Defense Intelligence Reference Document Metallic Glasses For Aerospace Applications

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and error. The potential benefits are significant, however, because there exists the
possibility of making materials with exceptionally high strength, fracture toughness,
and fatigue resistance.
Summary and Recommendations
Metallic glasses combine some of the advantageous mechanical properties of metals-
strength, stiffness, and in some cases toughness-with the processing flexibility usually
associated with thermoplastic polymers. The absence of crystalline defects allows
metallic glasses to be much stronger than conventional alloys but also means they have
near-zero tensile ductility and poor fatigue resistance. In structural applications,
therefore, metallic glasses are most likely to be useful in the form of composites
consisting of ductile crystalline dendrites in a metallic glass matrix. These dendritic
composites sacrifice some strength but can have exceptionally high fracture toughness,
as well as good fatigue resistance, and could replace high-strength steels in certain
load-limited structural components in aerospace vehicles where space is limited.
Because they are true glasses, thermoplastic forming near the glass transition
temperature affords metallic glasses tremendous flexibility in processing. For instance,
metallic glass components can be formed in a single step {for example, by injection
molding) in complex geometries that would be difficult or impossible to produce with
conventional alloys. In addition, metallic glass foams can be made with relative ease,
raising the possibility of making structural foams with high strength and stiffness.
Finally, because they lack a crystalline grain structure, metallic glasses can be used to
form nanoscale features with high fidelity. This may make metallic glasses useful in a
variety of micro-electromechanical systems (MEMS) applications.
Metallic glasses also have significant limitations for aerospace applications, however.
Foremost among these is a lack of good glass-forming alloys; in particular, there are no
good aluminum-rich glass-forming alloys, the known titanium-based alloys are either
relatively dense {owing to high concentrations of alloying elements) or contain
beryllium, and the known magnesium- and iron-based alloys are all qurte brittle, wrth
low fracture toughness. Although metallic glass matrix composites can have
outstandfng properties (particularly strength and fracture toughness), the number of
good composite systems known at present is also quite limited.
For metallic glasses (and their composites) to be of broad utility in aerospace structural
applications, progress in the following areas is required:
• Development of new lightweight alloys and composite systems, preferably by
computational and/or combinatorial approaches rather than by trial and error.
• Understanding of mechanical behavior, especially:
- The effect of alloy composition and structure on plastic deformation.
- Microstructural design of composites for optimal toughness.
• Development of processing techniques, including thermophysical processing of
complex and/or nanoscale features as well as production of metallic glass foams.
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