• Addition of trace lanthanum (5.0×10-5 to 3.4×10-4 wt.%) to a second-generation nickel-based single crystal superalloy reduces elemental segregation and increases γ/γ′ eutectic content in the as-cast state.
• Lanthanum promotes the formation of strip-shaped clusters in interdendritic regions, composed of Ni5La and Ni3Ta, with the Ni5La proportion increasing with La content.
• Heat treatment induces incipient melting in cluster regions, increasing microporosity, and transforms cluster morphology from strip to elliptical while altering composition to include MC carbides.
• The study provides a metallurgical basis for optimizing La content and heat-treatment parameters to enhance high-temperature oxidation resistance and mechanical properties of single crystal superalloys.