Wednesday, July 20, 2016

Bulk Metallic Glass

Bulk Metallic Glass are a class of materials that offer many desirable and unique processing routes and properties. An improving universal description of the structure and behavior of BMGs should lead to better tools for their design and for commercialization. Metals and alloys normally exist as crystalline materials. Compared to crystalline, bulk metallic glasses often exhibits very high strength, corrosion resistance, and elasticity. BMGs with the best glass-forming abilities are Zr- and Pd-based, are often reported in terms of a “critical casting diameter”. 
Small changes in composition can lead to large changes in properties. Several rules exist correlating a single physical parameter with glass-forming ability. However, only some of these correlations are useful for predicting glass-forming ability. These rules still require knowledge of properties of the alloy to allow prediction of glass-forming ability. A better understanding of the atomic structure of BMGs is developing through analyses of the simplest binary alloys. The BMG structure is a randomly packed assembly of the different atoms. However, although BMGs lack long range atomic order, they do exhibit short and medium range order over several atomic lengths.
Compared to other engineering materials, BMGs are used in small volumes, but most are very strong in compression. Many engineered parts require good toughness and strength in tension rather than compression. Similarly, BMG composites have been well designed by matching the micro-structural length scale of the crystalline phase to crack length scales. It resulted in high tensile ductility and fracture toughness. A unique property of BMGs is their ability to reversibly transform from the low-temperature glassy state into a supercooled liquid state above a glass transition temperature. Many BMGs are not well adapt to conventional forming at room temperature. Forming in the SLR has been successful on most systems. Extrusion, closed-die forming of micro-components, and many more innovative techniques such as blow molding have also been demonstrated.
BMGs can be formed into very precise shapes while in the SLR.
Production technologies for crystalline metals have matured, but BMGs are in their first stage. A variety of casting techniques and fluxing strategies are now used to produce BMGs. It is said that BMG properties are dependent on cooling rate, so the choice of manufacturing route can be important. Gas atomization or mechanical alloying can be used to produce amorphous powders.

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