What are metallic glasses?

The word ‘glass’ as we normally use it refers to window glass. As we all know, this is a brittle, transparent material with vanishingly small electrical conductivity. It is in fact a material in which the constituent molecules are arranged in a disordered fashion as in a liquid but not moving around; that is to say, each molecule keeps its same neighbours and the glass behaves like a solid. Most of the solids that physicists have hitherto dealt with are crystalline i.e. their atoms or molecules are arranged in strictly ordered arrays. This is the essential difference between a so-called ‘glass’ and a crystal: a glass has no long-range order. Although the word ‘glass’ was originally used to designate only window glass it has now taken on this generalised meaning of what we may call an amorphous solid.

Electrically insulating glasses have been studied for a long time and it was generally thought that in order to form a glass by cooling a liquid it was necessary to have a material composed of fairly complicated molecules so that, on cooling through the temperature range at which crystallisation would be expected to occur, the molecules would have difficulty in getting into their proper places and could be, as it were, frozen in a disordered pattern at lower temperatures without the thermal energy necessary to get into their ordered positions. This general picture is correct and helpful although the expectations based on it have proved in some respects wrong. It was thought that because metals and alloys are usually of simple atoms, it would be impossible to form a glass from such constituents.