Effect of solution cooling rate on the microstructure and creep deformation mechanism of a rhenium-free second-generation single crystal superalloy

Abstract

• The cubic degree and size of γ′ phases were inversely proportional to the solution cooling rate. • Air cooling method significantly improved the creep lifetime at 800 °C/750 MPa and 1100 °C/137 MPa. • Air cooling facilitated the formation of dislocation locks and retarded γ′ phases to coarsen at 800 °C/750 MPa. • The influence mechanism that air cooling facilitated the denser interface dislocation networks was explored in combination with the long-term ageing test. • The nucleation and precipitation of secondary γ' phase might well be influenced by the solution cooling rate. Various cooling scenarios (water, oil, air and furnace) were employed to study the impacts of the solution cooling rate (SCR) on the microstructure and creep behavior of a novel single-crystal (SX) superalloy. The results showed that the cubic degree and size of the γ′ phases were inversely proportional to the SCR. The creep life first increased and then dropped dramatically with a reduction in the SCR. The creep life of the sample cooled with air cooling (AC) was the highest, up to 144.90 h at 800 °C/750 MPa and 160.15 h at 1100 °C/137 MPa. During creep at 800 °C/750 MPa, the improved creep life of the AC sample was mainly attributed to the fine cubic γ′ phases, which decreased the rate of γ′-phase coarsening and favoured plastic deformation by promoting the active movement of dislocations. The AC helped the γ′ phases become rich in Al, Ti and Ta while depleted in Co and Cr, which enhanced its stacking fault energy, thus promoting the formation of dislocation locks. Meanwhile, the largest negative lattice misfit caused by AC induced denser γ/γ′ interface dislocation networks at 1100 °C/137 MPa, which efficiently reduced the minimum creep rate. The calculated average dislocation spacing results indicated that the smallest density of excess dislocations corresponded to the AC sample, proving its greatest creep resistance. Interestingly, the size of the secondary γ′ phases first decreased and then increased sharply with decreasing SCR during creep at 1100 °C/137 MPa, when fine secondary γ′ phases had a positive role in the blockage of dislocation movement in the matrix. Eventually, the comprehensive SCR effect was explored to provide more guidance in the design of Re-free SX superalloys.