Understanding cellular structure formation in rapidly solidified Al–Ni alloys by constructing solidification microstructure selection maps

The eutectic solidification strategy has been widely implemented in Al–Si, Al–Ni, and Al–Ce alloys to suppress cracking during laser powder bed fusion (LPBF) processes. More significantly, the resultant cellular structures that comprise cells and eutectic networks at the cell boundaries, have exhibited great potential for improving strength and ductility. However, the formation mechanism of cellular structures during rapid solidification (RS) has not been thoroughly understood, impeding the fine-tuning of cells and eutectic networks in additive manufacturing (AM). In this study, the solidification microstructure selection maps of the Al–Ni system for directional solidification are constructed based on the Al–Ni phase diagram, the eutectic coupled zone (ECZ), the Mullins–Sekerka stability criterion, and the limit of absolute stability. During the map construction process, holding the idea that the divorced eutectic reaction occurs when the width of the liquid entrapped at the α-Al cell boundaries is comparable to the Al–Al3Ni eutectic spacing, a model criterion evaluating the formation of Al–Ni divorced eutectic is proposed by integrating eutectic and dendritic growth models. Then the formation regions of the cellular structures containing Al3Ni divorced eutectic networks are determined in the maps. Finally, the formation of cellular structures in the Al–Ni system under LPBF conditions is elucidated with the help of the microstructure selection maps in composition–interface temperature (C0–Ti) space and temperature gradient–solidification velocity (G–V) space. The outcomes of this study may provide a basic idea for tailoring the cellular structures of eutectic Al alloys fabricated by the LPBF technique.