In-Situ Synthesis of Aluminum- Titanium Diboride Metal Matrix Hybrid Nanocomposite

Metal matrix nanocomposites (MMNC’s) are reported to have improved mechanical, thermal and electrical properties as compared to their respective base alloys. To date, these materials have been synthesized mainly by powder metallurgy or deformation processing. Solidification synthesis of MMNCs is a promising method, capable of economically producing large and complex shapes, however technical challenges including nanoparticle agglomeration, and poor interfacial strength have hindered the adoption of this technology. In-situ processing methods, in which the reinforcements are synthesized in liquid metals, typically via exothermic reactions offer the potential for improved dispersion and interfacial bonding between the reinforcement and the matrix, however this technique has been largely unexplored in the literature for metal matrix nanocomposites. The objectives of this research were to examine the feasibility of synthesizing nano or sub-micron size particulates in liquid aluminum using in-situ stir mixing and squeeze casting. An exothermic reaction was designed to synthesize Al2O3 and TiB2 from TiO2 particles and elemental boron in an aluminum melt. This dissertation investigates (i) the mechanism of aluminothermic and borothermic reduction of titanium oxide in the presence of molten aluminum and boron, (ii) in situ synthesis of micron and nano sized particles via solidification processing, and (iii) the effects of processing variables on the physical, microstructural, mechanical and tribological properties of in-situ MMNCs. Microstructural examination and theoretical analysis indicates that the reaction to form TiB2 and Al2O3 proceeds through several complex non-equilibrium reactions. A multi-stage reaction model is proposed to describe the process by which the TiO2 surface is reduced to form Al2O3 and TiB2. The effects of the powder particle size on the formation of reinforcing phases and microstructural evolution have been investigated and it was found that nanosized TiO2 powder promoted the formation of smaller size reinforcing phases. Furthermore, a solidification route has been designed to fabricate in-situ aluminum composites reinforced with submicron Al2O3 and TiB2 particulates. Experimental and theoretical analysis is presented that shows that the particle size and refining power of nanoparticles is controlled by the viscosity of the melt, rather than precipitation and growth. In addition, it was found that increasing the weight percentage of nanoparticles of TiO2 resulted in an increase in elastic modulus with good agreement to analytical models. Increasing the weight percentage of reinforcement up to 4 wt% resulted in an increase in the hardness greater than that predicted by the rule of mixtures or the Hall Petch relationship