Evolutionary Mechanism of Incipient Melting Microstructure of Single Crystal Superalloy DD6

The evolution of the incipient melting microstructure of the second-generation single crystal superalloy DD6 was studied during heat treatment at 1 330 ℃, slightly higher than its solution heat treatment window. After heat treated for 0.5 h, incipient melting occurs at interdendrites, and some of the incipient melting regions have a pore at the center; with the holding time increasing, incipient melting microstructure fades away gradually and totally disappears after 8 h. The experimental results are analyzed by thermodynamic and kinetic calculations. The solidus temperature of the interdendrite region of as-casting DD6 single crystal superalloy is apparently lower than that of the dendrite arm region because of severe dendrite segregation. During the heat treatment at a temperature slightly higher than the solidus temperature, incipient melting occurs initially; it fades away gradually with the improvement of element homogenization degree, and finally turns to normal microstructure.