Interfaces in as-extruded XD Al/TiC and Al/TiB 2 metal matrix composites
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A detailed study was conducted of the microstructure and particle-matrix interfaces in Al/TiCp metal matrix composites prepared by the XD process and subsequent extrusion. A study of the morphology of the TiC particles showed that the surfaces are low index (111) and (200) planes, the former being more common. Direct contact on an atomic scale is established between Al and TiC, allowing chemical bonds to form. Young's modulus is in the range expected for a composite of Al and TiC with good interfacial bonding and load transfer to the particles. No third element has been detected at the interfaces, showing that they are clean. Both incoherent and semicoherent interfaces are seen. The interface character depends on the size of the particles and their orientation with respect to the neighboring Al grains. "Special" interfaces with evidence for nearly periodic dislocations were observed in both XD Al/TiC and Al/TiB 2 composites, indicating the general tendency of in situ composites to lower their interfacial energy by forming such boundaries.
I. INTRODUCTION
II. BACKGROUND
Aluminum-based metal matrix composites are designed to combine the ductility of the Al matrix and the strength and modulus of the reinforcement phase. Considerable improvement can be obtained in the strength, stiffness, and wear resistance of composites compared to those properties in monolithic metals and alloys. However, often it is seen that the gains in the above properties fall short of theoretical expectations and the ductility is limited. Behavior may vary in the same composite system depending on the processing method. Load transfer from the matrix to the hard ceramic reinforcement, dislocation-particle interactions, and fracture depend on the properties of the interface region. Both efficiency of load transfer at a metal-ceramic interface and plastic deformation in the vicinity depend to a large extent on whether the bonding is mechanical or chemical. If chemical, the nature of the bond (ionic, covalent, or van der Waals) also is important. The character of the chemical bond is strongly influenced by the interfacial structure and chemistry on an atomic scale. Thus, in a study of composites it is important to characterize specific interfaces atomically and correlate the interface chemistry and structure to the macroscopic mechanical properties.
Many interfaces in composites such as those in Al/SiC, 1 A1/B4C,2 and Ti/SiC 3 can be categorized as multiphase interfaces because definite chemical reaction products are present at the interfaces. The wetting mechanism between the liquid metal and the ceramic particle is a chemical reaction between the matrix and the reinforcement producing additional phases. Such reaction products are often brittle in nature and detrimental to mechanical properties, especially room temperature ductility and fracture toughness. The presence of a reaction phase at the interface as well as contamination during processing prevent a direct low energy interface from being attained between the matrix and the dispersed p
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