An Evaluation of the Creep Properties of an Al 2 O 3 /Ni 3 A1 Composite and the Effect of Disorder on Mechanical Propert
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AN EVALUATION OF THE CREEP PROPERTIES OF AN Al203/Ni 3 A1 COMPOSITE AND THE EFFECT OF DISORDER ON MECHANICAL PROPERTIES P.C. BRENNAN*, W.H. KAO* AND J.-M. YANG** *Mechanics and Materials Technology Center, The Aerospace Corp.,El Segundo, CA 90245 **Department of Materials Science and Engineering, University of California, Los Angeles, CA 90024 ABSTRACT Ordered Ni 3 A1 alloys and their composites are attractive materials for elevatedtemperature structural applications due to their many favorable properties. The addition of alloying elements can significantly lower the Ni 3AI order-disorder transition temperature and also result in the formation of a Ni solid solution. As the percentage of Ni solid solution increases, the composite's room-temperature flexural strength increases. The effect of processing parameters on the material's microstructure is discussed. The complex matrix microstructure also has a significant effect on the composite's creep properties. Normal power-law creep was exhibited by the composite material when tested in compression. INTRODUCTION Future hypersonic aircraft and spacecraft will require advanced materials with temperature and strength capabilities far in excess of those possessed by the Ni-based superalloys in current use. Ordered intermetallic alloys and their composites are attractive candidates for many of these applications. The Ni 3Al intermetallic, which has a L12-type crystal structure, maintains its ordered structure to, or at least near, its peritectic melting point at 1395TC. The ordered structure means that dislocation motion is more complex, diffusion is more difficult, and, thus, creep deformation is much slower than in a similar disordered material. This is critical for materials to be used in high-temperature applications under load. The addition of certain alloying elements can lower the order-disorder transition temperature (TO for Ni 3 AI significantly below the melting point [1]. The alloying additions can also result in the formation of a Ni solid solution referred to as gamma. The ordered Ni 3AI intermetallic phase is often referred to as gamma prime. Most critical to a material to be used at high temperatures is its creep properties. Creep mechanisms in Ni3 AI depend on the stress orientation and whether the creep is occurring above or below T the temperature at which the yield strength peaks. When tested in tension at TTp, normal creep behavior with a steady-state creep regime is typically seen. It is the purpose of this paper to investigate the multi-phase microstructure of an A12 0 3 particulate-reinforced IC-221 composite and the effect of microstructure on room-temperature (RT) mechanical properties, and to evaluate the composite's compression creep properties. EXPERIMENTAL PROCEDURE The IC-221 matrix used in this study, developed by Oak Ridge National Laboratories, has a composition of Ni, 16.1 Al, 8 Cr, 1 Zr, and 0.1 B. The IC-221 powder was mixed with 25 vol. % A12 0 3 particulates in a Turbula Shaker Mixer. Billets were prepared by vacuum hot pressing at 900
