Abstract
Real-time in situ synchrotron X-ray tomographic microscopy was used to gain new insights into and quantify the nucleation mechanisms and growth kinetics of β-Al5FeSi intermetallics during solidification of an aluminium Al–7.5Si–3.5Cu–0.6Fe (wt.%) alloy. Three new insights were obtained. First, the plate-like β-intermetallics appeared to nucleate mainly on or near the primary aluminium dendrites and to a lesser extent off the oxide skin on the surface of the specimen. Second, for this alloy composition, β-intermetallic formation was largely complete before the formation of Al–Si eutectic. Third, the β-intermetallics formed via fast lateral growth, wrapping around and in between the primary dendrite arms. Further, the nucleation and growth dynamics of β-intermetallics were quantified as a function of undercooling in a functional form that could be easily used in microstructural simulations. The frequency of intermetallic interaction mechanisms, such as plate nucleation vs. impingement and branching, were also quantified.
Highlights
The size and morphology of secondary phases in many alloys are key to the resulting properties of the material, ranging from the strengthening effect of c0-precipitates via dislocation pinning in Ni-base superalloys [1] to grain refinement in steels [2]
A series of images from the slow-speed 4-D X-ray tomographic microscopy experiment are shown in Fig. 2 to qualitatively illustrate the microstructural evolution during the solidification of the Al–7.5Si–3.5Cu–0.6Fe specimen
The large number of b-intermetallics is shown in Fig. 2i and j (550 °C)
Summary
The size and morphology of secondary phases in many alloys are key to the resulting properties of the material, ranging from the strengthening effect of c0-precipitates via dislocation pinning in Ni-base superalloys [1] to grain refinement in steels [2]. Secondary phases can be harmful; for example, they can initiate fatigue failure [3], as well as accelerate solidification defect growth and propagation [4,5,6]. In Al–Si alloys, secondary phase formation provides one of the most widely used strengthening mechanisms, through age hardening, but it limits the use of material (P.D. Lee).
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