Interaction in growth and heterogeneous precipitation between θ′–Al₂Cu and Al₃Sc in an Al–Cu–Sc Alloy

Abstract

An Al–2.5 mass%Cu–0.23 mass%Sc alloy is strengthened by fine and uniform distribution of coherent spherical L1₂–Al₃Sc phase and semicoherent plate-like θ′–Al₂Cu phase, which precipitate during aging. These precipitates interact on growth and heterogeneous precipitation each other. In the present work, mechanisms of growth and heterogeneous precipitation between the two phases in the Al–2.5 mass%Cu–0.23 mass%Sc alloy aged at 623 K were investigated on the basis of TEM observation. On aging at 623 K, growth rate of θ′–Al₂Cu phase in the Al–Cu–Sc alloy is much larger than that in the Al–Cu alloy. Growth rate of Al₃Sc is almost same and very small in both of Al–Cu–Sc alloy and Al–Sc alloy and this is much smaller than that of θ′–Al₂Cu phase. This is considered to be closely related to the high binding energy between scandium atoms and vacancies. Coarsening process of Al₃SC and θ′–Al₂ Cu phases obey the LSW theory of diffusion controlled particle coarsening. The migration of scandium or copper atoms to the strain fields of the interface dislocations formed at θ′–Al₂Cu/matrix or Al₃Sc/matrix interface have a significant effect on the heterogeneous precipitation of Al₃Sc phase on θ′–Al₂Cu phase or θ′–Al₂Cu phase on Al₃Sc phase.