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

Abstract: 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 °C. The microstructure of the resulting AMCs was characterized by fine reinforcing ceramic phases with an average particle size of 0.24 µm, which were homogeneously distributed in Al matrix. Furthermore, no significant change in particle sizes could be found after extrusion, and ceramic particulate 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.