Metals usually exist in a crystalline structure, with the metal ions arranged in an orderly lattice structure. We learn in school how this structure gives rise to the characteristic properties of metals, such as malleability. However, in real metal samples, there are often imperfections in the lattice structure, causing potential weakness and fracture along these lines.
In comparison, glass has an amorphous structure, which means the atoms within a sample of glass are not orderly arranged in a lattice arrangement.
Bulk metallic glasses are made up of metal elements, but exist in an amorphous structure. Usually, when a metal is cooled from a liquid, it favours the formation of a crystalline solid lattice. However, when a metal is cooled very rapidly, an amorphous solid structure can result. Alternatively, if it is an alloy with different metallic elements present, the different sizes of the atoms can result in an orderly crystalline structure being more difficult to form, and an amorphous structure can result upon rapid cooling.
Many of the desirable properties of bulk metallic glasses arise due to the lack of grain boundaries. In crystalline metals, grain boundaries form at the point where two lattice structures with different directions of arrangement meet.
Grain boundaries give rise to weakness along these “fault lines”, and is also where corrosion is likely to occur. Due to the lack of crystalline structures in bulk metallic glasses, they do not have grain boundaries, leading to a greater strength.
Currently, though, it is difficult to synthesise large blocks of bulk metallic glasses since the tendency of metals is ultimately still to form crystalline structures. Bulk metallic glasses are usually used to make small components and synthesised under special laboratory conditions allowing for rapid cooling from the molten state.