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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//iiOA. OiiiiICI0I. Pili O111.V EX SITU COM POSITES There are two basic ways of making ex situ composites, in which the metallic glass matrix and the crystalline phase are combined physically, without a chemical reaction: • Add crystalline particles to a melt of a glass-forming alloy and then cast under conditions that allow the matrix to form a metalliic glass. • Make a preform of a crystalline phase (by packing fibers into a mold, for instance) and then cast the glass-forming alloy around the preform. Both approaches have lim itations. In the first, the addition of particles to the melt increases the viscosity (which is already quite high relative to non -g lass-forming alloys) considerably, ultimately to a point where casting becomes impossi ble. This limits the volume fraction of particles that can be added, which in turn limits the control one has over the microstructure and, in particular, the spacing of the particles. With a perform, the volume fraction of the crystalline phase can be much higher (up to about 80 percent by volume), but the problem then is how to infiltrate the high -viscosity melt into the preform without leaving voids and while still ensuring sufficiently rapid cooling to form a glassy matrix. With both approaches, interfacial reactions between the crystalline phase and the melt can cause partial or complete crystallization of the matrix, degrading the mechanical properties. IN SITU COMPOSITES The difficulty of making satisfactory ex situ composites has led to the development of a new approach in which the crystalline phase is precipitated directly from the melt, either during casting 28 or in a separate step prior to casting. 29 30 Precipitation during casting, although easier, is problematic from a practical standpoint because variations in the cooling rate (from the surface to the center of a casting, for instance) lead to significant variations in structure and, hence, in properties. One of the most promising recent advances in the metallic glass field is the development of in situ composites in which the crystalline phase is precipitated as dendrites, either during casting (Figure 8) or by holding the alloy at an elevated temperature prior to casting.31 By suitably choosing alloy composition, holding time, and temperature, the volume fraction, size, and spacing of the dendritic phase can be controlled. This control provides great flexibility in determining the mechanical properties of the resulting material. Because the crystalline phase is produced prior to casting, variation in the cooling rate across the casting is much less important, though the cooling rate must still be sufficiently high to ensure the matrix forms a glass during cooling. Once the glassy matrix is formed, the composite can be reheated above the glass transition temperature, allowing for thermoplastic forming in a manner similar to single-phase metallic glasses (as described above). Finally, the presence of the dendritic second phase allows for deformation processes (for example, by cold rolling or forging), similar to crystalline alloys. 32 14 UNCLASSIFIED/ /FOR OPPICIAL l:IS& 0PIL¥
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