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-5 Ti-1 B-modified Al-2 Fe alloys were revealed in this study. The results showed that the Al-5 Ti-1 B grain refiner as well as the applied pressure both resulted in reduction of the size and number of primary Al_(3)Fe phases and promoted the formation of eutectic Al_(6)Fe phases.The tomography results demonstrated that Al-5 Ti-1 B changed the three-dimensional(3 D) 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 Al_(3)Fe twins produce re-entrant corner on the twin boundaries along the growth direction. Moreover, the TiB_(2) provides possible nucleation sites for Al_6Fe 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 TiB_(2) and Al_(6)Fe phases was suggested, which promoted the transformation of Al_(3)Fe to Al_(6)Fe phases.