In Situ Neutron Diffraction Solidification Analyses of Rare Earth Reinforced Hypoeutectic and Hypereutectic Aluminum-Silicon Alloys

The recognised potential of rare earth (RE) additions such as cerium (Ce) and lanthanum (La) for strengthening aluminum alloys has led to an area of research focused on the development of new alloys, targeting powertrain applications that require high temperature strength and creep resistance. In an attempt to further improve the mechanical properties of the Al-Si system, this paper addresses the effects that RE additions have on the microstructure and phase evolution during solidification. This study presents the results of in situ solidification studies using neutron diffraction and microstructural analyses using scanning electron microscopy with energy-dispersive spectroscopy of Al7Si3.5RE and Al18Si8RE alloys, where numerical notation indicates composition in wt%. We find that the RE additions lead to the formation of globular Al20Ti2(Ce6-LaNd) and rod-like Si3Al2(Ce3La2Nd) intermetallics in the Al7Si3.5RE alloy. We also find that Si and Cu additions in the Al18Si8RE alloy transforms the solid structure of the rod-like Si3Al2(Ce3La2Nd) intermetallic to a fibrous twin-layered material comprised of alternating Si3Ce1Al1(-La6Nd3Cu2Pr) and Al5Si4CeCu(La6Nd3Pr) constituents. Furthermore, the high RE content in the Al18Si8RE alloy leads to a prolonged solidification range which may increase the alloy's susceptibility to porosity formation.