Thermodynamic Analysis on Microstructural Control of Ordered Phases in Nickel-base Superalloys

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The gamma (disordered fcc) and gamma' (ordered Ll_2 structure) phases are major constitutions in the Ni-base superalloys, while beta (ordered B2 structure) phase is used for surface coating. In spite of practical importance, however, the effect of alloying elements on the relative stability between gamma, gamma' and beta phases bas not been well established. The purpose of the present study is to clarify the partition behavior of alloying elements between gamma, gamma' and beta phases. Thermodynamic analysis of phase equilibria was also done by using regular solution approximation of gamma phase and sublattice model for gamma' and beta phases. The results obtained are as follows.1. The phase equilibria of gamma, gamma' and beta phases in the Ni-Al-X (X: Si, Ti, V, Cr, Mn, Fe, Co, Cu, Nb, Mo, Ta, W) systems were accurately determined between 700 and 1300ﾟC. 2. The partition behavior of alloying elements can be classified into 6 categories. 3. The gamma phase is stabilized by alloying of Cu, Co, Fe and Cr, while Mo, W, V, Ti, Nb, Ta and Si stabilize gamma' phase and Mn stabilizes beta phase. 4. The Gibbs energy of ordered gamma' and beta phases can be well described by the sublattice model. 5. The thermodynamic parameters in the gamma, gamma' and beta phases were evaluated. 6. The phase equilibria of Ni-Al-Fe, Ni-Al-Ti, Ni-Al-Cr and Ni-Al-Mn systems are computed in the Ni-rich portions, which would be most useful for the design of microstructures in the Ni-base superalloys.