These are alloys with an amorphous or glassy structure. A glass is a solid material obtained from a liquid, which does not crystallize during cooling. It is therefore an amorphous solid, which means that the atoms are packed in a more or less random fashion similar to that in the liquid state. The word glass is generally associated with the familiar transparent silicate glasses containing mostly silica and other oxides of aluminum, magnesium, sodium, and so on. These glasses are not metallic; they are electrical insulators and do not exhibit ferro-magnetism. One obvious answer to obtaining a glass having metallic properties is to start from a melt containing metallic elements instead of oxides.
Electrical and Superconductivity Properties
The electrical resistivity of metallic glasses is high, for example 100 |Q-cm and higher, which is in the same range as the familiar nichrome alloys widely used as resistance elements in electric circuits. Another interesting characteristic of the electrical resistivity of metallic glasses is that it does not vary much with temperature. The temperature coefficient is of the order of 10-4K-1 and can even be zero or negative, in which case the resistivity decreases with increasing temperature. Because of their insensitivity temperature variations, metallic glasses are suitable for applications in electronic circuits for which this property is an essential requirement.
Superconducting metallic glasses are much more stable, and some of them do not crystallize at temperatures as high as 500°C. Some superconducting metallic glasses contain only two metals, such as Zr75Rh25, and some are more complex alloys in which there is approximately 20% of metalloid elements, mostly boron, silicon, or phosphorus.
One of the main reasons for continuing research on new superconducting glasses is their projected usefulness in high-field electromagnets, which will be required to contain the high-temperature plasma in fusion reactors. The present requirement for these magnets is a field of at least 100,000 gauss, which is not attainable with conventional copper-wound electromagnets. Observations and results have shown that the electrical resistivity of metallic glasses is not affected by radiation.
Magnetic Properties
The ferromagnetic properties of metallic glasses have received a great deal of attention, probably because of the possibility that these materials can be used as transformer cores. One class of ferromagnetic metallic glasses is based on transition metals with zirconium or hafnium, for example, (Co,Ni,Fe)90Zr10.
Because metallic glasses do not have crystalline anisotropy, the core maker is able to anneal in a desired domain pattern. For example, metallic glasses can provide extremely high magnetic efficiency in motors, because their anisotropy can be built in during a final annealing to develop a low-reluctance path in the exact configuration required by the motor. The use of metallic glasses in motors can reduce core loss by as much as 90% as compared with conventional crystalline magnets. Alternatively, for transducer applications, one can just as well anneal in a domain structure that is transverse to the ribbon length by applying a transverse magnetic field during the annealing cycle. This will maximize the rotation of domains and hence the magnetostrain during any subsequent longitudinal excitation.
Mechanical and Other Properties
Although a metallic glass under tension will fracture with very small permanent deformation, scanning electron micrography reveals that a large local shear deformation occurs before rupture.
Metallic glasses thus are intrinsically ductile. The interest in the mechanical properties of metallic glasses is motivated by their high rupture strength and toughness. The fracture strength of metallic glasses approaches a theoretical strength that is about 1/50 of Young’s modulus.
Future and Applications
Considering the unusual physical and chemical properties of metallic glasses, there is no doubt that they will play an important role as an engineering material in the future. The melt-spinning process of producing metallic glasses should also result in substantial savings in labor and energy when compared with the present technology, because it can produce a thin sheet of material by direct casting from the liquid state.
Possible applications of metallic glasses have already been demonstrated on audio and video magnetic tape recording heads, sensitive and quick-response magnetic sensors or transducers, security systems, motors, and power transformer cores. The combination of excellent strength, resistance to corrosion and wear, and magnetic properties may lead to interesting applications, for example, the use of such glasses as inductors in magnetic separation equipment.
Besides the straightforward applications, there remain many less obvious potential applications of metallic glasses: the freedom from constraints imposed by the equilibrium phase diagram may well allow metallurgists to devise chemically interesting glassy alloys that could not be made otherwise in single-phase form. Glasses based on palladium-phosphorus alloys show a considerably higher catalytic activity for oxidation of methanol, and they are stabler than the fine-grained platinum electrodes. Another potential application of metallic glasses is their use when intentionally crystallized. Since glasses are highly supercooled liquids, crystallization can be effected via copious nucleation, which results in the formation of a fine-grained crystalline product that is microstructurally very homogeneous. An example is the use of a glass sheet of the alloy Ni69Cr6Fe3B14Si8 as a brazing filler metal; the glass sheet can be stamped to shape and inserted between the parts to be joined, yielding excellent control of the joint properties after fusion of the products.
Another development is an application of powder-metallurgy methods to the fabrication of amorphous alloys. In the process of "atomizing" liquid alloys into a very fine powder, the rate of cooling may be high enough to produce an amorphous powder.
By using explosive loading, it is indeed possible to subject the powder to very high pressure, and probably some rather high temperature, for a short enough time to avoid crystallization, and to achieve near-theoretical densities. Massive ingots of metallic glasses, susceptible of being forged and rolled into various shapes, may become available within the not-too-distant future. Finally, since metallic glasses may be regarded as liquids whose structure has been frozen, they constitute ideal materials for low-temperature transport and critical behavior studies, and they are most suited for the study of electrons in noncrystalline metals.
