LAUSR

Precipitation behavior of Al3(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 Al3Sc was the precursor of Al3(Sc,Zr) precipitate. The supersaturated solid solution containing Sc decomposed at 250 °C, forming Sc-rich clusters and gradually developing into Al3Sc nuclei. In multicomponent system containing Zr and Sc, the process of Al3Sc nucleus transforming to mature Al3(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 Al3Sc nuclei driven by chemical potential gradient, and then enrich around them to form Zr-rich thin layer, thus forming Al3(Sc,Zr) structure of Al3Sc 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 Al3(Sc,Zr) precipitates is about three orders of magnitude smaller than that of Al3Sc particles, showing much better thermal stability.