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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Even some metallic glasses with reasonable toughness may be embrittled by exposure
to elevated temperatures. This may occur in the heat-affected zone during weld ing (as
discussed above), or it may be a by-product of processing in the supercooled liquid
region (as in injection molding, for instance). The causes of embrittlement are also not
well understood, and there is no known way to reverse embrittlement once it occurs.
Fatigue
Fatigue is a process by which materials can experience incremental crack growth owing
to cycl ic loading, even at stresses well below the yield stress. If unabated, fatigue
cracks can grow to a critical length at which abrupt catastrophic fracture occurs. Up to
90 percent of failures of structural components in service are estimated to be caused by
fatigue, making fatigue resistance of obvious importance to designers.
The fatigue resistance of metallic glasses is not very good. A common measure of
fatigue resistance is the fatigue limit- the stress amplitude (range) below which no
fatigue failure will occur, regardless of the number of loading cycles the material
experiences. The fatigue limit for high-strength crystalline alloys is typically about 40
percent of the tensile strength, but for metallic glasses, it is only about S percent of the
tensile strength (Figure 5). The reason for th is difference has to do with the structure of
the material. In a crystalline alloy, there are microstructural features (such as grain
boundaries and precipitate particles) that can inhibit the growth of fatigue cracks. In
metallic glasses, the microstructure is completely featureless, and there is nothing to
prevent fatigue cracks from growing once they have been initiated.
The poor fatigue resistance of metallic glasses is a cr itical limitation for structural
applications in aerospace because it implies a need to overdesign components to keep
the stresses far below the yield stress. Thus, much of the advantage of having a high
strength material in the first place is lost. The desire to improve metallic glasses'
fatigue performance has led to the development of metallic-glass-matrix composites
with outstanding properties, as discussed below.
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