Phase stability and its effect on tensile properties of MAR-M247 alloy have been investigated during thermal exposure at 800-900 ℃ for up to 10,000 h.Detailed investigations reveal that the larger secondary γ'phase has no obvious growth,but the smaller tertiary γ'phase obviously coarsens and the coalescence occurs during thermal exposure at 850 ℃ and below.γ'coarsening behavior is consistent with the description of Ostwald ripening theory before γ'coalescence.Hf-rich blocky MC carbide shows excellent thermal stability,but Ta-rich script-type MC carbide gradually degenerates via reac-tion,MC + γ→ M23C6 + γ'and finally forms γ'film around MC and M23C6 carbides.With increasing thermal exposure time,the tensile strength decreases.The ductility first increases and then decreases during exposure at 800 ℃,but it decreases continuously at 900 ℃.In addition,the ductility keeps almost constant when the exposure time is longer than 5000 h.