Enhancing thermal shock resistance of Y2O3 via La3+/Nb5+ co-doping: Insights into site-selective substitution and microstructural modulation

Yttrium oxide is widely recognized as a critical refractory material for superalloy purification smelting, owing to its high melting point, excellent vacuum chemical stability, and low alloy wettability. However, its inherent brittleness and high thermal expansion coefficient lead to poor thermal shock resistance, which severely restricts its practical industrial applications. To address this limitation, this study utilized La 3+ /Nb 5+ co-doping to fabricate Y 2 O 3 -based composites and systematically investigated their phase structure, microstructure, and overall properties. XRD Rietveld refinement and density functional theory (DFT)-based first-principles simulations confirmed that La 3+ preferentially substitutes Y 3+ at the Y1 lattice site, while Nb 5+ occupies the Y2 site, inducing significant lattice distortion and generating cation vacancies for charge balance. Microstructural observations revealed that co-doping triggers bimodal grain growth-characterized by small, loosely bonded grains with low Nb 5+ content and large grains containing intergranular pores attributed to high Nb 5+ substitution-resulting in increased apparent porosity but enhanced material toughness. Mechanical and thermal shock tests showed that while room-temperature flexural strength decreases, thermal shock resistance is drastically improved. When the La 3+ /Nb 5+ co-doping molar ratio was Y 2 O 3 : La 2 O 3 : Nb 2 O 5  = 1:0.050:0.050 (corresponding to the YLN10 sample), the composite exhibited the optimal overall performance: its room-temperature flexural strength reached 45.57 MPa, residual strength retention after thermal shock was 58.43 %, and elastic modulus retention was 87.91 %, effectively balancing mechanical properties and thermal shock stability. These improvements are primarily attributed to pore-mediated thermal stress dissipation and crack deflection effects. This work demonstrates that La 3+ /Nb 5+ co-doping is an effective strategy to balance the mechanical rigidity and thermal shock stability of Y 2 O 3 , providing a valuable reference for optimizing refractory materials for high-temperature superalloy smelting.