Interactions and phase stability in Ni-rich binary alloys

Ni-based superalloys are the important materials for gas turbines in advanced aeroplane engines . The addition of refractory elements to these superalloys, such as rhenium and tungsten, can significantly improve the hightemperature performance by so-called solid-solution hardening. Although the strengthening effect of refractory elements in Ni-based superalloys have been known for a long time, the effective interactions among alloying components as well as the atomic ordering in the alloy systems are still under investigation and even under debate. In this work, we study these interactions and this ordering for two binary alloys, Ni-rich Ni-Re and Ni-rich Ni-W, by means of ab initio simulations and statistical mechanics simulations based on the Ising Hamiltonian. For the Ni-rich Ni-Re alloys, we show that the effective cluster interactions vary substantially depending on the temperature, concentration of the components and the magnetic state of the matrix. The strain-induced interactions have large contribution to the nearest-neighbor pair-interactions and some multisite cluster-interactions in the ferromagnetic and nonmagnetic states. The ordering tendency of binary Ni-Re alloy systems can be predicted in terms of ordering energy and enthalpy of formation. We show that the D1a ordered structure should be stable at the concentration of 20 at.% Re in the Ni-rich Ni–Re alloy system. The Monte Carlo simulations of Ni-Re random alloys show the existence with the D1a-Ni4Re ordered structure at low temperatures. We also calculated lattice parameters for different compositions of Ni-rich Ni-W alloys, and we find that lattice parameters of random Ni-W alloys increase linearly with the concentration of W. This is in good agreement with the Vegard’s law predictions and experimental data. We investigated phase stability of Ni-rich Ni-W alloys in terms of the enthalpies of formation and ordering energies. We find the chemical pair interactions are sensitive to the magnetic state and concentration. The calculated strain-induced interactions are quite large for the first coordination shell, which is due to a large size mismatch of Ni and W. Taking local lattice relaxation into account, the Ni-W systems were modeled by Monte Carlo method. The D1a-Ni4W ordered structure can be observed up to 22 at.% W. In higher concentrations of W, in our MC calculations, the DO22-Ni3W and Pt2Mo-Ni2W ordered structures can be observed in Ni-25 at.% W alloy and Ni-33 at.% W alloy, respectively.