Control of self-propagating high-temperature synthesis derived aluminum-titanium carbide metal matrix composites

Abstract

Self-propagating High-temperature Synthesis (SHS) is a combustion process that can be used to form Metal Matrix Composite (MMC) reinforcing phases in situ. Generally, the kinetic processes in these reactions are poorly understood but are affected by reactant particle size, reactant green density, reactant stoichiometry, reaction preheat temperature, and reaction product cooling rate. These reaction parameters also affect the microstructure of the reaction products because of changes in the rate of heat evolution, reaction rate, surface area available for heterogeneous nucleation, reaction temperature, and the stable phases during and after the reaction. Post-reaction processes affecting the microstructure and properties of the SHS products include densification, melt alloying (SHS reaction products are used as a master alloy), and die casting techniques. Matrix alloy additions should be controlled to prevent unwanted reactions between the matrix and the reinforcement. In the present study, Ti + C + X [right arrow] TiC + X (X = Al or TiC) is the SHS reaction system studied, with varying amounts of Al (10-50wt%) or TiC (0-20wt%) added to the reactants as a thermal diluent. Addition of these diluents decreases the reaction temperatures and decreases the TiC reinforcing particle size and interaction during particle growth. A method of direct thermal analysis of the self-heating behavior of diluted SHS reactions is developed and compared to existing methods used to measure the apparent activation energy of single step SHS reactions. The activation energies are used to determine a probable reaction path for Ti + C + Al [right arrow] TiC + Al. SHS reaction products of various diluent concentrations are analyzed for TiC particle size and shape. SHS reaction products containing 55v% TiC - 45v% Al are dispersed as a master alloy in aluminum melts; reaction products containing higher concentrations of TiC particles are difficult to disperse. To show compatibility with the TiC reinforcing particles, MMCs with aluminum alloy matrices of pure aluminum, Al-4.5Mg, and Al-4.5Mg-4.5Cu-1Mn-0.25Cr are coupled with TiC particle concentrations of 0, 10, and 20v%. MMC compositions were Thixocast at VForge in Lakewood, CO and squeeze cast at CWRU in Cleveland, OH. A pure aluminum matrix MMC with 55v% TiC was densified after the SHS reaction and thixocast, though the other MMCs with pure aluminum matrices were not thixocast because they lack a semisolid matrix condition required for thixocasting. The cast MMCs are tested for tensile, hardness, wear, and ballistic properties with properties apparently dominated by agglomerated TiC particles.