Microstructure and tensile properties of in-situ synthesized and hot-extruded aluminum-matrix composites reinforced with hybrid submicron-sized ceramic particles

Aluminum-matrix composites (AMCs) reinforced with submicron-sized ceramic particles of Al2O3, TiB2 and TiC were in-situ synthesized by reactive sintering of Al, TiO2, and B4C powder mixtures and further densified by hot-extrusion process. The reaction mechanisms for formation of the reinforcing particles, extrusion behavior, microstructure, and tensile properties of the AMCs have been investigated. The reactions of TiO2 and B4C with molten Al were a stepwise process, and there were many intermediate phases including oxygen deficient titanium oxides (Ti3O5, Ti2O3, and TiO), Al4C3, AlB2, and Al3Ti, before the expected reinforcing particles of Al2O3, TiB2, and TiC were formed. The results showed that hot-extrusion process was an effective means to densify reactive-sintered porous composites, and dense AMCs can be obtained through hot-extrusion in a temperature range of 480-550 degrees C. The microstructure of the resulting AMCs was characterized by fine reinforcing ceramic phases with an average particle size of 0.24 mu m, which were homogeneously distributed in Al matrix. Furthermore, no significant change in particle sizes could be found after extrusion, and ceramic particle content and extrusion temperature have small influences on the average particle sizes of the reinforcing phases. The presence of these sub-micron hybrid ceramic particles resulted in significant enhancements in yield and tensile strength of the AMCs. The yield strength improvement is mostly due to the coefficient of thermal expansion (CTE) mismatch between the ceramic particles and Al matrix, followed by Orowan strengthening, while the relative contributions of grain refinement and load-bearing effects are much smaller.