Defense Intelligence Reference Document Metallic Glasses For Aerospace Applications

UNCLASSIFIED//f8A 8FFI&l.9.k MliiE 8tIL¥
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 resolidifies 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
5
UNCLASSIFIED// rert 8PPil1Ak Ulilii 8fJla¥