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Fracture And Fatigue Of Amorphous Metals

2587 words - 10 pages

Introduction
A metallic or amorphous metal is one that lacks a traditional crystalline atomic structure and is instead composed of a microstructure that looks more like a liquid than a solid. As a liquid metal is cooled from the melt, the atoms typically rearrange from a random orientation, typical of a liquid, to a highly structured, lower energy crystalline lattice-type structure. Figure 1.1 a shows the typical crystalline structure of CuZr. Figure 1.1 b shows the same material in a glassy, amorphous arrangement. Figure 1.1 c shows the temperature versus potential energy for the CuZr system. The black data points show the potential energy of the system as the temperature of the system is increased. At about 1400K the system melts and you can see a rapid increase in the potential energy. The blue data points in Figure 1.1 c show the same material but during cooling. Typically, a metal would follow the same profile as it does during heating (the black points), however in this case the material was rapidly quenched at a rate of 1012 K/s. As Figure 2.1 illustrates, these are the sort of cooling rates typically needed to obtain amorphous microstructure in common metallic alloys. Once the material passes the point labeled Tg, the glass transition temperature, in Figure 1.1 c, the material has solidified, but because of the rapid cooling the atoms are unable to rearrange into their preferred lower energy state and the result is a microstructure shown in Figure 1.1 b. This random atomic microstructure is also higher in energy than the crystalline form, as is clearly highlighted in Figure 1.1 c.
It was discovered in the 1960s by Duwez et al. [8] that it was technically possible to cool a metal, in this case AuSi, rapidly enough to form a metallic glass. This, of course, sparked much interest in the possibility of producing other metallic glasses from different compounds. Early candidates typically required extremely high cooling rates and as a result special cooling techniques were developed. These cooling methods typically resulted in “splattered” or ribbon sized samples. More effort and research was put into discovering compounds that did not require such rapid cooling rates so that larger samples could be fabricated.
It took nearly 30 years from the first AuSi metallic glass before Zr-Cu-Ni based alloys were discover to exhibit properties enabling the formation of larger samples (greater than 1mm in thickness). These larger metallic glasses became referred to as bulk metallic glasses, or BMGs. Some of the best glass forming metallic system yet discovered as Pd-based alloys, capable of being formed into tens of millimeters in thickness [9].
Perhaps the most intriguing aspect of metallic glasses is their rather remarkable mechanical properties. They typically exhibit high strength, are very hard and have excellent surface finish. Figure 1.2 highlights some of the extreme properties capable of metallic glasses, exhibiting some...

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