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 8. Microstructure of In Situ Metallic Glass Matrix Composite. With ductile crystal line dendrites .
(a) Scanning electron micrograph showing the dendrites ( light gray) in the glassy matrix (dark gray) .
(b) Composite after plastic deformation; note the multiplicity of slip steps, ind icating extensive
interaction of shear bands with the dendrltes.33
The key limitation of these in situ composites is that not every alloy system is capable
of forming them. While any alloy will form crystalline phases at elevated temperatures,
usually the crystalline phases that form are brittle intermetallics that degrade rather
than enhance the mechanical properties. To be effective in controlling shear bands, the
precipitated phase needs to be ductile, have a shear modulus lower than that of the
glassy matrix, and (preferably) form as dendrites. To date, the only published reports
of systems that satisfy these criteria concern alloys based on early transition metals,
notably zirconium and titanium. Whether in situ composites can be developed in other
alloy systems remains to be seen.
MECHANICAL PROPERTIES OF COMPOSITES
The ability to produce mixed amorphous-crystalline microstructure provides the ability
to control the formation and propagation of shear bands. The resulting materials can
have good fracture and fatigue resistance while retaining the high strength and
processing flexibility associated with metallic glasses.
The origin of these effects is related to the development of a region of plastic
deformation at the tip of an advancing crack. For a crack opening under tensile loading,
the size of the plastic region is approximately given by:
(Equation 1)
where Kie is the plane-strain fracture toughness (mentioned above) and crv is the yield
strength. The size of the plastic zone varies from ~ 1 μm for "intrinsically brittlle"
metallic glasses to ,.., 1 mm for glasses capable of some plastic deformation. 34 If the
material has structure on this length scale (or if the sample itself is of this size), then
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