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HISTORY 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 Mega Kelvin per Second , 106 K·s-1) 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. Many amorphous alloys are formed by exploiting a phenomenon called the "confusion" effect. Such alloys contain so many different elements (often a dozen or more) that upon cooling at sufficiently fast rates, the constituent atoms simply cannot coordinate themselves into the equilibrium crystalline state before their mobility is stopped. In this way, the random disordered state of the atoms is "locked in". In 2004, two groups succeeded in producing bulk amorphous steel, one at Oak Ridge National Laboratory , the other at University Of Virginia . The Oak Ridge group refers to their product as "glassy steel". The product is non- Magnetic at Room Temperature and significantly stronger than conventional steel, though a long research and development process remains before the introduction of the material into public or military use. PROPERTIES An amorphous metal is usually an Alloy rather than a pure metal. 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, which does not have any of the defects (such as Dislocations ) that limit the strength of crystalline alloys. 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. REFERENCES | ||
| title | Glassy Steel |
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| journal | ORNL Review |
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| year | 2005 |
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| volume | 38 |
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| issue | 1 |
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| url | http://wwwornlgov/info/ornlreview/v38_1_05/article17shtml |
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