Microstructure and microhardness of nanostructured Al-4.6Cu-Mn alloy ribbons

The microstructural characteristics and microhardness of nanostructured Al-4.6Cu-Mn ribbons produced by melt spinning were investigated using field-emission gun scanning electron microscopy, transmission electron microscopy, and hardness testing, and the results were compared to those of similar ribbons manufactured by direct-chill casting. It is shown that the nanostructure of the as-melt-spun ribbons consists of α-Al dendrites with a secondary dendrite arm spacing of approximately 0.55-0.80 μm and ultrafine eutectic crystals of a nanosized scale of approximately 100-200 nm on dendritic boundaries. The solidification time and cooling rate of 46-μm-thick ribbons were estimated to be 1.3×10^-6 s and 4.04×10⁶ K·s^-1, respectively. At an aging temperature of 190℃, the coherent θ″ phase in aged ribbons gradually transforms into nanoscale θ'-phase platelets as the aging time is extended from 2 to 8 h; the rod-like morphology of the T (Al₂₀Cu₂Mn₃) dispersoid with 120-160-nm diameter also forms, which results in peak aging hardness. The precipitation behaviors of aged ribbons cannot be changed at the high cooling rates of as-cast ribbons. However, a finer and more uniformly distributed microstructure and a supersaturated solid solution at a high cooling rate can shorten the time required to obtain a certain aging hardness before peak hardness.