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Atomic size effect of alloying elements on the formation, evolution and strengthening of gamma'-Ni3Al precipitates in Ni-based superalloys

Mechanical properties of Ni-based superalloys strongly depend on phase and site preferences of alloying elements which influence bonding strength within gamma'-Ni3Al precipitates and microstructural characteristics of these unique class of materials. In the current work, therefore, besides disclosure of the phase partitioning behaviours of alloying X elements (i.e. X = Co, Cr, Nb, Ta or Ti) among gamma' and gamma phases, their site occupancy tendencies in gamma' precipitates (determined via first-principles ab initio calculations at 0 K) and effects on the microstructural evolution of Ni80Al15X5 alloy systems (exposed to aging at 800 degrees C for 4, 16, 64 and 256 h, respectively) have been examined. Bonding features of Ni-Al, Ni-X and Al-X atomic pairs within Ni3Al-X intermetallics have been simulated by utilizing charge density difference (CDD) method, which reveals site preferences of alloying X elements as well. Present theoretical and experimental investigations have shown that mechanical strength of Nibased superalloys is predominantly affected by bonding properties within precipitates. As atomic radii of alloying X elements become closer to that of Al atom, energy change parameter, E-Ni -> Al(X) values decrease and more Al sublattice sites are preferentially occupied in gamma' precipitates. Correspondingly, bonding strength of Ni-X atomic pairs along < 110 > directions of Ni3Al-X phases and micro-hardness properties of both as-cast and pre aged Ni80Al15X5 alloy systems enhance in the order of X = Co < Cr < Ti < Nb < Ta additions. Nevertheless, with increasing aging time, mechanical strength of alloys weakens in parallel with increasing size of gamma' precipitates simultaneously evolved from near-spherical to irregular forms.