Amorphous metals, or metallic glasses, are alloys with a disordered, non-crystalline atomic structure, similar to that of common glass. This is achieved by cooling the molten alloy at extremely high rates (e.g., [latex]10^6[/latex] K/s), preventing the atoms from organizing into a regular crystal lattice. Lacking grain boundaries, they exhibit unique properties like high strength, elasticity, and resistenza alla corrosione.
The creation of an amorphous metal requires bypassing crystallization during solidification. Every liquid, when cooled, has a termodinamico driving force to crystallize, but this process takes time for atoms to arrange themselves. By cooling the molten alloy faster than this critical rate, the liquid’s disordered structure is ‘frozen’ into the solid state. The first amorphous alloy, Au₇₅Si₂₅, was produced by Pol Duwez’s team using a ‘gun technique’ to propel a molten droplet onto a cold copper substrate, achieving the necessary rapid quenching.
A significant challenge was the limitation to very thin ribbons or foils, as heat could not be extracted quickly enough from thicker sections. This guidato to the development of Bulk Metallic Glasses (BMGs). BMGs are multi-component alloys (typically 3 or more elements) with large atomic size mismatches, which frustrates the crystallization process and dramatically lowers the required critical cooling rate, allowing for the casting of parts several centimeters thick. Unlike crystalline metals that deform plastically through dislocation motion, metallic glasses deform elastically to much higher strains. Once their elastic limit is exceeded, deformation localizes into narrow regions called shear bands, often leading to catastrophic failure with little to no overall ductility. This combination of extreme strength but limited toughness is a key area of ongoing research.
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