Morphology and Mechanical Properties of the T-Al₆Mg₁₁Zn₁₁ Phase in the Eutectic Microstructure of Al–Zn–Mg Ternary Alloys

We designed three Al-based cast alloys strengthened by T-Al₆Mg₁₁Zn₁₁ intermetallic phases using a eutectic reaction (liquid → α(fcc) + T) in the Al–Mg–Zn ternary system and then fabricated them using solidification processes. Thermodynamic assessment provided target alloy compositions of Al–26.5Zn–21.5Mg (at%), Al–23.5Zn–22.5Mg (at%), and Al–13Zn–28Mg (at%) with the α-Al (fcc) phase in equilibrium with the T phase with high fractions (>60%). All of the alloys exhibited regularly arrayed fibers of the α phase embedded in a T-phase matrix in α/T two-phase eutectic microstructures. A slight effect of phase compositions on the fibrous eutectic morphology was observed, whereas the volume fraction of the T phase could be controlled according to the thermodynamic assessments. The fibrous α phases had faceted interfaces of {111} planes with a surrounding T-phase matrix, suggesting a low interfacial energy of the α/T interfaces. The fabricated eutectic alloys exhibited a high Vickers hardness (>230 HV) and high yield strength at elevated temperatures above 250°C. The Mg-rich alloy with a composition of Al–13Zn–28Mg (at%) exhibited a relatively lower hardness, responsible for the low hardness of the T phase with a Mg-rich composition.