Microstructures of Al–Cu–Ag Ternary Eutectics Under Near-Isothermal Conditions

Abstract

The three-phase eutectic microstructures in Al–Cu–Ag ternary alloys are complex and only directional solidification (DS) structures have been systematically studied in the literature. In order to better understand the large variation of eutectic structures in this system, three off-eutectic alloys in the vicinity of the nonvariant eutectic composition were solidified in a differential scanning calorimeter (DSC). The DSC was operated between 450 and 550 °C and at a heating and cooling rate of 1 °C/min to produce near-isothermal conditions. The intermetallic phase ζ-Ag2Al is found to behave like pure metallic phase and forms primary dendrites. The ζ-Ag2Al phase has a regular morphology in eutectics, while the θ-Al2Cu exhibits big anisotropy of intermetallic nature and shows elongated morphology in the unconstraint grown eutectics. Three basic eutectic structures are categorized based on the relative phase arrangement: (1) semi-regular eutectic with elongated θ-Al2Cu; (2) regular eutectic with rod-like ζ-Ag2Al embedded in θ-Al2Cu and (3) irregular eutectic with θ-Al2Cu in ζ-Ag2Al. They have nearly the same phase fraction and composition but distinct phase size distribution. The DSC eutectic structures are compared with reported DS eutectic structures.