Influence of transition group elements on the stability of the δ- and η-phase in nickelbase alloys.

To improve the high-temperature capability of 718-type wrought nickel-base superalloys, the -phase () can be stabilized. However, this also reduces the size of the forging window because forging has to be done above the - and below the solvus temperature of the phase that is used to enable fine-grain forging, i.e. the δ-phase of type or the η-phase of -type. Understanding the influence of alloying elements on the formation of these phases is therefore important. In this paper, density functional theory calculations at 0 K are performed to determine the stabilizing effect of aluminium and of the transition group elements on the stability of the δ-phase and η-phase. Most of the transition group elements of 5th and 6th period stabilize the δ-phase, whereas the stabilizing effect on the η-phase is weaker. According to the calculations, Mo, Tc, W, Re, and Os may be expected to stabilize the δ-phase but not the η-phase, whereas Al and Zn strongly stabilize the η-phase. V, Zr, Ru, Rh, Pd, Ag, Cd, Hf, Ta, Ir, Pt, Au, and Hg stabilize both phases. For some elements (Cr, Mn, Fe, Co), magnetic effects in the δ and especially in the η-phase are shown to be significant at the concentrations studied here. [ABSTRACT FROM AUTHOR]