Unveiling growth and morphological transition of Al3Ni compounds during remelting cycles via synchrotron radiography

The microstructural evolution at the liquid Al/solid Ni interface during remelting and resolidification was dynamically observed by using synchrotron radiography. The formation and growth behavior of Al₃Ni₂ and Al₃Ni brittle intermetallic compounds (IMCs) under different conditions was investigated, and the formation mechanisms of dendritic Al₃Ni crystals with different morphologies were elucidated. The increasing remelting cycles accelerated the formation of Al₃Ni₂ layer and the growth of Al₃Ni IMCs. The increased-step heating temperatures and time promoted the morphological transition from faceted to non-faceted dendritic Al₃Ni, which was attributed to the enhanced undercooling during solidification and incompletely remelted Al₃Ni IMCs during remelting. The growth of regular Al₃Ni dendrites was dominated by coalescence of secondary dendrite arms, while the growth of irregular dendrites Al₃Ni was controlled by dendrite merging, radial melting and axial melting of secondary arms. The axially free dendritic Al₃Ni was attributed to the small distance between adjacent main trunks, and the dense secondary arms promoted the formation of local solute depletion regions.