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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.
UNCLASSIFIED//509 OFEJCIAP q55 ON! Y glasses in the true sense of the word, rather than melting abruptly (as crystalline metals do), they soften and flow over a range of temperatures in a manner akin to common (oxide) glasses. This creates opportunities for tremendous flexibility in the processing of metallic glasses. PROCESSING Glass-Forming Alloys The key to making a metallic glass is to retain the disordered, liquid-like atomic scale structure during cooling from the melt. All materials have a tendency to crystallize upon cooling because the crystalline state is the most stable structure at any temperature below the melting point. But crystallization takes time, so if the cooling is fast enough 1 it is possible to bypass crystallization and form an amorphous structure at the glass transition temperature (Figure 2(a)). Glass formation and crystallization are therefore competitive processes; which one will occur depends on the material and the processing conditions. (a) Temperature '.\lclling Jcmpcmlurc (ila,, lrnnsilion lcmpcrnlurc Liquid 0 Time (b) "':,2 ~ t.:: t/J -~ 'J .~ ·;: u ir/' 10~ l(t 10-1 10~ 100 ]()°~ 0 Pnrc- tiickc-1 - , "'Conventional"' metallic glnsscs I 0 :,. (mnx thickness l mm) \ 0 .... - '~loo', ,~o , '°,~ ~l'1l• .( 1J ,N1)',, Zr_,,1-;,,,cu,,,N1,)k~ _ ..,J 0.2 !J.3 0.4 05 0.n !L7 ILK Reduced glass trnn~ilmn temperature (T/T,,,J Figure 2. Critical Cooling Rate. (a) Effect of the cooling rate on glass formatlon - If the cooling rate is slow (path 1 ), then the melt crystallizes before going through the glass transition. If the cooling rate is fast enough (path 2), then the melt can form a glass. The critical coaling rate (path 3) is the slowest rate at which the melt can be cooled and still form a glass. (b) Critical coaling rates for various metallic alloys - The horizontal axis is the glass transition temperature normalized to the melting (liquldus) temperature. 1 For some materials, such as silica (silicon dioxide) and most thermoplastic polymers, the crystallization process is slow because the crystal structures are complex and the basic structural units (for example, segments of polymer chains) are slow to rearrange into a crystalline form. These materials can therefore be produced in glassy form even at very low cooling rates; in fact, it can be difficult to crystallize them at all. Metals and alloys are another matter because the crystal structures are relatively simple and the basic structural units are individual atoms, which are highly mobile. Metallic crystals nucleate and grow quickly, making production of a metallic glass more challenging. 2 UNCLASSIFIED/1PIHl BPPl@liillt 1191 8HL'f
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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.