterogeneity eliminates the preferential attack sites responsible for pitting corrosion, intergranular corrosion (see corrosion mechanisms), and galvanic attack at phase boundaries. The passive film that forms on corrosion-resistant metallic glasses (particularly Zr- and Fe-Cr-Mo-based compositions) is more chemically uniform and adherent than passive films on equivalent crystalline alloys.

Fe₄₈Cr₁₅Mo₁₄Er₂C₁₅B₆ amorphous alloy exhibits pitting potentials in 3.5 wt.% NaCl solution of +0.8 V_SCE — substantially higher than 316L stainless steel (+0.35 V_SCE) or even super-duplex stainless steel 2507 (+0.55 V_SCE). Zr-based BMGs in simulated body fluids (Hank's solution) show passive current densities below 0.5 μA/cm², competitive with pure titanium, supporting their use in biomedical implant applications.

8. Structural Relaxation and Stability

Metallic glasses are thermodynamically metastable. Even below T_g, atoms undergo slow structural relaxation — short-distance rearrangements that reduce enthalpy and free volume toward a more stable amorphous configuration. This relaxation is irreversible at sub-T_g temperatures and has measurable engineering consequences:

• Embrittlement: Reduced free volume suppresses shear band nucleation, decreasing compressive plasticity and fracture toughness (K_Ic can fall from ~40 MPa·m^0.5 to below 10 MPa·m^0.5 after annealing at T_g − 50 K).
• Dimensional change: Densification of ~0.1–0.5% accompanies relaxation, relevant to precision dimension-critical components.
• Magnetic property degradation: Coercivity of amorphous soft magnetic ribbons increases with prolonged sub-T_g annealing due to structural ordering around magnetic moments.

Rejuvenation — restoring free volume by mechanical deformation (shot peening, cold rolling), cryogenic thermal cycling, or brief annealing at T_g followed by quenching — can partially reverse structural relaxation, a topic of active research for restoring BMG ductility after service exposure.

9. Industrial Applications

9.1 Transformer Cores (Soft Magnetic Ribbon)

Amorphous metal distribution transformer cores represent the largest-volume commercial application of metallic glasses. ABB, Hitachi Metals (Metglas), and Vacuumschmelze produce toroidal and wound-core transformer designs in Fe-Si-B ribbon. The US Department of Energy estimates that widescale adoption of amorphous core transformers across the US grid would save approximately 70 TWh/year in no-load transformer losses.

9.2 Consumer Electronics and Precision Components

Liquidmetal Technologies has licenced BMG processing to Apple Inc. for small precision components (SIM card ejector pins, watch chassis). The combination of high hardness, near-net-shape castability, optical surface finish from TPF, and excellent corrosion resistance addresses requirements not met by conventional titanium or stainless steel components. Sporting goods include BMG golf club face inserts (Liquidmetal Golf) and tennis racquet frames exploiting the elastic energy storage advantage.

9.3 Medical Devices and Implants

Be-free Zr-based and Ti-based BMGs offer biocompatibility comparable to Ti-6Al-4V, higher hardness (important for wear-resistant articulating surfaces in orthopaedic implants), and MRI compatibility. TPF enables complex micro-textured surface features to promote osseointegration directly from the forming tool, without secondary machining.

9.4 Thermal Spray Coatings

Fe-Cr-Mo-C-B amorphous alloy powder is used as feedstock for High Velocity Oxy-Fuel (HVOF) and cold spray deposition of amorphous coatings on steel substrates. These coatings provide hardness of 900–1100 HV and pitting corrosion resistance superior to hard chrome plating, with potential as a hexavalent-chromium-free alternative in aerospace and oil and gas applications. See the corrosion protection coatings guide for context on coating selection and performance evaluation.

9.5 Brazing Filler Foils

Amorphous boron-containing Ni-Cr-Si-B and Au-Sn alloy ribbons are used as brazing filler metals. The amorphous structure provides uniform composition and thickness, predictable flow behaviour during brazing, and freedom from eutectic segregation that characterises cast filler metals. Applications include aerospace honeycomb panel brazing and electronic assembly. The relationship to heat treatment of brazed joints is discussed in the annealing and normalising guide.