In this work, a self-consistent thermodynamic database for the Mg-Cu-Ni-Y system has been constructed by combining the thermodynamic descriptions of the constituent binary and ternary sub-systems. The modified quasichemical model has been used to describe the liquid phase in this system, for the first time, which accounts for the short range ordering. Isothermal sections of the Mg-Cu-Y and Mg-Ni-Y systems at 673 K and Cu-Ni-Y system at 973 K have been constructed experimentally. Diffusion couples and key samples have been employed for establishing the phase equilibria. Microstructural characterizations using scanning electron microscopy (SEM), and phase identification using x-ray diffraction (XRD) and wave dispersive x-ray spectrometer (WDS) have been carried out. Twelve ternary intermetallic compounds have been identified in the Mg-Ni-Y system. Crystallographic prototypes of two of these compounds; 1 (MgNiY4) and 2 (MgNi2Y2) have been found and their lattice parameters have been determined using Rietveld analysis of the XRD patterns. Also, ternary solubility of 3 (MgNi4Y) has been found to be from 9.2 to 25.9 at.% Mg with constant 65 at.% Ni. Ternary solubility of the binary compounds in the Mg-Cu-Y, Mg-Ni-Y and Cu-Ni-Y systems has been determined. A complete mutual solubility between CuY-NiY and Cu4Y-Ni4Y in the Cu-Ni-Y system has been identified. Differential scanning calorimetry (DSC) experiments have been conducted for selected key samples of the Mg-Cu-Y, Mg-Ni-Y and Cu-Ni-Y systems to determine the liquidus and other phase transformation temperatures. These results have been compared with the thermodynamic modeling which showed good agreement. Thermodynamic calculations of the Mg-Cu-Ni system also show consistency with the literature data. 

Liquidus projections of the four ternary systems have been calculated based on the present thermodynamic modeling considering the current experimental results and data from the literature. All four ternary systems have been combined in one database for the Mg-Cu-Ni-Y quaternary.