TEM study of interfacial reactions and precipitation mechanisms in Al2O3 short fiber or high volume fraction SiC particle 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-0.5Ag alloy chosen for its promising mechanical properties at temperatures up to 200°C. Al2O3 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 Al2Cu, Al7Cu2Fe and Q-Al5Cu2Mg8Si6 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 Al2O3 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 Al2O3 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-Al5Cu2Mg8Si6 phase. They have both a hexagonal structure with a = 0.393 nm and c = 0.405 nm, and with a = 0.675 nm and c = 0.405 nm, respectively for QP and QC. A structural phase transition between the QP, QC, Q rod-shaped precipitates in the matrices of the composites is observed and studied by TEM, DF superstructure imaging and in-situ experiment techniques. The details of this transition are shown to bring a new understanding to the precipitation mechanisms in the 6xxx alloys (AlMgSi alloys) in general. These ones are widely used as medium-strength structural alloys. The structures of the metastable phases that precipitate in these alloys have been largely described in literature, but the precipitation mechanisms at atomic scale has not been well understood so far. In the present work, a model is developed from the crystallographic structure of the stable Q-phase determined by X-ray. It describes the QP, QC and Q structures as superordered structures formed by an order-disorder transition from a primitive phase named qp. The model predicts that a similar transition exists between all the metastable phases in the 6xxx alloys (β″, β′, B′, type-A, type-B). For example, β′ is supposed to be structurally similar to QC, with Si substituting Cu in the unit-cell. The latent lattices implied in the transitions are noted QP and βP for the matrices of the composites (AlCuMgSi alloys) and for the 6xxx alloys respectively. Microdiffraction patterns and superstructure DF images acquired on a CCD camera confirm the similarity between the QC and β′ phases. After refinement by comparison between the experimental and computed microdiffraction patterns, their crystallography is found to be hexagonal P6̄2m. Eventually, according to the model, the structural transitions in the AlCuMgSi and AlMgSi alloys are found to respectively follow the sequences qp →(QP →) QC → Q and βp → (βP →) β′ → Β′ corresponding to the breaking symmetry path P63/mmc → P6̄2m → P6̄. This sequence is respected during the cooling of the materials from the liquid state, and structurally mixed precipitates can be observed in the as-cast state, due to the slow kinetics of the transition (order/disorder transition). This sequence is also respected during the aging of the materials, since the small size of the precipitates is expected to reduce the critical temperature of transition. Monte Carlo simulations on an Ising lattice are computed to illustrate and confirm those effects.