Effect of remelting solution heat treatment on microstructure evolution of nickel-based single crystal superalloy DD5

The microstructure evolutions of nickel-based single crystal superalloy DD5 after solution treatment at different temperatures as well as heat treated with various routes were systematically investigated by differential scanning calorimeter (DSC), optical microscope (OM), scanning electron microscope (SEM) and electron probe microanalyzer (EPMA). The formation and elimination mechanisms of unfavorable incipient melting microstructure emerged during heat treatment were explored in detail. Microstructure observations reveal that a higher solution temperature can speed up the element diffusion, resulting in a more uniform element distribution. A reduced degree of lattice mismatch between γ phase and γ' phase would be obtained, which leads to an increase in the average size and content of γ' phase. However, excessively high solution temperature would cause the pre-melting occurred at the interface of γ matrix and γ' phase, causing the formation of incipient melting microstructure. The incipient melting microstructure can be eliminated by the subsequent homogenization process under the conditions that the solution temperature exceeds the solvus temperature of γ' phase. The formation course of incipient melting microstructure includes the pre-melting at the interface of γ matrix and γ' phase, the expansion of molten liquid, and the formation by the resolidification of molten liquid. The elimination of incipient melting microstructure begins with the dissolution of fine eutectic, followed by the dissolution of coarse γ' phase and the filling of pores, and eventually the dissolution of residual eutectic. Compared with standard heat treatment, remelting heat treatment can provide better homogenization effect of element for DD5 superalloy.