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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//POlll OPPIE!IAL WS& 0,u.v Current Challenges and Prospects for the Future ALLOY DESIGN A critical limitation of existing metallic glass technology (and related composites) is the relative dearth of alloys with good glass-forming ability. The best glass-forming alloys are either based on expensive elements (for example, palladium) or contain toxic elements (for example, beryllium in the best zirconium- and titanium-based alloys). For aerospace applications, the most glaring lack is that, despite significant alloy design efforts in the United States (through the DARPA Structural Amorphous Metals program), Japan, China, and elsewhere, there are no good glass-forming alloys based on aluminum. Attempts to make aluminum-based metallic glass components by consolidating amorphous powders have met with limited success. Similarly, all of the good iron-based metallic glasses contain considerable amounts of nonmetallic elements (notably carbon, boron, silicon, and/or phosphorus), which are thought to contribute to the very low fracture toughness of these alloys (Figure 9(b)). However, there is reason to expect that further progress is possible. Recent experimental results have shown that some of the empirical "rules" of glass-forming ability44 are actually quite flexible, and that glass-forming ability is much more sensitive to composition than had been previously appreciated. 45 So it is highly probable that some excellent glass -forming alloys compositions remain to be discovered, possibly including some low-density glasses based on aluminum. Identifying these good glass-forming alloys will be a challenge. Most alloy development to date has been done with a brute-force approach, but combinatorial techniques46 re likely to enable much more ra pid screening. One issue is identification of suitable metrics for glass-forming ability, since the combinatorial approaches use vapor deposited thin films, and it is not clear what characteristics of such a film correlate with glass-forming ability in the bulk. Similarly, continued development of ab initio molecular dynamics techniques should enable identification of candidate alloys from computer simulations, particularly as computers continue to increase in power. One area that has received insufficient attention is the influence of processing conditions on glass-forming ability. For instance, application of electromagnetic vibrations during cooling reportedly significantly enhances the glass-forming ability of magnesium-based metallic glasses. 47 This approach could, in principle, be applied to other alloys, possibly greatly extending the range of alloys and compositions that can be produced as bulk metallic glasses. THERMOPHYSICAL PROPERTIES AND THERMOPLASTIC PROCESSING Most of the practical interest in single-phase (monolithic) metallic glasses centers on the potential for thermoplastic processing near to or above the glass transition temperature. However, the t hermophysical properties and behavior of metallic glasses are not well understood. For instance, the viscosity of the metallic glass melt (or supercooled liquid) is of critical importance, but we do not know how and why alloy composition influences viscosity. From an engineering point of view, the practical aspects of molding of metallic glasses are just beginning to be explored. Certainly many 20 UNCLASSIFIED/ /iOlil OfiFiliCiili.t.k WS& 8HL¥
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