Ultrafine-grained (UFG) materials exhibit high strengths due to grain boundary strengthening, but grains can grow rapidly if post heat treatment is required, making it challenging to achieve grain boundary and precipitation strengthening simultaneously. Grain growth stagnation at 525 ℃ (0.87 T_m, melting point) was observed in a Mg-4Y-3RE alloy fabricated by additive friction stir deposition (AFSD), a novel solid-state additive manufacturing technology. The AFSD processing produced a UFG microstructure and two major second phases, Mg₄₁RE₅ and nanoparticles containing Y and O. After solid solution treatment (SST) at 525 ℃ for 72 h, no noticeable grain growth occurred. While Mg₄₁RE₅ particles dissolved into the matrix within 4 h of SST, the nanoparticles remained stable and unaltered. The observed grain growth stagnation is attributed to Zener pinning by these thermally stable nanoparticles. These new findings offer a novel approach to designing UFG materials with exceptional thermal stability for high-temperature applications.