Defense Intelligence Reference Document Metallic Glasses For Aerospace Applications

UNCLASSIFIED/s'Pen. 8PPIIIJIIL -..sl!! fJULY
deformation can proceed in a stable manner by generation and subsequent arrest of
shear bands. The key to composite design is to produce a microstructure with the
correct length scale to prevent propagating shear bands from becoming catastrophic
cracks. This turns out to be relatively difficult with ex situ composites, for reasons of
processing described above. As a result, the recently developed dendritic in situ
composites have the most promising properties, and we focus the remainder of our
discussion on them.
STRENGTH AND DUCTILITY: PLASTIC DEFORMATION
As with other composite materials, the yield strength of metallic glass matrix
composites can be approximated as a simple rule of mixtures based on the volume
fraction of the two phases. Because the ductile crystalline phases useful for limiting
shear band propagation are weaker than the amorphous matrix, in producing a
composite, some sacrifice in strength is inevitable. However, the gains in tensile
ductility can be significant. For instance, monolithic titanium-based metallic glasses
(like all metallic glasses) have essentially zero tensile ductility, but in situ composites
based on titanium have been reported with tensile elongation as large as 12 percent. 35
This is comparable to the ductility of Ti-6Al-4V (the most common conventional
titanium alloy), but in a material with about 30 percent greater strength. The properties
of metallic glass matrix composites and more conventional materials are further
compared below.
FRACTURE AND FATIGUE
The development of a stable plastic zone means additional energy is required for crack
propagation, making in situ composites much more resistant to fracture and fatigue
than are single-phase glasses. For instance, the plane-strain fracture toughness of
some zirconium-based in situ composites can exceed 170 MPa m 112-7 times greater
than that of single-phase glasses and greater than that of virtually any other metallic
alloy.36 This resistance to crack propagation is also manifested as improved fatigue
performance. The fatigue strength of the zirconium-based in situ composites is 20-30
percent of the tensile strength; in comparison, monolithic metallic glasses have a
fatigue strength of only ~ 5 percent of the tensile strength. 37 The fatigue strength of
the in situ composltes is thus comparable to that of conventional structural alloys.
Aerospace Applications of Metallic Glasses
STRUCTURAL APPLICATIONS
The key properties of materials for structural applications in aerospace are:
• Strength.
• Stiffness (Young's modulus).
• Density (weight).
• Fracture toughness (damage tolerance).
16
UNCLASSIFil:D,'f Pl!Ut o,,1e11111: U81! 8HLY