TEM study of interfacial reactions and precipitation mechanisms in Al2O3 short fibers or SiC particles reinforced Al-4Cu-1Mg-0.5Ag squeeze-cast composites

After more than a quarter of a century of active research, metal matrix composites (MMCs), and more particularly aluminum matrix composites (AMCs), are beginning to make a significant contribution to aerospace, automotive, and electronic industrial practice. This is the consequence of progresses in the development of processing techniques, and the result of advances in the understanding of the relationship between composite structure and mechanical behavior In the present work, two kinds of AMCs were elaborated by direct squeeze-casting for the assessment of their mechanical performance in view of potential applications for the automobile and electronic industry. They are based on a specifically designed precipitation hardening Al-4Cu-1Mg-O.5Ag alloy chosen for its promising mechanical properties at temperatures up to 200°C. A12O3 Saffil short fibers (15%-vol) on the one hand and SiC particles (60%-vol) on the other hand act as reinforcements. In the aim of a better understanding of the mechanical properties of the composites, their microstructure has been studied by transmission electron microscopy (TEM). The grain morphology and size, microsegregation and precipitation states of the composites have been investigated and compared with those of the unreinforced matrix alloy. Microsegregation, mainly of A12Cu, A17Cu2Fe and Q-A15Cu2Mg8Si6 phases, is observed in the as-cast composites at the interfaces between the matrix and the reinforcements. However, a solution heat treatment at 500°C for 2 hours leads to a significant dissolution of these phases. Although the unreinforced alloy was free of Si, this element is detected in the matrices of both composites. After a TEM study of the interfaces, it was deduced that Si is released from different interfacial reactions: (i) for the A12O3 reinforced composites, from a reaction between the Mg from the matrix alloy and the SiO2 from the Saffil fibers and the silica binder of the preform, and (ii) for the SiC reinforced composites, from a direct reaction between Al and the SiC particles with an indirect but important contribution of Mg to the reaction kinetics. As consequence of the chemical modification of the alloy, the precipitation state in the matrices of the composites has drastically changed. It was shown by energy dispersive spectrometry chemical analyses (EDS), high resolution electron microscopy (HREM), dedicated scanning transmission electron microscopy (DSTEM) and microdiffraction techniques that the usual Ω and S' hardening phases of the matrix alloy are substituted by a fine and dense precipitation of nano-sized QP rods and θ' plates. The θ' plates lie on nano-sized rod-shaped precipitates identified as Si phase in the A12O3 short fiber reinforced composite, and as QC phase in the SiC particle reinforced composite. The QP and QC phases are shown to be precursors of the stable Q-A15Cu2Mg8Si6 phase. They have both a hexagonal structure withal a = 0.393 nm and c = 0.405 nm, and with a = 0.675 nm a