Interfaces in XD processed TiB 2 /NiAl composites
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I.
INTRODUCTION
IT is generally accepted that excellent bonding between the metal matrix and the ceramic reinforcement can be achieved by the XD* (exothermic dispersion) *XD is a trademark of Martin Marietta Corporation, Bethesda, MD,
process for several of the metal matrix composites reinforced with TiB2 particles, such as TiB2-NiA1, tl,2] TiB2-TiA1, t3j and TiB2-A1 alloy, taJ During the XD process, the TiB2 particles are formed in situ in the molten metal matrix, so they have clean, nonoxidized interfaces and typically are single crystals of high purity. Usually~ the crystal structures of the metal matrix and the ceramic particles are so different that no simple topotaxial relationship exists t3~ in XD-processed metal-matrix composites. Recently, it was found that during high-temperature deformation, dislocations can be generated from a matrixparticle interface into both the matrix and the particles in XD-processed TiB~/NiA1 composite, t5.6] Therefore, interface structures might be very important in controlling the mechanical properties at these composites. However, another intriguing observation concerning the TiB2/NiA1 composites is the lack of dislocation generation due to cooling from the high-temperature (1673 K) a n n e a l i n g . 15.61 The predicted thermal residual stress should be very large, and this stress should be relieved by dislocation generation, tT~ It is possible that an interface layer exists between the TiB2 and the NiA1 which can act as a stress absorber, t71 The purpose of the current investigation was to determine if an interface layer exists and then determine if it is of sufficient thickness a n d also to determine if any consistent crystallographic relationships could be found between the matrix and the reinforcement. II.
EXPERIMENTAL PROCEDURE
Composites of 0, 10, and 20 vol pet TiB2/NiA1 were purchased from the Martin Marietta Corporation L. WANG, Graduate Student, and R.J. ARSENAULT, Professor, are with the Metallurgical Materials Laboratory, Department of Materials and Nuclear Engineering, University of Maryland, College Park, MD 20742-2115. Manuscript submitted January 30, 1991. METALLURGICAL TRANSACTIONS A
(Bethesda, MD). A series of compression tests at 1033, 1143, and 1273 K were performed in the strain-rate range from 10 -6 t o 10 -2 S- I . The transmission electron microscopy (TEM) foils were obtained from the compressivetested sample by electric discharge machining disks with a thickness of 0.5 mm and then dimple grounding to a thickness of 0.07 to 0.1 mm at the center. Foils for conventional TEM were prepared by twin-jet electrolytic polishing in a 5 pct perchloric acid-ethanol solution, whereas samples for high-resolution electron microscopy (HREM) were prepared in the same way as for conventional TEM except they were ion milled from 10 minutes to 1 hour to clean the surface as the final procedure. Most of the conventional TEM work was carried out on the 1 MeV high-voltage electron microscope (HVEM) at the Argonne National Laboratory (Argonne, IL). Highresolution elec
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