Amorphous metal

An amorphous metal is a metallic material (usually an alloy rather than a pure metal) with a disordered atomic-scale structure. In contrast to most metals, which are crystalline and therefore have a highly ordered arrangement of atoms, amorphous alloys are non-crystalline. Materials in which such a disordered structure is produced directly from the liquid state during cooling are called "glasses", and so amorphous metals are commonly referred to as "metallic glasses" or "glassy metals". However, there are several other ways in which amorphous metals can be produced, including physical vapor deposition, solid-state reaction, ion irradiation, and mechanical alloying. Amorphous metals produced by these techniques are, strictly speaking, not glasses, but scientists commonly consider amorphous alloys to be a single class of materials, regardless of how they are prepared.

The first metallic glass was a gold-silicon alloy, produced at Caltech by Pol Duwez in 1957. This and other early glass-forming alloys had to be cooled extremely rapidly (on the order of one megakelvin per second) to avoid crystallization. An important consequence of this was that metallic glasses could only be produced in a limited number of forms (typically ribbons, foils, or wires) in which one dimension was small so that heat could be extracted quickly enough to achieve the necessary cooling rate. As a result, metallic glass specimens (with a few exceptions) were limited to thicknesses of less than one-tenth of a millimeter.

In the 1990s, however, new alloys were developed that form glasses at cooling rates as low as one kelvin per second. These cooling rates can be achieved by simple casting into metallic molds. These "bulk" amorphous alloys can be cast into parts of up to several centimeters in thickness (the maximum thickness depending on the alloy) while retaining an amorphous structure. The best glass-forming alloys are based on zirconium and palladium, but alloys based on iron, titanium, copper, magnesium, and other metals are also known.

Amorphous alloys have a variety of potentially useful properties. In particular, they tend to be stronger than crystalline alloys of similar chemical composition, and they can sustain larger reversible ("elastic") deformations than crystalline alloys. Amorphous metals derive their strength directly from their non-crystalline structure, because this structure has no defined boundaries there are no especially weak planes or points within the metal, providing a uniform and mutually-reinforced response to stresses. One modern amorphous metal, known as Vitreloy, has a tensile strength that is almost twice that of high-grade titanium. However, metallic glasses at room temperature are not ductile and tend to fail suddenly when loaded in tension. Therefore, there is considerable interest in producing composite materials consisting of a metallic glass matrix containing particles or fibers of a ductile crystalline metal.

Perhaps the most useful property of bulk amorphous alloys is that they are true glasses, which means that they soften and flow upon heating. This allows for easy processing, such as by injection molding, in much the same way as polymers. As a result, amorphous alloys have been commercialized for use in sports equipment, medical devices, and as cases for electronic equipment.

External links

• "Metallic glass: a drop of the hard stuff" (http://www.newscientist.com/channel/mech-tech/mg18624931.000) at New Scientist