High-temperature slow-strain-rate compression studies on CoAI-TiB 2 composites
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I.
INTRODUCTION
C o A l with a B2 structure and a large range of stoichiometry has received considerably less attention than its Ni-base counterpart, NiA1, because CoAl is much more brittle than NiA1. However, CoAl has a comparable density [~] and melting point [2] and a relatively higher modulus. [3] Both single-crystal [4] and polycrystalline tS] CoAl have been examined for their compressive creep response over a range of temperatures and strain rates. In addition, the role of stoichiometry in influencing high-temperature strength has been elucidated.~5] Whittenberger's studies showed that polycrystalline CoAl [5] has a higher compressive creep strength than NiA1 [6] at relatively high strain rates for all the temperatures examined, although at the lower strain rates, the strengths converge. Recently, Yanley and Nix [71 compared the hightemperature deformations of polycrystalline CoAl and NiA1 by strain-rate change techniques and showed that CoAl has a higher strength than NiA1 over the range of temperatures and strain rates investigated. They attributed this higher strength to greater lattice friction effects. Pease ts] showed that although both CoAl and NiA1 exhibit strong second neighbor interaction energies at 7 K, CoAl continues to retain this energy at ambient temperature, whereas NiA1 does not. It is possible that this difference in otherwise similar compounds contributes to the excessively brittle nature of CoAl. Recent work on particulate-reinforced NiA1-TiB2 composites has shown that the addition of 20 vol pet TiB2 improves the compressive strength of NiA1 at 1300 K by a factor of 3, [91 increases the elastic modulus, tl~ decreases the density, [11] and does not affect the roomtemperature toughness. [HI However, this increment in strength is still insufficient to compete with conventional superalloys, even though the latter are more dense. Since CoAl is stronger than monolithic NiA1, it was hoped that higher strength could be obtained by the addition of TiB2 S.K. MANNAN, Associate Scientist, and K.S. KUMAR, Senior Scientist, are with Martin Marietta Laboratories, Baltimore, MD 212273898. J.D. WHITTENBERGER, Materials Engineer, is with the NASA-Lewis Research Center, Cleveland, OH 44135. Manuscript submitted January 18, 1990. METALLURGICAL TRANSACTIONS A
to CoAl. Further, it was hoped that grain refinement realized in the composite could improve ambient toughness. In this investigation, results on the compressive deformation of particulate-reinforced CoA1-TiB2 composites in the temperature range of 1100 to 1300 K are presented. Hot-pressed and postdeformation microstructures were examined by optical microscopy and transmission electron microscopy (TEM). Hightemperature compressive behaviors of CoAI-TiB2 and NiA1-TiB2 composites are compared. II.
EXPERIMENTAL PROCEDURE
Powders of stoichiometric CoAl with various volume fractions of TiB2 were prepared by the XD ~M, process.[12] *This process was developed at Martin Marietta Laboratories, Baltimore, MD.
These were then hot-pressed in vacuum at 1723 K an
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