Dislocations in continuous filament reinforced W/NiAl and Al 2 O 3 /NiAl composites
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I.
INTRODUCTION
AMONG the various aluminides, NiAl has many attractive features, e.g., a very high melting point (1912 K),[1] high modulus[2,3] (ENiAl . E of cast or wrought superalloys),[2,3] low density (5.9 g/cm3),[4] and excellent oxidation resistance.[5,6] The major drawbacks are its low strength at high temperatures and inadequate ambient-temperature toughness.[7] The use of high-strength reinforcements has been shown to improve the high-temperature properties; e.g., addition of 5-mm TiB2 particulates improves the creep resistance of NiAl.[8–13] During a microstructural examination of a 20 vol pct 5mm TiB2/NiAl composite, it was observed that the dislocation density in the annealed composite was only slightly higher than that of NiAl with 0 vol pct TiB2.[10] Due to the presence of the TiB2 particulates, it was expected that there would be a reasonably high dislocation density in the matrix resulting from the relaxation of the thermal residual stress (TRS).[14] This TRS, which develops during cooling of the sample from the annealing or processing temperature to room temperature, is caused by the difference in the coefficient of thermal expansion (DCTE) between the matrix and the reinforcement. Continuous filament-reinforced composites are currently being explored as an alternative means of improving creep resistance of NiAl. It has been demonstrated that singlecrystal Al2O3 filaments are capable of improving both the creep and fatigue resistance of NiAl.[15] A recent investigation,[16] using both neutron diffraction and the finite element method (FEM), has shown that plastic relaxation of the TRS should occur in the NiAl matrix during the cooling process after annealing the composite at temperatures L. WANG is with the Materials Engineering Branch, Goddard Space Flight Center, NASA, Greenbelt, MD 20771. K. XU is with the Ceramics Division, NIST, Gaithersburg, MD 20899. R.R. BOWMAN is with NASA–Lewis Research Center, Cleveland, OH 44135. R.J. ARSENAULT is with the Department of Materials and Nuclear Engineering, University of Maryland, College Park, MD 20742-2115. Manuscript submitted October 27, 1995. METALLURGICAL AND MATERIALS TRANSACTIONS A
higher than 1473 K. The calculated elastic stresses and strains agree with the experimental diffraction data. The FEM-predicted plastic strain was found to be as high as ;0.7 pct in W/NiAl and ;0.5 pct for the Al2O3/NiAl composites, respectively, when the sample was cooled from the processing temperature (1533 K) to room temperature. In the FEM simulation, however, a perfect interfacial bond and a perfect filament were assumed. In reality, the interfacial bond strength in these continuously reinforced composites is strongly dependent upon the processing methods.[15] This difference is significant since thermal residual stresses and strains are directly related to the bond strength and, therefore, the magnitude of plastic relaxation. Dislocation generation and motion are predicted in both the discontinuous and continuous NiAl matrix composites due to the TRS; however,
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