Owing to the rapid advancement of composite materials technology, composites are not only replacing conventional materials in many applications but also creating new areas unique to themselves. In recent year, Metal/ceramic composites are even more attractive because they have improved properties over conventional materials and are relatively simple to fabricate. Recently, in situ processes for metal matrix composites have emerged as novel processing techniques. In situ techniques involve a chemical reaction resulting in the formation of a very fine and thermodynamically stable reinforcing ceramic phase within a metal matrix. In particular, the reinforcement surfaces are likely to be free from contamination and a strong bonding between matrix and reinforcement can be achieved.
Aluminum composites have a great potential because of their high strength-to-weight ratio. The TiC-Al composite system has been studied by a number of researchers, which shows high strength, high wear resistance and high stiffness. The TiC-Al composite can be made either by adding TiC powders directly to the melt, or by in situ reactions between carbon sources such as $Al_4C_3$ particles and $CH_4$ gas in Al-Ti melts. In addition, the XD process could make the in situ TiC-Al composite by using an exothermic reaction.
It is well known that the strength of metal matrix composites can be increased by suitably controlling the dispersion parameter, i.e. either by increasing the volume fraction or by decreasing the particle size of reinforcement. However, the processing of composites with high volume fractions (greater than 30vol.%) of reinforcements is difficult and usually lead to structures with defects (cavities and cracks). In this Ph.D. thesis, it was suggested that high volume TiC reinforced aluminum composites could be synthesized by using a dipping exothermic reaction process (DERP) as a newly suggested process technique. DERP involves exothermic reactions like other in-situ processes. In this process, however, the concept of the addition of a dilution agent is introduced to control or reduce the explosive exothermic reaction in order to obtain an improved microstructure of high volume reinforced in-situ composites. DERP is carried out in liquid metal. Therefore, a one-step process is possible for fabricating high volume reinforced in-situ composites without pore structure, which is an advantage of the dipping exothermic reaction process. Moreover, the originality of this technique has been confirmed by the United States Patent and Trademark Office (US patent No. 6,406,516).
First above all, in this thesis, the reaction mechanism of TiC reinforced Al composite was investigated by a novel in-situ process, Dipping Exothermic Reaction Process (DERP). Reaction sequences of DERP are as follows: (1) the first infiltration of molten aluminum into porous Ti-C-Al-TiC preform, (2) exothermic reaction between Ti and C to form TiC, and (3) the second infiltration of molten aluminum into porous skeleton of TiC-Al composites. TiC powder was added as a dilution agent to control the explosive reaction and provided a critical clue to the microstructure variation. In the preform, aluminum powder was used as an activator for the first infiltration. After the initial infiltration, the high exothermic reaction similar to typical combustion reaction was followed. The high temperature of the exothermic reaction accelerated the second infiltration. By increasing the content of the dilution agent, which is the major parameter of a dipping exothermic reaction process (DERP), microstructure homogeneity improved. In this thesis, it was suggested that the homogeneous TiC-Al composites could be synthesized with the value of _log (υ/$T_c$) = 4.9 to 5.3 through a dipping exothermic reaction process.
Mechanical properties of the high volume TiC reinforced composites fabricated by DERP were measured. The yield strength and the ultimate tensile strength of the extruded composite were 223MPa and 351MPa respectively. These values are relatively high although the alloying element was not added. As the results of measuring the elastic modulus, the optimum enhancement of E can be obtained according to increasing the content of reinforcement. Also, thermal treatment for the improvement in the mechanical properties was evaluated. Through the thermal treatment, it was possible to change the microstructure from metal matrix composite (MMC) of aluminum matrix to ceramic matrix composite (CMC) of $TiAl_3$ matrix. In particular, wear of the high volume reinforced in-situ Al-TiC composite was studied using a ball-on-reciprocating flat apparatus. For comparing the wear properties, SiC reinforced aluminum alloy (A359-20vol.% SiC) was used as a reference sample. As-extruded composite showed the abrasive mild wear behavior and as-dipped composite and A359-20vol.% SiC composite showed the severe wear behavior. From the variation of the wear loss, the wear property of as-extruded composite was superior to as-dipped composite and A359-20vol.% SiC composite. TiC particles of as-extruded composite could serve as hard barriers that enhance the resistance to plastic deformation.
$Ti_3AlC_2$ of whisker shape as an intermediate phase was observed in the in situ TiC-Al composite prepared by the exothermic reaction process in liquid aluminum. $Ti_3AlC_2$ is a member of a class of ternary carbides that recently have been shown to posses an unusual combination of properties. It was the first time to observe the $Ti_3AlC_2$ of whisker shape during the reaction sequences of the Ti-C-Al system. Also, the formation mechanism of $Ti_3AlC_2$ phase was suggested in this thesis.
