Abstract: The incorporation of REEs, particularly Y, has become significant in enhancing ultrahigh-temperature performance of nickel-based single-crystal superalloys (Ni-SXs). However, the relationship between Y doping and creep resistance remains unclear. To address this knowledge gap, Y was added to DD5 Ni-SXs with its content ranging from 0 to 1900 ppm (0-0.19 wt.%), which was quantitatively correlated to the rafting evolution and creep performance. Results reveal a critical Y threshold of approximately 320 ppm (equivalent to 0.032 wt.%): below this level, creep rupture life increases by 165 %, while exceeding it results in a 44 % lifespan reduction from its peak value. Through multiscale characterization, we attribute this dual behavior to the competing effect: improved creep resistance at low Y concentrations results from inhibited atomic diffusion and reduced γ/γ′ lattice misfit stress, while excessive Y promotes micropore nucleation, which was distributed in the inter-dendritic region. A mathematical model for creep life prediction and a mechanistic dominant diagram is further established, illustrating the transition between rafting refinement and defect-mediated damage, which are quantitative frameworks for optimizing REEs in next-generation Ni-SXs.

Key words: Creep properties, Yttrium, Rafting microstructure, Single crystal superalloy, Atomic diffusion