Documents / Report
The Defense Intelligence Agency issued this Defense Intelligence Reference Document (DIA-08-0911-012), dated 14 December 2009, as one of its FY 2009 advanced technology reports under the Advanced Aerospace Weapon System Applications (AAWSA) Program. It reviews how metallic glasses are structured and processed, their mechanical properties, and metallic glass matrix composites. It concludes that dendritic composites could replace high-strength steels in some aerospace parts. It adds that wide aerospace use depends critically on developing new lightweight glass-forming alloys.
UNCLASSIFIEDl,4FGR: GFFl61ilill WSE eP•tY Casting and Molding Like other alloys, metallic glasses can be cast into net-shape or near-net-shape geometries. Die casting into a permanent (metal) mold-because it provides the rapid heat transfer needed to meet the requirement for relatively rapid cooling-is the most common casting technique. In most cases, casting is done in either a vacuum or an inert atmosphere to prevent formation of oxide particles that promote crystallization. Conventional casting, however, does not take advantage of the flexibility afforded by the glassy nature of these alloys. If a metallic glass is heated to a temperature above its glass transition temperature, it becomes a supercooled liquid. In this state, the viscosity drops with increasing temperature over a wide range, making it possible to control the viscosity by controlling the temperature. 1 This ability to control the viscosity enables many of the processing techniques commonly used in molding thermoplastic polymers to be applied to metallic glasses (Figure 3). 200 μ111 Figure 3. Examples of Processing of Metallic Glasses. (a) Microspring produced by lithography and (b) thin- walled bottle produced by blow molding. Images are courtesy of Professor Jan Schroers (Yale University). There are two important limitations on processing of metallic glasses in the supercooled liquid region. First, supercooled liquids are metastable and have a tendency to crystallize, so there is a limited window of time (typically on the order of minutes) in which the processing must be completed if the glassy structure is to be maintained. Second, the viscosity of many glass-forming alloys near the glass transition temperature is too high for convenient processing. The viscosity can be reduced by increasing the processing temperature, but higher temperatures promote crystallization 1 A crystalline metal, in contrast, melts abruptly, going from a rigid solid to a low-viscosfty fluid very quickly. 4 UNCLASSIFIED,61SF8R 8FFlliiil"1k ltOli IHIL/.t
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Report, from the dia collection. The PDF is mirrored here; the original link is above. 30 pages are in the text index: search them above, or from the library's search.