Defense Intelligence Reference Document Metallic Glasses For Aerospace Applications

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Current Challenges and Prospects for the Future
ALLOY DESIGN
A critical limitation of existing metallic glass technology (and related composites} is the
relative dearth of alloys with good glass-forming ability. The best glass-forming alloys
are either based on expensive elements (for example, palladium) or contain toxic
elements (for example, beryllium in the best zirconium- and titanium-based alloys). For
aerospace applications, the most glaring lack is that, despite significant alloy design
efforts in the United States (through the DARPA Structural Amorphous Metals program),
Japan, China,. and elsewhere, there are no good glass-forming alloys based on
aluminum. Attempts to make aluminum-based metallic glass components by
consolidating amorphous powders have met with limited success. Similarly, all of the
good iron-based metallic glasses contain considerable amounts of nonmetallic elements
(notably carbon, boron, silicon, and/or phosphorus), which are thought to contribute to
the very low fracture toughness of these alloys (Figure 9(b)).
However, there is reason to expect that further progress is possible. Recent
experimental results have shown that some of the empirical "rules" of glass-forming
ability44 are actually quite flexible, and that glass-forming ability is much more sensitive
to composition than had been previously appreciated. 45 So it is highly probable that
some excellent glass-forming alloys compositions remain to be discovered, possibly
including some low-density glasses based on aluminum.
Identifying these good glass-forming alloys will be a challenge. Most alloy development
to date has been done with a brute-force approach, but combinatorial techniques46re
likely to enable much more rapid screening. One issue is identification of suitable
metrics for glass-forming ability, since the combinatorial approaches use vapor-
deposited thin films, and it is not clear what characteristics of such a film correlate with
glass-forming ability in the bulk. Similarly, continued development of ab initio molecular
dynamics techniques should enable identification of candidate alloys from computer
simulations, particularly as computers continue to increase in power.
One area that has received insufficient attention is the influence of processing
conditions on glass-forming ability. For instance, application of electromagnetic
vibrations during cooling reportedly significantly enhances the glass-forming ability of
magnesium-based metallic glasses. 47 This approach could, in principle, be applied to
other alloys,. possibly greatly extending the range of alloys and compositions that can
be produced as bulk metallic glasses.
THERMOPHYSICAL PROPERTIES AND THERMOPLASTIC PROCESSING
Most of the practical interest in single-phase (monolithic) metallic glasses centers on
the potential for thermoplastic processing near to or above the glass transition
temperature. However, the thermophysical properties and behavior of metallic glasses
are not well understood. For instance, the viscosity of the metallic glass melt (or
supercooled liquid) is of critical importance, but we do not know how and why alloy
composition influences viscosity. From an engineering point of view, the practical
aspects of molding of metallic glasses are just beginning to be explored. Certainly many
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