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 0 '' to 0 ' during T7 aging and can effectively increase the coarsening resistance of 0 ' 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 0 ', both the yield strength and creep resistance at 300 degrees C were remarkably improved. Under the T7A condition, the alloy with the combined Zr + V addition achieved the highest yield strength at 300 degrees 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 x 10-9 s-1 compared with 2.0 x 10-7 s- 1 for the base alloy, owing to the co-existence of thermally stable 0 ' 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.