The synergetic effect of Si and Sc on the thermal stability of the precipitates in AlCuMg alloy

The effect of Si and Sc on the evolution of the precipitates during aging in Al4.0Cu0.5 Mg alloys was investigated by means of microhardness tests and transmission electron microscope. The results show that Si can significantly refine the size of θ′ phases, because addition of Si introduces a considerable amount of small, uniformly dispersed Al–Cu–Mg–Si (Q) quaternary phases, which provides nucleation sites for the θ′ phases. Sc atoms segregate at the interfaces between the matrix and the θ′-Al2Cu precipitates in AlCuMgSiSc alloy. The segregation of Sc atoms inhibits the growth of the θ′ phases and improves the thermal stability of the θ′ phases. At the same time, the segregation of Sc atoms in Q phases restrains the coarsening of the Q phases, which also contributes to the thermal stability of the θ′ phases. The co-addition of Si and Sc thus leads to a combination of high strength and high thermal stability of the Al–Cu–Mg–Si-Sc alloy.