With the upper temperature limit of the Ni-based superalloys
attained, a new class of materials is required. Intermetallics
appear as likely candidates because of their attractive physical
properties. With a relatively low density, high thermal
conductivity, excellent oxidation resistance, high melting point,
and simple crystal structure, nickel aluminide (NiAl) appears to be
a potential candidate. However, NiAl is limited in structural
applications due to its low room temperature fracture toughness and
poor elevated temperature strength. One approach to improving these
properties has been through the application of eutectic composites.
Researchers have shown that containerless directional
solidification of NiAl-based eutectic alloys can provide
improvement in both the creep strength and fracture toughness.
Although these systems have shown improvements in the mechanical
properties, the presence of refractory metals increases the density
significantly in some alloys. Lower density systems, such as the
carbides, nitrides, and borides, may provide NiAl-based eutectic
structure. With little or no information available on these
systems, experimental investigation is required. The objective of
this research was to locate and develop NiAl-carbide eutectic
alloys. Exploratory arc-melts were performed in NiAl-refractory
metal-C systems. Refractory metal systems investigated included Co,
Cr, Fe, Hf, Mo, Nb, Ta, Ti, W, and Zr. Systems containing carbides
with excellent stability (i.e., HfC, NbC, TaC, TiC, and ZrC)
produced large blocky cubic carbides in an NiAl matrix. The
carbides appeared to have formed in the liquid state and were
randomly distributed throughout the polycrystalline NiAl. The Co,
Cr, Fe, Mo, and W systems contained NiAl dendrites with a two-phase
interdendritic microconstituent present. Of these systems, the
NiAl-Mo-C system had the most promising microstructure for in-situ
composites
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