The precipitation mechanism during aging treatment and its effect on mechanical properties of 2124Al alloy and $SiC_p$/2124Al metal matrix composites(MMC), which were fabricated by powder metallurgy process using vaccum hot press, were investigated. The differential scanning calorimetry(DSC) analysis and microstructure observation by transmission electron microscopy were performed to investigate the precipitation behavior. The tensile tests were performed to evaluate the mechanical properties. The major precipitate phase was identified as S'($Al_2CuMg$) phase and was grown along $<100>$ direction of matrix in both 2124Al alloy and $SiC_p$/2124Al MMC. Due to a large difference in thermal expansion coefficients between SiC reinforcement and 2124Al matrix, the dislocation density was abruptly increased in $SiC_p$/2124Al after solution treatment and quenching. The GP zone, which forms at the vacancy clusters, was not observed in $SiC_p$/2124Al MMC, because the abundant dislocations in $SiC_p$/2124Al acted as vacancy sinks. The high dislocation density accelerated the aging process and resulted to form finer precipitates by providing faster diffusion path with more nucleation sites of precitates in $SiC_p$/2124Al. Therefore, at overaged condition, the finer and increased number of lath type S' precipitates were observed in $SiC_p$/2124Al MMC, while, in 2124Al alloy, the precipitates were transformed from S' to S which go with precipitate breaking due to the difference of unit cell volume between S' and S. An incubation time of about 2hrs was observed for the precipitation of S', and the yield strengths were kept constant during the incubation time. However, the yield strength of $SiC_p$/2124Al MMC was decreased for the first 2hrs of aging by decreasing of dislocation density as annealing. The peak yield strength was obtained after 8hrs aging at $190^\circ C$ in both 2124Al alloy and $SiC_p$/2124Al composites. Comparing the precipitation behavior of $SiC_p$/2124Al and 2124Al alloy, the volume fraction of precipitates increase more rapidly in $SiC_p$/2124Al composites, while the size of precipitates increased slower in $SiC_p$/2124Al composites. It is analyzed that the precipitates influence on the yield strength by two different strengthening mechanisms, i.e. shearing of precipitates and dislocation bowing between precipitates. The peak strength was observed when those strengthening effects are balanced. The slower precipitation size increasing during aging treatment and faster volume fraction increasing results in taking more aging time to balance those two strengthening effect, which means peak aging condition, despite of fast aging process in $SiC_p$/2124Al MMC. That is why the peak aging time of $SiC_p$/2124Al MMC is same with that of 2124Al alloy at $190^\circ C$ even it including incubation time. Therefore, volume fraction increasing rate and size increasing with varying aging condition can change the peak aging time of $SiC_p$/2124Al to be larger or shorter relative to 2124Al alloy.
