AAWSAP DIRD, Metallic Glasses for Aerospace Applications, December 2009

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Even some metallic glasses with reasonable toughness may be embrittled by exposure
to elevated temperatures. This may occur in the heat-affected zone during welding (as
discussed above), or it may be a by-product of processing in the supercooled liquid
region (as in injection molding, for instance). The causes of embrittlement are also not
well understood, and there is no known way to reverse embrittlement once it occurs.
Fatigue
Fatigue is a process by which materials can experience incremental crack growth owing
to cyclic loading, even at stresses well below the yield stress. If unabated, fatigue
cracks can grow to a critical length at which abrupt catastrophic fracture occurs. Up to
90 percent of failures of structural components in service are estimated to be caused by
fatigue, making fatigue resistance of obvious importance to designers.
The fatigue resistance of metallic glasses is not very good. A common measure of
fatigue resistance is the fatigue limit- the stress amplitude (range) below which no
fatigue failure will occur, regardless of the number of loading cycles the material
experiences. The fatigue limit for high-strength crystalline alloys is typically about 40
percent of the tensile strength, but for metallic glasses, it is only about S percent of the
tensile strength (Figure 5). The reason for this difference has to do with the structure of
the material. In a crystalline alloy, there are microstructural features (such as grain
boundaries and precipitate particles) that can inhibit the growth of fatigue cracks. In
metallic glasses, the microstructure is completely featureless, and there is nothing to
prevent fatigue cracks from growing once they have been initiated.
The poor fatigue resistance of metallic glasses is a critical limitation for structural
applications in aerospace because it implies a need to overdesign components to keep
the stresses far below the yield stress. Thus, much of the advantage of having a high­
strength material in the first place is lost. The desire to improve metallic glasses'
fatigue performance has led to the development of metallic-glass-matrix composites
with outstanding properties, as discussed below.
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