An amorphous metal, also known as bulk metallic glass (BMG) is a solid metal alloy with disordered atomic-scale structure. These metal alloys are special in that they have a unique ability to stay amorphous when cooling from their molten liquid phase to solid phase as long as they are cooled fast enough. The material structure of the alloy stays disordered, putting it in what’s called a glassy state. These alloys are composed of a special combination of metals that give them good glass-forming ability (GFA).
Creating a glass is akin to racing against a thermodynamic clock. The clock begins ticking when a liquid is cooled below its melting temperature TM. Typically, as the supercooled liquid’s temperature decreases, so do its enthalpy and volume, as illustrated by the plot in figure 1a. (Water, which has a negative thermal expansion coefficient below 4 °C, is a notable exception.) At what’s known as the glass transition temperature TG, the atoms in the liquid become so tightly packed that they can no longer easily rearrange themselves. In that glassy state, the system behaves like a solid, although liquid-like disorder is “frozen” into its structure. The challenge, then, is to supercool the liquid from TM to TG so fast that there’s no chance for a crystalline phase to nucleate and grow.