Revealing the nucleation and growth mechanisms of Fe-rich phases in Al–Cu–Fe(-Si) alloys under the influence of Al–Ti–B

Al–5Ti–1B is commonly added to refine the grain size of Al alloys. However, its presence affects the type, size, morphology, and formation sequence of Fe-rich intermetallic phases. The underlying nucleation and growth mechanisms of Fe-rich phases under the influence of Al–Ti–B during the solidification of Al–Cu–Fe(-Si) alloys has been systematically studied by thermodynamic calculation and experimental techniques: scanning electron microscopy (SEM), X-ray diffraction (XRD), differential scanning calorimetry (DSC), focused ion beam (FIB), transmission electron microscopy (TEM), and in-situ synchrotron X-ray radiography. The SEM and in-situ synchrotron X-ray radiography experiments revealed that the Al–Ti–B addition significantly reduce the size and number of primary Al3(CuFe) phases, and refine the size of α-Fe in Al–Cu–Fe–Si alloy without changing their type. This was caused by Ti solute atoms in Al melts, which limits the nucleation and growth of primary Al3(CuFe) phases during solidification. The FIB and TEM results showed that the priorly formed TiB2 particles were nucleate sites for Al6(CuFe) phase and α-Fe phase in Al–Cu–Fe alloy and Al–Cu–Fe–Si alloy, respectively. The Edge-to-Edge model and the orientation relationship further confirmed the experimental results. This study may provide fundamental understanding about the influence of Al–Ti–B on the nucleation and growth of Fe-rich phases.