Precipitation Behavior of Al3(Sc,Zr) Particles in High-Alloyed Al–Zn–Mg–Cu–Zr–Sc Alloy During Homogenization

Precipitation behavior of Al 3 (Sc,Zr) particles in a new high-alloyed Al–Zn–Mg–Cu–Zr–Sc aluminum alloy during homogenization was investigated by use of three-dimensional atom probe, transition electron microscope and high-resolution transition electron microscope. The results indicate that Al 3 Sc was the precursor of Al 3 (Sc,Zr) precipitate. The supersaturated solid solution containing Sc decomposed at 250 °C, forming Sc-rich clusters and gradually developing into Al 3 Sc nuclei. In multicomponent system containing Zr and Sc, the process of Al 3 Sc nucleus transforming to mature Al 3 (Sc,Zr) particle was controlled by diffusion. In the early stage of nuclei growth, Zr atoms did not have long-range diffusion capacity, but only Sc atoms diffused freely. With the increase in temperature, Zr atoms began to diffuse over long distance and approach to Al 3 Sc nuclei driven by chemical potential gradient, and then enrich around them to form Zr-rich thin layer, thus forming Al 3 (Sc,Zr) structure of Al 3 Sc core + Zr-rich shell. During isothermal annealing at 440 °C for different holding hours, the mean diameters of the Sc-containing particles after 1 h, 10 h, 36 h and 100 h were 10.9 nm, 15.5 nm, 14.3 nm and 16.7 nm, respectively. The coarsening coefficient for Al 3 (Sc,Zr) precipitates is about three orders of magnitude smaller than that of Al 3 Sc particles, showing much better thermal stability.