Grain boundary reconstruction and property improvement of MgO-CaO ceramics via co-doping with Y2O3 and Al2O3

MgO-CaO ceramic cores are considered promising for precision superalloy casting due to their high melting points, chemical compatibility with molten alloys, and water-assisted removability. However, the difficult sintering and limited mechanical properties hinder their further engineering application. In this work, Y 2 O 3 -Al 2 O 3 composite precursors were synthesized via a low-temperature solid-state route and employed as reactive additives to regulate the densification behavior and grain-boundary chemistry of MgO-CaO ceramics. The effects of Y 2 O 3 /Al 2 O 3 ratio, additive state (direct precursor vs. post-calcined powder), and addition level on the phase evolution, microstructure, densification, flexural strength, and refractoriness were systematically investigated. The results suggested a precursor-directed reactive sintering mechanism with two coupled contributions: At low Y 2 O 3 /Al 2 O 3 ratios, a transient Ca-Al-O-rich liquid associated with Ca 3 Al 2 O 6 promoted mass transport and pore elimination, while at higher ratios, the in-situ precipitation of lamellar CaYAlO 4 at grain boundaries contributed to reconstruct the intergranular network, strengthened interphase bonding, and suppressed grain coarsening via a pinning effect. Due to the balanced densification and grain-boundary stabilization, the optimal sample containing 1.5 mol.% uncalcined precursor with a high Y 2 O 3 /Al 2 O 3 ratio achieved a room-temperature flexural strength of 126.16 MPa and refractoriness of 1698 °C after sintering at 1600 °C for 3 h. As a result, post-calcined additives led to coarser and less interconnected intergranular structures with inferior densification. This work demonstrates that tailoring precursor chemistry provides an effective route to switch and coordinate liquid-assisted densification and CaYAlO 4 -dominated grain-boundary reinforcement, offering a viable strategy for designing high-performance MgO-CaO ceramics potentially used for advanced superalloy casting cores.