Microstructural evolution of reactive-sintered aluminum matrix composites

Aluminum matrix composites reinforced with TiB2 and Al2O3 particulates were synthesized via reactive sintering of Al–B–TiO2 three-component powder mixtures. The effects of powder particle size and reactive sintering temperature on formation of reinforcing phases and microstructural evolution have been investigated. The reduction reaction of TiO2 by molten Al has been proved to be a stepwise process. Four oxygen deficient titanium oxides can be formed and Ti2O3 was a main intermediate phase. In addition to the oxygen deficient titanium oxides, there also existed several transitional phases, such as AlB2, γ-Al2O3, and Al3Ti. As the reinforcements in the composites, Al2O3 is formed by a series of aluminothermic reactions, while the formation of TiB2 depends on AlB2 transitional phase. AlB2 was gradually transformed to a mixed diboride (Al,Ti)B2, and subsequently to TiB2, through displacement of Al in the metallic sublattice of AlB2 by Ti atoms. Small-sized TiO2 powder promoted the formation of the reinforcing phases at lower temperatures, and the resultant reinforcing particulates exhibited smaller sizes.