AAWSAP DIRD, Metallic Glasses for Aerospace Applications, December 2009

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Casting and Molding
Like other alloys, metallic glasses can be cast into net-shape or near-net-shape
geometries. Die casting into a permanent (metal) mold-because it provides the rapid
heat transfer needed to meet the requirement for relatively rapid cooling- ls the most
common casting technique. In most cases, casting is done in either a vacuum or an
inert atmosphere to prevent formation of oxide particles that promote crystallization.
Conventional casting, however, does not take advantage of the flexibility afforded by
the glassy nature of these alloys. If a metallic glass is heated to a temperature above
its glass transition temperature, it becomes a supercooled liquid. In this state, the
viscosity drops with increasing temperature over a wide range, making it possible to
control the viscosity by controlling the temperature. 1 This ability to control the viscosity
enables many of the processing techniques commonly used in molding thermoplastic
polymers to be applied to metallic glasses (Figure 3).
200 µm
Figure 3. Examples of Processing of Metallic Glasses. (a) Microsprlng produced by lithography and (b) thin­
walled bottle produced by blow molding. Images are courtesy of Professor Jan Schroers (Yale University).
There are two important limitations on processing of metallic glasses in the supercooled
liquid region. First, supercooled liquids are metastable and have a tendency to
crystallize, so there is a limited window of time (typically on the order of minutes) in
which the processing must be completed if the glassy structure is to be maintained.
Second, the viscosity of many glass-forming alloys near the glass transition
temperature is too high for convenient processing. The viscosity can be reduced by
increasing the processing temperature, but higher temperatures promote crystallization
1 A crystalline metal, In contrast, melts abruptly, going from a rigid solid to a low-viscosity fluid very quickly.
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