Evolution of eutectic microstructure and strength in an Al-Ce-Ni-Mn-Sc-Zr alloy fabricated by laser powder-bed fusion

We characterize the evolution of microstructure and mechanical properties during thermal exposure of a strong and creep-resistant Al-11.5Ce-3.4Ni-0.6Mn-0.11Sc-0.34Zr (wt%) alloy fabricated by laser power-bed fusion (L-PBF). The alloy composition is based on a cast, near-eutectic alloy (Al-10.4Ce-3.5Ni-0.80Mn-0.25Sc-0.12Zr, wt%) with extreme creep- and coarsening-resistance for high-temperature applications. The as-fabricated L-PBF alloy exhibits a continuous network of fine, eutectic Al₁₁Ce₃ and Al₂₇Ce₃Ni₆ phases. The compositions of these phases are non-stoichiometric in the peak-aged alloy, but shift to the stoichiometric compositions during long-term thermal exposure. Upon aging at 300–400°C, L1₂-Al₃(Sc,Zr) nanoprecipitates form in the α-Al matrix and at the matrix/eutectic interface; Mn solutes are present in the Al matrix, but to a lesser extent than in the cast alloy. The refined eutectic phases are the dominant strengthening mechanism in the L-PBF alloy, and their evolution controls the loss of strength and creep resistance at elevated temperatures. During long-term thermal exposure at 300–400°C, the continuous eutectic network fragments into discontinuous elongated particles, which then spheroidize and coarsen. The initial eutectic fragmentation is associated with a significant decrease in room-temperature hardness and work-hardening capacity; the subsequent particle coarsening is slower and results in a more gradual decline in room-temperature strength and hardness. At 300°C, the alloy demonstrates excellent creep resistance, with dislocation creep threshold stresses of 109–149 MPa, depending on the aging condition and eutectic microstructure. Lastly, we demonstrate via analytical and numerical (finite-element) modelling that inhibition of dislocation motion, rather than load transfer, is the dominant strengthening mechanism imparted by the eutectic precipitates.