Combustion synthesis of Al–Cr preforms activated in microwave field

A method of manufacturing porous preforms of intermetallic Al–Cr compounds intended for local reinforcement of composite materials was developed. Cylindrical specimens, cold pressed of powdered Al and Cr mixture with stoichiometric ratio 7:1, 4:1, 9:4 and granularity −10, −44, −74 μm were combusted in a microwave field. Microwave energy was transferred from a magnetron via a standard waveguide and focused by a tuner on the specimen under argon shield. After synthesis initiation (microwave-activated combustion synthesis = MACS), the microwave field was turned off and the reaction comprised the entire specimen. Analysis of the synthesis course on the ground of temperature diagrams revealed that the reaction temperature increased with increasing Cr content, more energy was released in shorter time and other phase transitions occurred.The obtained XRD patterns showed that the reaction proceeded between solid Cr and liquid Al to create intermetallic phases with residue of starting and transient phases. Microscopic examination of fractures revealed porous structure of the preforms with rounded Al–Cr precipitates covered with tiny cuboid particles. EDS analysis discovered reaction zones around partially reacted Cr particles. A distinct envelope of Al9Cr4 on Cr particle was next enclosed with a layer of Al4Cr compound. Following the reaction progress, pure chromium slowly passed to the envelope that finally disappeared. Selected specimens with uniform structure and sufficient interconnected porosity were subject to infiltration with liquid aluminum alloy. The castings were reinforced by intermetallic structures using the direct-squeeze casting method and typical process parameters.