JOURNAL OF MATERIALS SCIENCE 38 (2 0 0 3 ) 2073 – 2077 Combustion synthesis and subsequent sintering of titanium-matrix composites A. CHRYSANTHOU ∗,† , Y. K. CHEN † , A. VIJAYAN † , J. O’SULLIVAN ‡ Department of † Aerospace, Automotive and Mechanical Engineering and ‡ Design Technology and Management, University of Hertfordshire, Hatfield, Herts AL10 9AB, UK Titanium-matrix composites containing up to 50 wt% TiC particles were prepared by combustion synthesis using elemental powders. The products were subsequently sintered at 1160 ◦ C for various periods of time. SEM and X-ray diffraction were used to assess the changes that took place during the two stages of processing. Products of combustion synthesis containing in excess of 25%TiC contained large cracks as well as agglomerates of carbide particles that were undesirable from the point of view of reinforcing the metal. The carbide obtained by combustion synthesis had a higher carbon content than that expected according to the Ti-C equilibrium phase diagram, due to the non-equilibrium nature of the reaction. During the sintering stage, the carbide of non-equilibrium composition reacted with titanium to yield the carbide of equilibrium composition. The composition changes were investigated and their significance on the sintering process is discussed. C  2003 Kluwer Academic Publishers 1. Introduction Titanium is a lightweight material with a strength to weight ratio superior to that of both steel and alu- minium. For this reason, titanium has found a wide range of applications in the aerospace industry. De- mands for improvements in mechanical properties and service temperatures have in recent years led to re- search in the development of titanium-matrix compos- ites. Prospective uses of these materials include aircraft engine components [1, 2], automotive valves, wear- resistant parts like gears and tubing, sports equipment as well as military applications. A number of problems have been identified that may restrict the development of titanium matrix composites. These problems include (i) the high reactivity of Ti with most ceramics, leading to instability of the reinforce- ment phase during processing and service and (ii) ma- terials and processing costs. A number of studies have focused on using SiC as the reinforcement material in a titanium alloy matrix [3, 4]. However, titanium and SiC are chemically non-compatible and react together to form titanium silicides and TiC as can be verified by inspection of their Gibbs Free energy data. Similarly, B 4 C will react with titanium to form titanium boride and TiC, while a titanium/TiB 2 combination will yield TiB. The choice of reinforcement in this study was TiC, which according to the Ti-TiC equilibrium phase dia- gram, coexists with Ti over a range of compositions [5]. Using vacuum plasma spraying, Dearnley and Roberts [4, 6] prepared titanium matrix composites re- inforced with SiC, B 4 C and TiB 2 , all of which are ther- modynamically unstable with the matrix. Although this ∗ Author to whom all correspondence should be addressed. technique may be useful in avoiding interfacial reac- tions during processing, the problem still exists that these reinforcements will react with Ti at high tem- peratures during service. SiC has also been used as a reinforcement of titanium-based composites that were prepared by shock wave consolidation [3]. A cold and hot isostatic pressing (CHIP) technique has been de- veloped by Dynamet Technology Inc. [7]. This pro- cess can successfully reinforce Ti-6A1-4V alloys with up to 10 vol%TiC. An investigation has shown that this route is unable to successfully produce composites with 20 vol%TiC because of clustering of particles and porosity problems [8]. The capital investment costs are quite high and the process will be at a disadvantage when competing with more conventional techniques. Chen et al. [9] produced Ti-TiC composites by intro- ducing graphite powder into molten titanium followed by casting. This led to the in-situ formation of TiC in the metal. This route has the advantage of being simple to operate and is capable of producing complex shaped components. The objective of the work which is reported here was to produce Ti-TiC composites by means of com- bustion synthesis and to subsequently sinter these to near-net-shape products. The process involves ignition of an exothermic reaction where the evolution of heat raises the temperature of the reactants and is sufficient to self-propagate the reaction. Its advantages include a low energy requirement and short reaction times. It has also been reported that the products of combustion synthesis are more easily sintered than those obtained by conventional methods [10]. 0022–2461 C  2003 Kluwer Academic Publishers 2073