DOW-UAP-D117 — AAWSAP DIRD, Metallic Glasses for Aerospace Applications, December 2009

File: DOW-UAP-D117
Agency: Department of War
Incident date: 12/14/09
Location: Las Vegas, Nevada
Released in: Release 06 (2026-09-18)
File type: PDF

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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TL;DR

December 14 2009, the only day metallic glass got a spa day in Las Vegas.

DIRD on metallic glasses, Defense Intelligence Agency, Dec 2009, Las Vegas, Nevada
Reports high strength but poor ductility; composites improve toughness; outlines processing challenges
Concludes composites most promising; broader use requires 20‑50 years progress

AI summary

This is an unclassified report from the Defense Intelligence Agency, prepared by the Acquisition Support Division (DWO-3) on December 14, 2009, with a codicil dated December 1, 2009. It discusses the status and prospects of metallic glasses for aerospace applications. The document outlines the structure, processing, and properties of metallic glasses, noting their strength and processing flexibility but also their lack of ductility and fatigue resistance. It suggests that metallic glasses may be most useful in composite forms and identifies challenges in developing suitable alloys and processing techniques for aerospace use. The report was part of the Advanced Aerospace Weapon System Applications program.

Full text

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Defense
Intelligence
Reference
Document
Acquisition Threat Support
14 December 2009
[COD : 1 December 2009
DIA-08-0911 -012

Metallic Glasses: Status and
Prospects for Aerospace
Applications

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Metallic Glasses: Status and Prospects for Aerospace
Applications

Prepared by:
Acquisition Support Division (DWO-3)
Defense Warning Office
Directorate for Analysis
Defense Intelligence Agency
Author:

AAP Person 63
Administrative Note
COPYRIGHT WARNl NG: Further dissemination of the photographs in this publication is not a uthorized .

This product is one in a series of advanced technology reports produced in FY 2009
under the Defense Intelligence Agency, Defense Warning Office's Advanced Aerospace
Weapon System Applications (AAWSA) Program. Comments or uestions pertaining to
, AAWSA Prog ram
this document should be addressed to !AAP Person 1
Manager, Defense Intelligence Agency, ATTN: CLAR/DWO-3, Bldg 6000, Washington,
DC 20340-5100.

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Contents
Summary.................................................................................................................v
Metallic Glasses ...................................................................................................... 1
Structure ............................................................................................................ 1
Processing.......................................................................................................... 2
Glass-Forming Alloys ...................................................................................... 2
Casting and Molding ....................................................................................... 4

Joining ........................................................................................................... 5
Foams ............................................................................................................. 5
Thin Films and Coatings ................................................................................. 5
Mechanical Behavior Near Room Temperature ............................................... 5
Stiffness: Elastic Deformation ........................................................................ 6
Strength and Ductility: Plastic Deformation ................................................... 6
Fracture Toughness ........................................................................................ 8
Fatigue ........................................................................................................... 9
Wear Resistance........................................................................................... 10
Corrosion and Stress-Corrosion Cracking ..................................................... 10
Mechanical Behavior at Elevated Temperature ............................................. 11
Other Properties: Magnetic, Electrical, Optical, Thermal, and Acoustic ........ 12
Metallic Glass Matrix Composites ......................................................................... 13
Processing and Structure of Composites .......................................................... 13
Ex Situ Composites........................................................................................... 14
In Situ Composites ........................................................................................... 14
Mechanical Properties of Composites ............................................................... 15
Strength and Ductility: Plastic Deformation ..................................................... 16
Fracture and Fatigue ........................................................................................ 16
Aerospace Applications of Metallic Glasses .......................................................... 16
Structural Applications ..................................................................................... 16
Other Applications............................................................................................ 19

iii

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Current Challenges and Prospects for the Future ................................................. 20
Alloy Design ..................................................... ................................................ 20
Thermo physical Properties and Thermoplastic Processing ............................... 20
Composites and the Quest for Ductility ........................................... ......... .... .... ... 21
Summary and Recommendations ................................................................ ......... 22

Figures
1. Amorphous Versus Crystalline Structure ............................................................ 1
2. Critical Cooling Rate ........................................................................................... 2
3. Examples of Processing of Metallic Glasses ........................................................ 4
4. Shear Bands ....................................................................................................... 8
5. Fatigue Limit of Metallic-Glass-Matrix Composites ........................................... 10
6. Deformation Map for a Metallic Glasses ............................................................ 11
7. Cast Metallic Glass Wedge ................................................................................ 13
8. Microstructure of In Situ Metallic Glass Matrix Composite ................................ 15
9. Materials Property Charts ................................................................................. 18

Tables
1. Selected Bulk Glass-Forming Alloys.................................................................... 3
2. Comparison of Strengths of Amorphous and Crystalline Aluminum Alloys ........ . 7

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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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•

□The effect of alloy composition and structure on plastic deformation.

0 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.

It is highly likely that continued work over the next 20-50 years will result in
significant advances in all these areas, and that metallic glasses and metallic
glass matrix composites will see increasing acceptance as structural materials.
Whether or not they achieve widespread use in aerospace applications,
however, depends critically on the development of new, lightweight alloys.

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Metallic Glasses
STRUCTURE
The atomic-scale structure of most metals and alloys is crystalline; that is, the atoms
are arranged in a highly ordered manner on a lattice that is periodic in three
dimensions, as depicted in Figure l(a). In contrast to this crystalline structure, metallic
glasses lack the long -range order of a lattice and are therefore said to be amorphous,
as depicted in Figure l(b) . Although the word "amorphous" implies a complete lack of

structural order, in fact the atomic structure of metallic glasses is not truly random.
Constraints on atomic packing provide strong short-range order; for instance, on
average the atoms have a particular number of nearest atomic neighbors at a well
defined distance. But this short-range order persists only over distances of a few
atoms; there is no long -range order as there is in a crystalline alloy. In many ways, the
atomic-scale structure of metallic glasses more closely resembles the highly disordered
structure of a liquid than the structure of a crystalline alloy.

Cry ta ll in

Amorph u (gla s)

Figure 1, Amorphous Versus Crystalline Structure. Schematic atomic-scale structure of crystalline
(a) and amorphous (b) metals. In a crystalline structure, order persists over long distances (many
atomic dimensions}. In a glass, there is short range order but no long-range order.

A corollary of this difference in structure is that the nature of structural defects is quite
different between crystalline and amorphous alloys. Crystalline alloys, for example,
have extended linear defects in the crystal structure, called dislocations, that are (in
large part) responsible for determining mechanical behavior. The lack of crystalline
order precludes the existence of dislocations in metallic glasses, but other sorts of
defects can be present and may influence properties and behavior.
From an applications point of view, the amorphous structure of metallic glasses has two
principal implications. First, the mechanical properties of amorphous alloys are
significantly different from those of their crystalline counterparts; some of these
differences are advantageous, but others are not. Second, because metallic glasses are

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glasses in t he true sense of the word, rather than meltin g abruptly (as crysta ll ine
metals do), they soften and flow over a range of temperatures in a manner akin to
common (oxide) glasses. This creates opportunities for tremendous flexibi lity in the
processing of metallic glasses.

PROCESSING

Glass-Forming Alloys
The key to making a meta ll ic glass is to retain the disordered, liq uid-like atomic sca le
structure during cooling from the melt . All materials have a tendency to crystal lize upon
coo lin g because the crystalline state is the most stable structure at any temper ature
below the melting point. But crystallization takes time, so if the cooling is fast enough,
it is possib le to bypass crysta ll ization and form an amorphous structure at the glass
transition temperature (Figure 2(a)) . Glass formation and crysta llization are therefore
competitive processes; wh ich one will occur depends on the material and the processing
conditions.

(a)

(b)
Temperature

0 Purt: nickel

101(1
Liquid
~

g_"',.,

Me ll ii1g
1ompcrarurc

i§

- , " onventional' metall ic gla scs
I0
, (max Lh1clme < I mm)

10~

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10<•

"'1)

10"

10 2

·o

lass lr:msiti on

Bu lk metallic glasses
(max lb icknes > l mm)

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temperatu.re

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Zr,.,T1 1,.Cu,,,N1" Bc,.,,

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0

'x , o \
~\ Pd., u.,N1,P,,

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...._

_,,

Reduced glass tran ition temperature (T ,IT,,.)
Figure 2. Critica l Cooling Rate. (a) Effect of the coo ling rate on glass formation - If the cooling rate l s slow (path
1), th en the melt crysta llizes befo re going through the glass transition . I f the cooling rate is fast enough (path 2),
then the me lt can form a glass. The critica l cooling rate (path 3) is the slowest rate at which the melt can be cooled
and still form a glass. (b) Criti ca l cooling rates for various metall ic al loys - Th e horizonta l axis is the glass tra nsition
temperature nor malized to the melting (liquidus) temperature. 1

For some materials, such as silica (s il icon dioxide) and most thermoplastic polymers,
the crystallization process is slow because the crystal structures are complex and the
basic structura l units (for example, segments of polymer chains) are slow to rearrange
into a crystalline form. These materials can therefore be prod uced in glassy form even
at very low cooling rates; in fact, it can be difficult to crystallize them at all. Metals and
alloys are another matter because the crystal structures are relatively simple and the
basic structura l units are individual atoms, which are highly mobi le. Metal lic crystals
nucleate and grow quickly, making production of a meta lli c glass more challenging.

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One way to quantify the ability of a metallic alloy to be produced in glassy form is
through the critical cooling rate - the slowest rate at wh ich a metallic liquid may be
cooled and still produce a fully amorphous structure, as shown in Figure 2(a). The
critical cooling rate for a variety of metallic glass-forming alloys is shown in Figure 2(b).
Early metallic glasses (discovered in the 1960s and 1970s) were binary alloys with
critical cooling rates typically on the order of 104 to 107 K/s. Achieving such high
cooling rates requires specialized techniques (such as melt spinning) and limits the
maximum thickness of the metallic glass to < 100 µm because of the need to rapidly
extract heat from the melt. As a result, these early metallic glasses could be produced
in only a limited range of forms, including ribbons, foils, wires, and powders.
Extensive research efforts in alloy design over the past two decades have resulted in
the development of multi-component alloys with much lower critical cooling rates (0.1
K/s or even lower). This has enabled the production of metallic glass specimens in
larger sizes-in some cases exceeding 1-cm section thickness. Common practice in the
field is to refer to any alloy capable of being cast into a section at least 1-mm thick as a
"bulk" metallic glass. These alloys may be cast or molded into forms suitable for
structural applications.
At present, it is not possible to predict a priori the glass-forming ability of an alloy of
arbitrary composition. A variety of empirical rules for selecting alloying elements and
compositions have been proposed, and techniques have been demonstrated for efficient
searching of composition space. But identification of alloys with good glass-forming
ability is still mostly a matter of trial and error. As a result, the number of truly
outstanding glass-forming alloys (loosely defined as being able to be cast as a glass to
a thickness of at least 1 cm) is quite limited (see Table 1).
Table 1. Selected Bulk Glass-Forming Alloys. Selected alloys reported to have
excellent glass forming ability, quantified here as the maximum thickness of a
fully amorphous casting. 2 3 4 s 6 7 s
Composition
MgGsCu 1sAgsPdsGd 10
Zr41.2Ti 13.sCu 12.sNi 10 Be22.s
Pd40Cu30Ni10P:w
CU41Zr4sAg4Al4
Pts1.sCU14_7Nis.J P22.s
Ti40Zr2sNi3Cu 12Be20
Fe4sCr1sM014Er2C1sBG

Maximum Thickness
(mm)
10
50
72
10
16
14

Reference
2
3
4
5
6
7
8

Moving from the laboratory to industrial practice, it is important to note that factors
besides alloy composition can affect glass-forming ability. In particular, some alloys are
sensitive to the presence of impurities; for example, the glass-forming ability of some
zirconium-containing alloys is dramatically reduced by the presence of oxygen.
Processing conditions also influence the ability to make a glass; these may include the
material and surface finish of the mold and the temperature of the liquid prior to
casting . Finally, glass-forming ability can be quite sensitive to small variations in
composition, which may be difficult to control in industrial practice.

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Casting and Molding
Like other alloys, metallic glasses can be cast into net-shape or near-net-shape
geometries. Die casting into a permanent (metal) mold-because it provides the rapid
heat transfer needed to meet the requirement for relatively rapid cooling - ls the most
common casting technique. In most cases, casting is done in either a vacuum or an
inert atmosphere to prevent formation of oxide particles that promote crystallization.
Conventional casting, however, does not take advantage of the flexibility afforded by
the glassy nature of these alloys. If a metallic glass is heated to a temperature above
its glass transition temperature, it becomes a supercooled liquid. In this state, the
viscosity drops with increasing temperature over a wide range, making it possible to
control the viscosity by controlling the temperature. 1 This ability to control the viscosity
enables many of the processing techniques commonly used in molding thermoplastic
polymers to be applied to metallic glasses (Figure 3).

200 µm
Figure 3. Examples of Processing of Metallic Glasses. (a) Microsprlng produced by lithography and (b) thin
walled bottle produced by blow molding. Images are courtesy of Professor Jan Schroers (Yale University).

There are two important limitations on processing of metallic glasses in the supercooled
liquid region. First, supercooled liquids are metastable and have a tendency to
crystallize, so there is a limited window of time (typically on the order of minutes) in
which the processing must be completed if the glassy structure is to be maintained.
Second, the viscosity of many glass-forming alloys near the glass transition
temperature is too high for convenient processing. The viscosity can be reduced by
increasing the processing temperature, but higher temperatures promote crystallization
1 A crystalline metal, In contrast, melts abruptly, going from a rigid solid to a low-viscosity fluid very quickly.

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and thus reduce the window of time available for molding. In practice, therefore,
successful molding requires careful control of the processing conditions.

Joining
Structural applications inevitably require joining of components, for instance by
mechanical fasteners or adhesives or by welding, soldering, or brazing. The use of
fasteners and adhesives is much the same for metallic glasses as for any other metal.
Techniques such as welding, soldering, and brazing are potentially problematic because
they involve heating the glassy alloy, running the risk of crystallization (which could
make the joint more brittle) . In welding, for instance, the metal to be joined is actually
melted and then resolid ifies upon cooling. In the case of a metallic glass, care must be
taken to ensure the cooling rate is fast enough to avoid crystallization. There is also a
risk that the glassy material in the heat-affected zone (near to but not in the molten
region) might crystallize. Laboratory tests of a variety of welding techniques have been
performed on several glass-forming alloys with mixed results, and it is clear that much
remains to be done in this area.

Foams
One particularly promising recent development is the ability to produce metallic glass
foams. Here, the relatively high viscosity of glass-forming alloys is an advantage in
producing a stable foam structure that can be solidified, leaving a high-porosity foam
with metallic glass ligaments. 9 These foams have high specific strength (that is,
strength normalized to density) and specific stiffness and could have excellent damage
tolerance, although this has not been demonstrated.

Thin Films and Coatings
The discussion above focuses on the processing of free-standing metallic glasses, with
an emphasis on structural applications. However, it is also possible to produce
amorphous alloys as thin films or coatings using techniques such as physical vapor
deposition or electrodeposition. Although the thicknesses of material that can be
produced in this way are limited, they are useful for making amorphous alloy coatings
(for wear and corrosion resistance) or for thin films for magnetic or micro
electromechanical system (MEMS) applications. A distinct advantage of the thin film
techniques is that because the effective cooling rates during vapor deposition are
extremely high, a much wider range of alloys can be produced in amorphous form than
is possible with casting. This allows the alloy composition to be tailored for optimization
of functional properties, with less concern about glass-forming ability.

Mechanical Behavior Near Room Temperature
When a material is subjected to a stress, it can experience both elastic and plastic
deformations. Elastic deformation occurs at lower stresses and is recoverable when the
applied stress is removed. The limit of elastic deformation is defined by the yield
stress- the point at which plastic (nonrecoverable) deformation begins. Much of the
current interest in metallic glasses arises because their yield stresses (that is, their
strengths) can be much higher than those of crystalline alloys of similar composition;
this difference is a direct result of the novel atomic-scale structure of metallic glasses.
The fracture and fatigue characteristics of metallic glasses are also different from those

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of conventional alloys. In this section, we review the mechanical behavior of metallic
glasses, with particular attention to properties of interest for aerospace applications. We
consider actual properties in detail in the section below on applications, where we
compare the properties of metallic glasses with those of other advanced structural
materials.

Stiffness : Elastic Deformation
Stiffness is the resistance of a material to elastic deformation and is quantified by either
the elastic modulus (for tensile or compressive loads) or the shear modulus (for shear
loading). Metallic glasses tend to be somewhat (20-30 percent) less stiff than
crystalline alloys of similar composition. The lower modulus is a consequence of the
amorphous structure, in which atoms are (on average) slightly farther apart than in a
crystalline alloy, enabling certain atomic relaxations that are not possible in a crystal.
The lower modulus of amorphous alloys is clearly a concern in applications where
stiffness is a primary criterion, but it does present some advantages. For instance,
some applications (springs, for example) require the ability to store elastic strain
energy (resilience), and here metallic glasses do quite well. Resilience is also a key
figure of merit for snap-fit assembly of materials without fasteners. Overall, however,
for structural applications, the low stiffness of metallic glasses is a disadvantage.

Strength and Ductility: Plastic Deformation
The theoretical strength of perfect, defect-free crystalline metals is several orders of
magnitude larger than strengths measured in typical laboratory experiments. The
difference exists because metallic crystals inevitably have crystalline defects
(dislocations) that are able to move at relatively low stresses and cause plastic
(nonrecoverable) deformation . Because dislocations cannot exist in an amorphous
structure, in principle the strength of amorphous alloys should approach theoretical
limits based on the inherent strength of the atomic bonds. As shown in Table 2, the
strength of aluminum - based metallic glasses can be two or three times greater than
those of conventional (crystalline) high-strength aluminum alloys. Similarly high
strengths are seen for other amorphous alloys; for instance, the best iron -based alloys
have a strength of approximately 4 GPa-again, two or three times greater than those
of conventional high-strength steels. 10 Such high strengths create great interest in
potential structural applications of metallic glasses.

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Table 2. Comparison of Strengths of Amorphous and Crystalline
Aluminum Alloys. Compared with the theoretical maximum strength
(taken to be µ/30, where µ is the shear modulus of pure aluminum).
Yield Stress (MPa)

Theoretical
Strength (Defect
Free Crystal)
Typical High
Strength
Aluminum Alloy
(7xxx Series) 11
Best Crystalline
Aluminum Alloy 12
Aluminum-Based
Metallic Glass 13

% of Theoretical
Stren th

1,600

400- 500

25- 31%

770

48%

1,280

80%

Unfortunately, the lack of dislocations in amorphous alloys is also their Achilles' heel. In
crystalline alloys, dislocations move and multiply in response to applied stresses,
resulting in dislocation tangles that increase the resistance to further dislocation
motion . Th is process, called strain harden ing, is of crucial importance because It makes
plastic deformation stable. If one region of a crystalline material yields and begins to
plastically deform, the deforming region strain hardens, and so another region will
deform instead. The result is that the plastic deformation is not concentrated but rather
spreads through a large volume of material. Metallic glasses, lacking dislocations, do
not strain harden and in fact strain sohen in response to plastic deformation . Th is
means t hat as soon as any one region yields, any further deformation will occu r in the
same region. This process, known as shear localizati1on, leads to the formation of shear
bands (Figure 4 ). In any loading geometry where the metallic glass experiences
significant tensile loading, fracture occurs on a single dominant shear band with
essentially zero tensile ductility. 2 Metallic glasses therefore fracture in an abrupt,
apparently brittle manner on the macroscopic scale (even though there can be
significant plasticit y on a microscopic scale). This lack of ductility is of obvious concern
to designers interested in structural applications. Furthermore, it limits the ability to
fabricate metallic glasses into different shapes by deformation processing (by rolling or
forging, for instance) aher casting.

2 Th is assumes there is no geometrical constrai nt preventing fractur,e. Some geometries (such as si mple bending)
can involve tensile load ing, but there can still be significant plastic deformation because the geometrica l constraints
Inhibit propagating of shear bands across th e specimen.

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200µm
Figure 4. Shear Bands. Produced by bend ing of a zirconium -based metallic glass. v,

Fracture Toughness
Fracture toughness is a measure of a material's resistance to growth of cracks, a critical
property for structural materials subjected to tensile loading. In very tough metals, the
toughness usually results from plastic deformation that occurs near the tip of the
advancing crack; plastic deformation requires energy, and the need to provide this
energy translates into resistance to crack growth. 3 Despite their lack of tensile ductility,
at least some metallic glasses are not brittle in the same sense that ceramics are, for
example, because they can experience significant plastic deformation around the crack
tip during fracture. For instance, the fracture toughness (Krc) of zirconium-based
metallic glasses is about 20 MPa -m 112 15 -somewhat lower than the "' 55 MPa -m 112
typical of crystalline zirconium alloys 16 but much greater than the fracture toughness of
ceramics (typically 1-5 MPa-m 112 ). The fact that metallic glasses are reasonably tough
despite their lack of tensile ductility suggests structural applications are not out of the
question.
However, some metallic glasses appear to be intrinsica lly brittle in that they fracture
with only limited plastic deformation near the crack top and thus have very low values
of fracture toughness. For this reason, some alloys t hat would otherwise be highly
desirable, such as iron-based metallic glasses (for their high strength and low cost) and

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