The effects of Ti on the precipitation evolution and age-hardening behavior of Al–7Si–Mg alloy

The effect of Ti addition on the precipitation evolution for Al–7Si–Mg alloy was systematically investigated by combing high-resolution transmission electron microscopy (HRTEM) and first-principles calculations. At the early aging stage, the rod-shaped Al3Ti and ellipsoid Ti5Si3 phases precipitated firstly, and the stronger Si capture ability of them suppressed the precipitation of the Mg–Si phases (β'' and β′), leading to the deceleration of the aging hardening kinetics. With prolonging aging time, one Al atom in Al3Ti was easily replaced by Si atom to form a more stable Al2SiTi phase. It was demonstrated by Energy Dispersive Spectrometry (EDS) and the calculation of formation enthalpy. The accurate atomic structure of the Al2SiTi phase was also obtained, which provided a resonable speculation on the structure of Al3Ti two-dimensional compound (2DC) containing Si on the surface of TiB2. This shed light on a deep understanding of Si poisoning on grain refinement of Al–Si/Al–5Ti–B system. At the over-aged stage, the Mg–Si phases began to precipitate, preventing the decrease of the hardness. Finally, a new precipitation sequence in the Ti-rich Al–7Si–Mg alloy can be obtained: SSSS → Al3Ti + Ti5Si3 → Al2SiTi + Ti5Si3 → Al2SiTi + Ti5Si3 + β" → Al2SiTi + Ti5Si3 + β"+ β'.