Defense Intelligence Reference Document Metallic Glasses For Aerospace Applications

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parallels can be drawn with thermoplastic forming of polymers, but there are certain to
be many differences as well.
Continued developments in this area are highly likely to result in the ability to produce
complex net-shape parts in a single processing step. This ease of processing could
offset the higher raw materials costs for metallic glasses, making them competitive in a
much wider range of applications. Furthermore, as noted above, the ability to replicate
extremely small features ( < 20 nanometers) in metallic glasses is likely to be exploited
in the manufacture of nanostructured devices.48 49
Finally, development of metallic glass foams will continue and will be aided by improved
understanding of thermophysical properties. It is highly likely that foams will be
produced in a wide range of glass-forming alloys, and that techniques will be developed
for precise control of the porosity, pore size, ligament size, and connectivity. This will
allow the properties of these foams to be tailored to particular applications.
COMPOSITES AND THE QUEST FOR DUCTILITY
From the point of view of structural applications, localization of plastic deformation into
shear bands is the single biggest challenge because this tendency limits the tensile
ductility, fracture toughness, and fatigue crack resistance of metallic glasses. There
may well be no solution to this problem for monolithic metallic glasses, for the simple
reason that they lack any microstructure to interact with shear bands.
Progress is likely to occur on two fronts. First, it is now well established that some
alloys are inherently brittle, in the sense that they experience very little plastic
deformation around a crack tip, while other alloys show extensive plastic deformation
(albeit localized into shear bands). The precise reason for this difference is not
understood at present, but it seems likely that it will be resolved with continued work
on fundamental aspects of plastic deformation and fracture. This is likely to lead to
development of new alloys with reasonable fracture toughness, although not to tensile
ductility. However, even this will be an important step if such alloys can be used as
matrices for composites.
Second, in order to achieve tensile ductility, it appears to be necessary to have some
microstructural features to interact with shear bands. Furthermore, the length scale of
the microstructure is clearly a critical parameter in arresting shear band propagation.
Again, the precise reasons for this are not known, but continued research quite likely
will lead to an improved understanding of the interactions between second-phase
particles and shear bands.
At present, the most promising approach to producing composite materials with the
proper microstructural length scale is the formation of dendritic composites, as
discussed above. A critical limitation is that this process has been demonstrated in only
two, closely related alloys and does not appear to be a general phenomenon.
Unfortunately, our understanding of thermodynamics and phase formation in complex
multicomponent alloys is not such that we can predict a priori which alloys are capable
of producing ductile dendrites in a glass-forming matrix. Until that understanding is
developed, discovery of new dendritic composite materials will remain a matter of trial
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