AAWSAP DIRD, Metallic Glasses for Aerospace Applications, December 2009

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Metallic Glasses: Status and Prospects for Aerospace
Applications
Summary
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 processi'ng. 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 fide ity. 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 quite brittle, with low fracture
toughness. Although metallic glass matrix composites can have outstanding
properties (particularly strength and fracture toughness), the number of good
composite systems known at present is also quite limited.
Therefore, in order 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:
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