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This Defense Intelligence Reference Document, dated 14 December 2009, was prepared by the Defense Intelligence Agency's Defense Warning Office under its Advanced Aerospace Weapon System Applications program. It is a technical review of metallic glasses that covers their structure, processing, mechanical behavior and possible aerospace uses. It concludes that composites with ductile dendrites in a glass matrix hold the most promise for structural use. It also finds that widespread aerospace adoption depends on developing new lightweight glass-forming alloys.
From the source:Release of 2026-09-18 Incident: 12/14/09, Las Vegas, Nevada. Released with redactions. This document is a Defense Intelligence Reference Document (DIRD), a technical reference format used by the Defense Intelligence Agency (DIA) to capture baseline knowledge on a specific topic for later analytic use. DIRDs are best understood as reference and synthesis products rather than as original research. It is one of 38 DIRDs produced under the Advanced Aerospace Weapon System Applications Program (AAWSAP) between 2009 and 2011. Because AAWSAP’s scope permitted a broad range of supporting topics, not every DIRD in the series directly concerns aerospace systems or future threat assessment. The following summary reflects the DIRD’s scope and framing at the time of writing and should not be read as implying current validation of the concepts discussed. This DIRD surveys metallic glasses as a potentially important class of aerospace materials and describes their amorphous structure as offering very high strength and unusual manufacturing advantages, but also significant drawbacks, especially poor ductility and fatigue resistance. The document concludes that the most promising aerospace applications are likely to come from metallic-glass-matrix composites rather than single-phase glasses, because these composites can retain high strength while greatly improving fracture toughness and fatigue performance, potentially enough to substitute for high-strength steels in some space-limited structural uses. At the same time, the report judges that broader aerospace use will depend on substantial progress over the next 20–50 years in alloy design, processing, and especially the development of lightweight systems, including aluminum-based options.
UNCLASSIFIED/ ,'POI\ OP'P'ICIAL USE ONLf parallels can be drawn with thermoplastic forming of polymers, but there are certain to be many differences as well . Continued developments in this area are highly likely to result in the ability to produce complex net-shape parts in a single processing step. This ease of processing could offset the higher raw materials costs for metallic glasses, making them competitive in a much wider range of appl ications. Furthermore, as noted above, the ability to r eplicate extremely small features ( < 20 nanometers) in metallic glasses is likely to be exploited in the manufacture of nanostructured devices.48 49 Finally, development of metallic glass foams will continue and will be aided by improved understanding of thermophysical properties. It is highly likely that foams will be produced in a wide range of glass-form ing alloys, and that techniques will be developed for precise control of the porosity, pore size, ligament size, and connectivity. This will allow the properties of these foams to be tailored to particular applications. COMPOSITES AND THE QUEST FOR DUCTILITY From the point of view of structural applications, localization of plastic deformation into shear bands is the single biggest challenge because this tendency limits the tensile ductility, fracture toughness, and fatigue crack resistance of metallic glasses. There may well be no solution to this problem for monolithic metallic glasses, for the simple reason that they lack any microstructure to interact with shear bands. Progress is likely to occur on two fronts. First, it is now well established that some alloys are inherently brittle, in the sense that they experience very little plastic deformation around a crack tip, while other alloys show extensive plastic deformation (albeit localized into shear bands). The precise reason for this difference is not understood at present, but it seems likely that it willl be resolved with continued work on fundamental aspects of plastic deformation and fracture . This is likely to lead to development of new alloys with reasonable fracture toughness, although not to tensile ductility . However, even this will be an important step if such alloys can be used as matrices for composites. Second, in order to achieve tensile ductility, it appears to be necessary to have some microstructural features to interact with shear bands. Furthermore, the length scale of the microstructure is clearly a critical parameter in arresting shear band propagation. Again, the precise reasons for this are not known, but continued research quite likely will lead to an improved understanding of the interactions between second-phase particles and shear bands. At present, the most promising approach to producing composite materials with the proper microstructural length scale is the formation of dendritic composites, as discussed above. A critica 'I limitation is that this process has been demonstrated in only two, closely related alloys and does not appear to be a general phenomenon . Unfortunately, our understanding of thermodynamics and phase formation in complex multicomponent alloys is not such that we can predict a priori which alloys are capable of producing ductile dendrites in a glass-forming matrix. Until that understanding is developed, discovery of new dendritic composite materials will remain a matter of trial 21 UNCLASSIFIED/ /FOR OFFICIO! 11 SF AN! X
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