Influence of heat treatments at temperatures below the γ′ solvus on γ′ phase in nickel-base superalloy GH720Li

The microstructurM evolution of γ′ phase was investigated in difficult-to-deform nickel-base superalloy GH720Li subjected to subsolvus solution heat treatment, 650℃ for 24 h → air cooling or 650 ℃ for 24 h→ air cooling + 760 ℃ for 16 h → air cooling after subsolvus solution treatment, and long time aging at 870 ℃ for 3000 h. It is found that the primary γ′ phase is affected most significantly by subsolvus solution treatment, the degree of partial dissolution increases with increasing solution temperature, and aging treatment makes the primary γ′ phase turn to sphere or near sphere. However, the secondary and the tertiary γ′ phase fully dissolve into the matrix during subsolvus solution treatment, re-precipitation during the aging is obvious, and the amount and zone increase with increasing solution temperature. When long time aging at 870 ℃ the microstructure uniformity gradually improves, and the secondary and tertiary γ′ phase dissolve. The primary γ′ phase at grain boundaries coarsens a little after 500 h. Besides, the hardness decreases with the aging time prolonging at 870℃ before keeping unchanged.