Effect of Ag Content on the Microstructure and Crystallization of Coupled Eutectic Growth in Directionally Solidified Al-Cu-Ag Alloys

A series of coupled eutectic growths along the univariant eutectic groove in the ternary Al-Cu-Ag alloy was studied to investigate the effect of Ag on the microstructure and crystallization of directionally solidified Al-Cu-Ag alloys. The results indicated that the eutectic morphology and orientation relationship (OR) between eutectic phases were modified as the Ag content in the Al-Cu-Ag alloys increased. At a lower growth velocity (R <= 1 mu m/s), a banded structure formed and the interlamellar spacing decreased with the increasing Ag content. At a higher growth velocity (R >= 3 mu m/s), the eutectic cell spacing decreased with increasing Ag content. Increasing the Ag content in the Al-Cu-Ag alloys enhanced the enrichment of the Ag solute in the liquid ahead of the quenched liquid/solid interface. In addition, increasing the Ag content in the Al-Cu-Ag alloys promoted the transformation from a "Beta 6" OR to an "Alpha 4" OR between eutectic phases. Modifications of the eutectic morphology and the OR during directional solidification were attributed to the enrichment of Ag content at the solid/liquid interface and the changes in the interfacial energy due to the increase in Ag solubility in the alpha-Al phase. (C) The Minerals, Metals & Materials Society and ASM International 2018