Enhanced thermal stability of precipitates and elevated-temperature properties via microalloying with transition metals (Zr, V and Sc) in Al–Cu 224 cast alloys

The evolution of precipitates and the elevated-temperature mechanical and creep properties of Al–Cu 224 cast alloys with the microalloyed transition metals (TMs) of Zr, V, and Sc were investigated. The results showed that the addition of TMs can generally delay the transformation from θ″ to θ′ during T7 aging and can effectively increase the coarsening resistance of θ′ during prolonged thermal exposure (T7A), especially in the alloys with combined TM additions. Meanwhile, thermally stable L1₂-Al₃M dispersoids were observed in alloys with Zr and Sc + Zr additions. With the increased thermal stability of the predominant strengthener θ′, both the yield strength and creep resistance at 300 °C were remarkably improved. Under the T7A condition, the alloy with the combined Zr + V addition achieved the highest yield strength at 300 °C with an increase of 37% relative to the base alloy. The alloy microalloyed with Sc + Zr exhibited the best creep resistance with a minimum creep rate of 5.5 × 10⁻⁹ s⁻¹ compared with 2.0 × 10⁻⁷ s⁻¹ for the base alloy, owing to the co-existence of thermally stable θ′ and Al₃(Sc, Zr). The relationship between the evolution of the microstructure and the mechanical properties under the T7A condition was rationalized using the proposed strengthening mechanisms.