Metallic glasses
Metallic glasses
Alloys having amorphous or glassy structures. 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 ferromagnetism. Glass having metallic properties is obtained from a melt containing metallic elements instead of oxides. However, liquid metals and alloys crystallize so rapidly on cooling that it was not until 1960 that the first true metallic glass, an alloy of gold and silicon (Au80Si20), containing 80 at. % Au and 20 at. % Si, was obtained.
The effect of adding solute atoms to a pure metal, especially if they are of a size and chemical character different from those of the host atoms, is to suppress the freezing temperature, so that the probability of solidifying the melt without crystallization is increased. Accordingly, the alloy systems for which glass formation occurs most readily are those manifesting either one or more eutectics. Those compositions with the lowest liquidus temperature, that is, near eutectic compositions, thus form a glass most easily. The known glass-forming families are alloys of transition metals or noble metals that contain about 10–30% semimetal [for example, platinum/phosphorus (Pt75P25) or iron/boron (Fe80B20)], alloys of early-transition metals only [for example, zirconium/palladium (Zr70Pd30) or niobium/rhodium (Nb60Rh40)], alloys containing metals from group 2 in the periodic table [for example, magnesium/zinc (Mg70Zn20) or calcium/magnesium (Ca70Mg30)], and alloys of rare-earth metals and transition metals [for example, gadolinium/cobalt (Gd70Co30) or yttrium/iron (Y60Fe40)]. In a few cases, the glass-forming composition does not fall at the eutectic point but in a composition range richer in the minor element, such as alloys of aluminum (Al) and rare-earth metals (for example, Al90Y10 or Al90Gd10). Binary alloy glasses can be obtained only as thin foils about 0.002 in. (50 μm) thick, because a critical quenching rate of 1.8 × 105 °F/s (105 K/s) is required to retain the glassy phase. When further solute is substituted, the stability and glass-forming tendency can be drastically enhanced. Ternary alloy glasses, for example, palladium/copper/silicon (Pd77Cu6Si17), palladium/nickel/phosphorus (Pd40Ni40-P20), and platinum/nickel/phosphorus (Pt60Ni15P25), have been prepared as cylindrical rods of 0.100 in. (2.5 mm) in diameter at a quenching rate of 1.8 × 102 °F/s (102 K/s) or less.
The electrical properties of crystalline metals and alloys are generally well understood. The absence of a crystal lattice in metallic glasses results in substantial changes in their electrical properties and has theoretical applications in studies of transport properties in solids.
The electrical resistivity of metallic glasses is high, for example 100 μ&OHgr;-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. Because of their insensitivity to temperature variations, metallic glasses are suitable for applications in electronic circuits for which this property is an essential requirement.
The first superconducting metallic glass was reported in 1975. This was an alloy containing 80 at. % lanthanum (La) and 20 at. % Au. 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 B, Si, or P. 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 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.
Ferromagnetic amorphous alloys had been prepared before the technique of rapid cooling from the liquid state was developed. Electrolytic deposits of NiP alloys are slightly ferromagnetic for P concentrations less than 17 at. %. Amorphous CoP alloys can be electrodeposited in the amorphous state for P concentrations from 18 to 25 at. % Co and are also ferromagnetic. Ferromagnetism was also measured in alloys of Co with Au in the form of vapor-deposited thin films. These results suggested that it should be possible to obtain a ferromagnetic metallic glass from a liquid alloy containing a high enough percentage of ferromagnetic metals. The choice of alloying elements was guided by trying to satisfy the low-melting-point eutectic composition of the original AuSi glass, and Fe was the most obvious choice for the metal constituent.
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. Iron-based glasses have a fracture strength of 5 × 105 lb/in.2 (3.4 GPa), which is comparable to the best hard-drawn piano wires. Remarkably, despite their high strength, metallic glasses exhibit a high toughness contrary to the brittle behavior inherent in nonmetallic and high-strength crystalline metals. The ductility and toughness of Fe-based glasses, however, are very sensitive to thermal annealing. A complete loss in ductility of Fe glasses may occur after annealing without crystallization. In contrast, glass-forming alloys of Ni, Pd, and Pt as well as metal-metal alloys (Nb-Ni,Zr-Cu) remain ductile even in a partially crystalline state. The causes of embrittlement are still not clear. See Young's modulus
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.