On the microstructure developed in as-cast and homogenized 7010 aluminium alloy containing scandium

Grain refinement in the as-cast microstructure and the nature of the Sc-bearing primary and secondary phase particles present in as-cast and homogenized Al-Zn-Mg-Cu-Zr base 7010 alloys, containing Sc, have been examined by a combination of light microscopy, scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDXS) and transmission electron microscopy (TEM). It is shown that addition of 0.25 wt.% Sc to the base alloy 7010 gives rise to the desired grain refinement in the as-cast microstructure. Further additions of Sc (investigated up to 0.5 wt.%) reduce the as-cast grain size further, although increased amounts of Sc additions give rise to an increased density of Sc-bearing coarse primary particles that always have an L1(2) crystal structure, dissolve Zr, and survive the homogenization treatment. The compositions of the Sc-bearing primary phase particles vary depending on their morphologies, i.e. cuboidal and irregular shape. The cuboidal shape of the Sc-bearing primary particles is the dominant one and such particles are mostly located within the grains. The noticeable features of the homogenized microstructure are (1) the changes in the composition of the Sc-bearing primary particles, (2) the formation of fine, spherical, coherent Al3ScxZr1-x base secondary precipitates having an L1(2) structure and (3) the stabilization of the refined as-cast grain structure.