Near-Surface Microstructure on Twin-Roll Cast 8906 Aluminum Alloy

The microstructure of the near-surface region of twin-roll cast foil stock 8906 AlFeSi alloy has been investigated. It was revealed that a near-surface layer with microstructure differing from that of the underlying bulk alloy was generated due to severe surface shear strain during twin-roll casting. Scanning and transmission electron microscopy showed that the near-surface layer consists of nano-sized grains with diameters varying in the range of 50 to 600 nm. Energy dispersive X-ray spectroscopy revealed that grain boundaries within the near-surface layer were decorated by aluminum oxide particles and carbon-containing lubricant inclusions. Preferential precipitation of a high population density of cubic α-AlFeSi dispersoids, of 30 to 150 nm diameter, was observed within the near-surface layer. The near-surface layer was associated with shingles on the twin-roll cast alloy surface and the maximum layer thickness of approximately 6 µm was observed at the shingles. Other areas of the surface were either free of the near-surface layer or had a thinner near-surface layer.