Both the addition of 0.6% Sc and simultaneous addition of 0.2% Sc and 0.1% Zr exerted a remarkable effect on grain refinement of as-cast Al–Mg alloys, changing typical dendritic microstructure into fine equiaxed grains. Such effect was found to be related to the formation of primary particles, which acted as heterogeneous nucleation sites for α-Al matrix during solidification. Primary particles formed in Al–Mg–Sc–Zr alloy could be identified as the eutectic structure consisting of multilayer of ‘Al3(Sc,Zr) + α-Al + Al3(Sc,Zr)’, with a ‘cellular-dendritic’ mode of growth. In addition, an attractive comprehensive property of as-cast Al–5Mg alloy due to the addition of 0.2% Sc and 0.1% Zr was obtained.
The effect of erbium addition at different ratios (0.2 wt.%, 0.4 wt.%, 0.6 wt.%) on microstructure evolution and mechanical properties of the AlÀ Si-MgÀ Cu alloy have been investigated. The results show that adding erbium can refine the primary αaluminum grains in the as-cast alloys to a diverse extent. In addition, the coarse and long plate-like eutectic silicon can be modified into a fine short rod-like structure, and the distribution of eutectic silicon can be optimized. During the solidification of the alloy, erbium and other elements will form primary erbium-rich precipitates of different sizes and shapes, including primary erbium-silicon-rich and erbium-copper-silicon-rich precipitates, which can change the distribution of solute atoms and promote the evolution of microstructure. But the coarse primary precipitates will embrittle the alloys and consume more solute atoms (erbium, copper, magnesium), having a certain inhibitory effect. After heat treatment, the solid solution decomposes to form secondary Al 3 Er precipitates, which further strengthens the alloy. In particular, the alloy with the highest erbium content shows the best comprehensive tensile properties at as-cast and heat-treatment conditions.
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