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

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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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 WARNlNG: Further dissemination of the photographs in this publication is not authorized.
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 1uestions pertaining to
this document should be addressed to!AAP Person 1 , AAWSA Program
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
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Current Challenges and Prospects for the Future ................................................. 20
Alloy Design ..................................................................................................... 20
Thermophysical 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.
- 0Microstructural 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 the true sense of the word, rather than melting abruptly (as crysta lline
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 metallic glass is to retain the disordered, liquid-like atomic scale
structure during cooling from the melt. All materials have a tendency to crystallize upon
cooling because the crystalline state is the most stable structure at any temperature
below the melting point. But crystallization takes time, so if the cooling is fast enough,
it is possible to bypass crysta llization and form an amorphous structure at the glass
transition temperature (Figure 2(a)). Glass formation and crystallization are therefore
competitive processes; which one will occur depends on the material and the processing
conditions.
(a) (b)
Temperature 0 Purt: nickel
101(1
Liquid
- , " onventional' metallic gla scs
g~"_' 10~ 0 ,
Me llii1g ,., I \ cg \( max Lh1clme < I mm)
1ompcrarurc
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\ O \ Bulk metallic glasses
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u ' 0 -- - o' '
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lass lr:msition ·c: \~ '
temperatu.re u 'x,o~\ \ Pd., u.,N11,P,,
1011
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0
Time 01 OJ OA 05 Oh 0~ OB
Reduced glass tran ition temperature (T ,IT,,.)
Figure 2. Critical Cooling Rate. (a) Effect of the cooling rate on glass formation - If the cooling rate ls slow (path
1), then the melt crystallizes before going through the glass transition. If the cooling rate is fast enough (path 2),
then the melt can form a glass. The critical cooling rate (path 3) is the slowest rate at which the melt can be cooled
and still form a glass. (b) Critical cooling rates for various metallic alloys - The horizontal axis is the glass transition
temperature normalized to the melting (liquidus) temperature. 1
For some materials, such as silica (silicon dioxide) and most thermoplastic polymers,
the crystallization process is slow because the crystal structures are complex and the
basic structural units (for example, segments of polymer chains) are slow to rearrange
into a crystalline form. These materials can therefore be produced 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 structural units are individual atoms, which are highly mobile. Metallic crystals
nucleate and grow quickly, making production of a metallic 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 which 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 196