a metallic glass
Ordinary metals are crystals: cool molten metal and its atoms leap almost instantly into a neat repeating lattice of grains, which is why a snapped steel bar shows a grainy face. Metals crystallise so eagerly that for a long time everyone assumed a metal simply could not be made into a glass. A metallic glass proves otherwise: it is a metal or alloy cooled so fast that its atoms are frozen in the disordered, liquid-like tangle of a glass before they can ever line up into crystal grains. It is a genuine metal — shiny, conducting, often magnetic — but with no crystal lattice inside.
Structurally, a metallic glass is best pictured as a frozen dense random packing of atoms: no unit cell, no grains, no long-range order, just a jammed disordered arrangement with real short-range order (each atom has a well-defined shell of nearest neighbours, often in five-fold, icosahedral clusters). Because metal atoms are roughly like hard spheres with no strong directional bonds, they crystallise very easily, so the challenge is speed. The first metallic glass, a gold-silicon alloy made by Pol Duwez in 1960, needed cooling of about a million degrees per second to beat crystallisation — achievable only as a thin splat or ribbon a few tens of micrometres thick. The trick that later allowed thicker 'bulk' metallic glasses was chemistry: mixing several atoms of very different sizes (as in zirconium-copper-nickel-aluminium alloys) jams the packing and makes crystallisation so awkward that the melt vitrifies at far gentler cooling rates, letting glasses be cast centimetres thick.
Metallic glasses matter both scientifically and practically. Scientifically they are the cleanest real example of random close packing and of geometric frustration resisting crystallisation. Practically, the absence of grains and dislocations gives them unusual behaviour: some are extremely strong and springy, and amorphous iron-based ribbons are superb soft magnets used in efficient transformer cores. One honest limitation to keep straight: 'metallic glass' names a STRUCTURE (amorphous metal), the disordered atomic arrangement — it does not, by itself, tell you the properties, and metallic glasses are also metastable, meaning that if reheated enough they will crystallise and lose their glassy state.
Molten gold-silicon alloy sprayed as a fine jet onto a cold spinning copper wheel is chilled at roughly a million degrees per second, freezing into a metre-long amorphous ribbon in a fraction of a second — the 1960 experiment that first proved a metal can be a glass. Modern zirconium-based bulk metallic glasses, by contrast, vitrify at only tens of degrees per second and can be cast as solid rods a centimetre across.
A metal frozen too fast to crystallise: amorphous inside, yet shiny, conducting, and metallic.
Metallic glass names the amorphous STRUCTURE, not a property, and the glass is metastable: heat it enough and it crystallises, reverting to an ordinary grainy metal. 'Bulk' metallic glasses are not glassier — they simply resist crystallisation well enough to form at slower cooling rates.