Designing homogenization–solution heat treatments for single crystal superalloys

The conventional stepwise solutionizing method is studied for an experimental Ni-base single crystal superalloy using metallography, eutectic fraction analysis and electron microprobe analysis. The temperature range for solutionizing the alloy is determined by combinations of holding time and temperature. The effects of long isothermal holding within and above this solutioning temperature range are presented. Heat treatment steps below the γ′-solvus temperature stabilize the eutectic phase, while steps above the solvus temperature improve the homogenization and reduce eutectic phase fraction. There is a finite nucleation time for incipient melting which is a function of holding temperature above the equilibrium solidus of the alloy. A prolonged isothermal holding above the equilibrium solidus temperature causes up-hill diffusion at the eutectic region leading to incipient melting. A new homogenization–solution heat treatment approach with continuous heating between solvus and solidus is proposed.