The chemical compatibility and tensile behavior of an Ni 3 Al-based composite
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The chemical compatibility and tensile properties of a powder processed intermetallic matrix composite (IMC) were investigated in the temperature range 298-1373 K. The matrix alloy selected for this study was the ORNL developed advanced nickel-aluminide IC-221 (Ni-16Al-8Cr-lZr-0.05B at. %). The composite contained 25 vol. % TiC particulate reinforcement. TiC/IC-221 compatibility samples were heat treated at 1373 K for up to 1000 h. A layer enriched in Zr and Ti formed at the TiC/IC-221 interface after heat treatment. In addition, Ti was observed to diffuse into the matrix. The composite exhibited higher yield strength and lower ductility than a similarly processed matrix alloy at all test temperatures. Heat treatment of the composite improved the tensile properties due to particulate/matrix interaction, resulting in improved load transfer. SEM fractography revealed that tensile failure occurred at the matrix/particulate interface. Remnants of the matrix were observed on TiC particles on the fracture surface, suggesting good matrix/particulate bonding. The mechanical properties of the composite were very competitive with Ni-base superalloys.
I. INTRODUCTION
Ductilization of the Fe- , Ni- , and Ti-aluminides has stimulated a great deal of interest in these materials for elevated temperature structural applications.1"8 The boron-doping ductilization of Ni3 Al has resulted in alloy development programs yielding several high strength, high ducility advanced aluminides. In addition to the boron micro-alloying, these Ni-aluminides contain additions cf Cr (for high temperature oxidation resistance) and Zr (or Hf, for solid solution strengthening).1 Although developed to compete with Ni-base superalloys for high temperature applications, the Nialuminides have not yet exhibited a clear-cut advantage over the superalloys. In order to become more competitive with the superalloys, the strength of the advanced Ni-aluminides must be increased further. Ni-aluminides have many unique properties that make them excellent candidates for the matrix material for a high temperature intermetallic matrix composite (IMC). For example, Ni-aluminides have an anomalous yield strength behavior with a peak in yield strength at approximately 900-1100 K. In addition, Ni-aluminides exhibit high modulus, good oxidation resistance, and a density approximately 7% lower than Ni-base superalloys. Further, the selection of the appropriate reinforcement could reduce the density of the composite while increasing the mechanical properties. a)
Current address: General Electric Company, P.O. Box 1072, Schenectady, New York 12301.
J. Mater. Res., Vol. 5, No. 8, Aug 1990
Previous investigators have reported that the advanced Ni-aluminides react extensively with SiC,9 B4C,10 and TiB 2 . n Very little reaction was observed with AI2O3, and preliminary results indicated limited TiC/IC-221 interaction.101213 However, A12O3 did not adequately bond to the Ni-aluminide matrix, which resulted in poor mechanical properties.121314 In this present study, TiC particula
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