Interfaces in continuous filament-reinforced Al 2 O 3 /NiAl composites
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I.
INTRODUCTION
THE use of continuous filaments as composite reinforcements is currently being considered as a means of improving the creep resistance of intermetallics. In particular, it has been demonstrated that single-crystal A1203 filaments are capable of improving both the creep and fatigue resistance of NiA1. tlj The interfacial bond strength, which is strongly dependent upon the processing methods, plays a key role in the control of the mechanical properties of these continuously reinforced composites, oj The strength of the interfacial bond is important because it strongly influences the thermal residual stresses (TRS), which develops during cooling due to the coefficient of thermal expansion (CTE) mismatch between the filaments and the matrix. In a previous investigation,t2] it was found that if an A1203/NiA1 composite with an as-fabricated interfacial bond strength (measured by push-out testing) in the range of 90 to 180 MPa was thermally cycled 10 times in a temperature range of 373 to 1373 K, matrix cracking occurred. On the other hand, cracks were not observed after identical thermal cycling in those samples that had an interfacial bond strength in a range of 35 to 120 MPa. Also, the interfacial bond strength of these weakly bonded sampies dropped to 5 to 30 MPa in the post-thermal-cycled condition. The mechanism(s) responsible for the difference in interfacial bond strength was not clear. It was also confirmed that the NiA1 matrix was plastically deformed as a result of the relaxation of the TRS. Although dislocation densities in the region adjacent to the interface were found to be very high, dislocation generation L. WANG, formerly Graduate Student, Department of Materials and Nuclear Engineering, University of Maryland, College Park, MD 20742-2115, is Senior Materials Engineer, Code 313/Materials Branch, GSFC/NASA, Greenbelt, MD 20771. K. XU, Graduate Student, and R.J. ARSENAULT, Professor and Director, are with the Department of Materials and Nuclear Engineering, Metallurgical Materials Laboratory, University of Maryland, College Park, MD 20742-2115. R.R. BOWMAN, Research Engineer, is with NASA Lewis Research Center, Cleveland, OH 44135. Manuscript submitted June 21, 1994. METALLURGICALAND MATERIALS TRANSACTIONSA
in the interface region was not necessarily related to the interfacial bond strength since dislocation activity was observed in both weakly and strongly bonded composites. In the present study, an electron microscopy investigation was undertaken to determine the interface structures of the single-crystal A1203 filament-reinforced NiA1 composites that had a relatively weak interfacial bond strength, i.e., 35 to 120 MPa, and the effect of thermal cycling on the interface structure of the composites. Simulations of transmission electron microscopy (TEM) diffraction contrast images based upon a threedimensional (3D) finite element method (FEM) analysis were generated to understand the residual strain at regions adjacent to the interface.
II. MATERIALS AND EXPERIMENTAL METHODS The composi
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