Nucleation and growth of Fe-rich phases in Al-5Ti-1B modified Al-Fe alloys investigated using synchrotron X-ray imaging and electron microscopy

Plate-like Fe-rich intermetallic phases directly influence the mechanical properties of recycled Al alloys; thus, many attempts have been made to modify the morphology of these phases. Through synchrotron X-ray imaging and electron microscopy, the underlying nucleation and growth mechanisms of Fe-rich phases during the solidification of Al-5Ti-1B-modified Al-2Fe alloys were revealed in this study. The results showed that the Al-5Ti-1B grain refiner as well as the applied pressure both resulted in reduction of the size and number of primary Al3Fe phases and promoted the formation of eutectic Al6Fe phases. The tomography results demonstrated that Al-5Ti-1B changed the three-dimensional (3D) morphology of primary Fe-rich phases from rod-like to branched plate-like, while a reduction in their thickness and size was also observed. This was attributed to the fact that Ti-containing solutes in the melts inhibit the diffusion of Fe atoms and the Al3Fe twins produce re-entrant corner on the twin boundaries along the growth direction. Moreover, the TiB2 provides possible nucleation sites for Al6Fe phases. The nucleation mechanism of Fe-rich phases is discussed in terms of experimental observations and crystallography calculations. The decrease in the lattice mismatch between TiB2 and Al6Fe phases was suggested, which promoted the transformation of Al3Fe to Al6Fe phases.