AAWSAP DIRD, Metallic Glasses for Aerospace Applications, December 2009

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.

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Figure 4. Shear Bands. Produced by bend ing of a zirconium -based metallic glass. v,
Fracture Toughness
Fracture toughness is a measure of a material's resistance to growth of cracks, a critical
property for structural materials subjected to tensile loading. In very tough metals, the
toughness usually results from plastic deformation that occurs near the tip of the
advancing crack; plastic deformation requires energy, and the need to provide this
energy translates into resistance to crack growth. 3 Despite their lack of tensile ductility,
at least some metallic glasses are not brittle in the same sense that ceramics are, for
example, because they can experience significant plastic deformation around the crack
tip during fracture. For instance, the fracture toughness (Krc) of zirconium-based
metallic glasses is about 20 MPa -m112 15-somewhat lower than the "' 55 MPa -m 112
typical of crystalline zirconium alloys16 but much greater than the fracture toughness of
ceramics (typically 1-5 MPa-m112). The fact that metallic glasses are reasonably tough
despite their lack of tensile ductility suggests structural applications are not out of the
question.
However, some metallic glasses appear to be intrinsica lly brittle in that they fracture
with only limited plastic deformation near the crack top and thus have very low values
of fracture toughness. For this reason, some alloys t hat would otherwise be highly
desirable, such as iron-based metallic glasses (for their high strength and low cost) and
magnesium-based glasses (for their low density), fa II into this category. The physical
origins of the difference between intrinsically brittle metallic glasses and those capable
of limited plastic deformation (and thus some toughness) are not well understood.
3 In other materials, notably polymer-matrix composites, other mechanisms of toughening can be more important.
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