Defense Intelligence Reference Document Metallic Glasses For Aerospace Applications

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EX SITU COMPOSITES
There are two basic ways of making ex situ composites, in which the metallic glass
matrix and the crystalline phase are combined physically, without a chemical reaction:
• Add crystalline particles to a melt of a glass-forming alloy and then cast under
conditions that allow the matrix to form a metallic glass.
• Make a preform of a crystalline phase (by packing fibers into a mold, for instance)
and then cast the glass-forming alloy around the preform.
Both approaches have limitations. In the first, the addition of particles to the melt
increases the viscosity (which is already quite high relative to non-glass-forming alloys)
considerably, ultimately to a point where casting becomes impossible. This limits the
volume fraction of particles that can be added, which in turn limits the control one has
over the microstructure and, in particular, the spacing of the particles. With a perform,
the volume fraction of the crystalline phase can be much higher (up to about 80
percent by volume), but the problem then is how to infiltrate the high-viscosity melt
into the preform without leaving voids and while still ensuring sufficiently rapid cooling
to form a glassy matrix. With both approaches, interfacial reactions between the
crystalline phase and the melt can cause partial or complete crystallization of the
matrix, degrading the mechanical properties.
IN SITU COMPOSITES
The difficulty of making satisfactory ex situ composites has led to the development of a
new approach in which the crystalline phase is precipitated directly from the melt,
either during casting 28 or in a separate step prior to casting. 29 30 Precipitation during
casting, although easier, is problematic from a practical standpoint because variations
in the cooling rate (from the surface to the center of a castingr for instance) lead to
significant variations in structure and, hence, in properties.
One of the most promising recent advances in the metallic glass field is the
development of in situ composites in which the crystalline phase is precipitated as
dendrites, either during casting (Figure 8) or by holding the alloy at an elevated
temperature prior to casting. 31 By suitably choosing alloy composition, holding time,
and temperature, the volume fraction, size, and spacing of the dendritic phase can be
controlled. This control provides great flexibility in determining the mechanical
properties of the resulting material. Because the crystalline phase is produced prior to
casting, variation in the cooling rate across the casting is much less important, though
the cooling rate must still be sufficiently high to ensure the matrix forms a glass during
cooling. Once the glassy matrix is formed, the composite can be reheated above the
glass transition temperature, allowing for thermoplastic forming in a manner similar to
single-phase metallic glasses (as described above). Finally, the presence of the
dendritic second phase allows for deformation processes (for example, by cold rolling or
forging), similar to crystalline alloys. 32
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