Synthesis and characterization of (Al,Si)₃(Zr,Ti)-D0₂₂/D0₂₃ intermetallics: Understanding the stability of silicon substitution

(Al,Si)₃(Zr,Ti)-D0₂₂/D0₂₃ are phases that may form in aerospace and automotive aluminium alloys. The substitution of Zr/Ti in these solid solutions is widely reported in the literature; however, it remains relatively unexplored for Si. In this work, in situ precipitation of (Al,Si)₃(Zr,Ti)-D0₂₂/D0₂₃ intermetallics was performed using Al-Si-Zr-Ti alloys. The precipitation, sedimentation and concentration of numerous intermetallic particles were accomplished by filtrating the residual molten aluminium using a temperature/pressure-controlled vessel adapted with a PoDFA filter. A combination of SEM, TEM, XRD and EMP analysis allowed the identification of (Al,Si)₃(Zr,Ti)-D0₂₂/D0₂₃ intermetallics concentrated within α-FCC matrices of non-Si-doped (sample S2) and Si-doped (samples S4 and S6) alloys. EDS analysis confirmed that Zr and Ti substitute each other in the D0₂₂ and D0₂₃ phases, whereas Si substitutes in Al sites. Acceptance of Si inside the D0₂₃ phase was not expected according to FTlite (FactSage) and TCAL7 (Thermo-Calc) databases. Additionally, Si was found to enhance the formation of (Al,Si)₃(Zr,Ti)-D0₂₂ intermetallics with high Zr-content, contrary to FactSage 7.3 predictions. TEM results showed intermetallic/FCC crystal coherency for samples S2 and S6, implying that these intermetallics acted as nucleation sites for the Al-phase due to their small lattice mismatch. Furthermore, Si site occupancy was calculated for both (Al,Si)Ti-D0₂₂ and (Al,Si)₃Zr-D0₂₃ phases via DFT, showing that sites 2b and 4e are the most favorable for Si occupation, respectively. Finally, a thermodynamic model is derived to describe Si substitution upon solidification. Experimental and numerical examinations indicate that Si substitution preferentially occurs in the D0₂₂ intermetallics compared to the D0₂₃ phase.

Mots clés

Intermetallics; PoDFA; Silicon substitution; Aluminium alloys; Grain refiner; DFT simulations; Phase coherency