Thermo-Calc and Dictra simulation of constitutional liquation of gamma prime (γ′) during welding of Ni base superalloys

The softwares Thermo-Calc and Dictra, based on the one hand on the calculation of phase diagrams (CALPHAD), and on the other hand on the finite difference simulation of diffusion in multicomponent alloys, are used to simulate the out-of-equilibrium formation of a liquid film during the rapid heating or welding of nickel-based superalloys. The softwares rely on the thermodynamic and mobility databases TTNI7 and MOB2, respectively. When temperature increases, γ′ particles of Ni3(Al,Ti,Nb) dissolve progressively. If temperature reaches the own melting point of the γ′ phase before it has completely dissolved, the particle melts (liquation); if the particle dissolves completely before temperature reaches its melting point, no γ′ liquation is possible. However, γ′ dissolution leads to a local enrichment of the γ matrix in Al, Ti and/or Nb, which decreases locally the melting point. If temperature reaches this melting point before the alloy has been completely homogenised, a sort of “γ liquation” can take place, the material melting locally at a temperature inferior to the equilibrium solidus of the alloy. As the kinetics of precipitate dissolution is a major aspect of the phenomenon, the latter depends strongly on precipitate size. Results are in good agreement with published experimental observations.