Defense Intelligence Reference Document Metallic Glasses For Aerospace Applications

UNCLASSIFIEDl,'P811 8FPl81tf.tls Y&& &ftlsl.f
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 applicat1ons of metallic glasses.
6
UNCLASSIFIED/i'F81i1 QFFICIOh. : ■ ss ctr· X