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 the Al alloys for grain refinement, which also affects the type, size, morphology, and formation sequence of Fe-rich intermetallic phases. The underlying nucleation and growth mechanism of Fe-rich phases under the influence of Al-5Ti-1B during the solidification of Al-Cu-Fe(-Si) alloys were systematically studied by thermodynamic calculation, and with scanning electron microscopy (SEM), X-ray diffraction (XRD), differential scanning calorimetry (DSC), focused ion beam (FIB) coupled with 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-5Ti-1B addition significantly reduced the size and number of primary Al3(CuFe) phases. and refined 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 limiting the nucleation and growth of primary Al3(CuFe) phases during solidification. The FIB and TEM results showed that the prior formed TiB2 particles or/and newly-formed aluminium oxide 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 orientation relationship further confirmed the experimental results. In addition, V diffusion into TiB2 particles in the Al melts to form (TiV)B2 particles and lowering the interfacial energy enhance the heterogenous nucleation for Fe-rich phases.