Assembling dual precipitates to improve high-temperature resistance of multi-microalloyed Al–Cu alloys

Abstract Co-stabilization of multiple precipitates has attracted growing attention as a promising approach to improve the performance of Al-based alloys at elevated temperatures. In this paper, three microalloying strategies, including 0.18 at.% Sc, 0.18 at.% Sc with 0.05 at.% Si, and 0.18 at.% Sc with 0.05 at.% Zr, are adopted to acquire an assembly of θ′-Al2Cu and Al3Sc precipitates in Al-1.08 at.%Cu alloys. The differences in chemical compositions are found to have huge impact on the stability of the precipitates and thus on the softening resistance of the alloys during thermal exposure at 300 °C. The results show that compared with Sc or Sc–Si microalloying, Sc–Zr microalloying in Al–Cu alloy is far more effective to co-stabilize the dual precipitates and thus improve the high-temperature resistance. Using transmission electron microscopy (TEM), the underlying mechanisms responsible for the distinct microalloying effects of the three strategies are revealed by: (i) the limited improvement of high-temperature resistance in Sc–Si microalloyed Al–Cu alloy results from a cumulative effect of the retarded θ′-Al2Cu coarsening and the accelerated Al3Sc growth. (ii). the combined Sc–Zr microalloying simultaneously stabilizes θ′-Al2Cu and Al3Sc precipitates, which dramatically suppresses the softening in Sc–Zr microalloyed alloy. Further experimental evidences of atom probe tomography (APT) suggest that the θ′-Al2Cu stabilization induced by combined Sc–Zr or Sc–Si microalloying is actually related to a modified multiple solute segregation at θ′-Al2Cu/α-Al interface to reduce the interfacial free energy. And the suppression and acceleration of Al3Sc coarsening kinetics induced by Sc–Zr and Sc–Si microalloying are attributed to Si-accelerated Sc diffusion and the formation of core-shell structural precipitates, respectively.