Characterization of structure and hardness at aging of the A319 type aluminum alloy with Sn trace addition

The effect of 0.1 wt% (0.02 at%) Sn trace addition on the structure and phase composition of Al-Si-Cu based alloy has been studied using thermodynamic calculations (Thermo-Calc software) and experimental techniques (transmission electron microscopy (TEM), atom probe tomography (APT), hardness and specific electrical conductivity measurements). The Vickers hardness measurement made after aging at 175 °C revealed that the peak hardness of the Sn-containing alloy is about 22% higher than that of the Sn-free alloy (135 HV vs 115 HV). Moreover, the peak hardness is achieved in a much shorter aging time (4 h vs 16 h). Considerably finer precipitates of the θ′ phase (average length less than 60 nm and thickness 3–5 nm) with a substantially higher number density are observed in the Sn-containing alloy. Tiny, rounded particles most of which are associated with the θ′ phase precipitates are also observed. Atom probe tomography analysis confirmed that the observed nanoparticles consist of tin. Quantitative APT analysis revealed that the number density of the copper-containing precipitates is at least twice that of the tin-containing ones (7.6·10 ⁴ vs 3·10 ⁴ µm ^-3 ). APT data also revealed a clear evidence of noticeable dissolution of Sn (average concentration from 0.05 to 0.10 at%) and silicon (average concentration from 2.2 to 2.8 at%) at the core of the θ′ phase precipitates, which is consistent with the fact that Sn and to a lesser extent Si act as catalysts in the nucleation of this phase. The data obtained using the proxigram technique suggest that Sn atoms show no tendency to segregate at the coherent θ′/(Al) heterophase interface upon aging, whereas localized Sn segregation is observed at the semi-coherent interface. Both Sn-free and Sn-containing alloys in their peak aged states have been subjected to uniaxial compression tests at an elevated temperature of 250 °С. The results suggest that the yield strength of the Sn-containing alloy is about 1.5 times that of the Sn-free alloy (250 MPa vs 175 MPa). The revealed difference is very big and testifies to a high potential of the new alloys as materials for new-generation engine parts.