Investigation of precipitation hardening behaviour of Ni-based superalloys by thermo-calc simulation

The augmenting performance conditions of new generation power systems involve innovative materials with improved high-temperature mechanical properties. In advanced power plants or land-based gas turbines, increasing the service temperature results directly in enhanced efficiencies, and thus, provides important operational cost savings. Polycrystalline Ni-based superalloys are considered as strategic materials for applications requiring a best combination of strength, corrosion-oxidation resistance, melting point and creep resistance at high temperatures. The various alloying elements are present in the chemical compositions in order to design and develop Ni-based superalloys having specific properties for energy applications including steam turbines or traditional land-based gas turbines (rotor, spacers, airfoils, etc.). Ni-based superalloys commonly exhibit a two phase γ/γ' microstructure, in which ordered γ' precipitates, of Ni3Al composition and FCC-L12 crystal structure, are embedded in a FCC γ (Ni) matrix. The γ' precipitates provide significant contribution to the strength to and their physical characteristics can be controlled through partitioning behavior of elements such as Al, Ti, Ta and Nb. Other alloying elements that partition to the matrix, such as Cr, Co, Mo and W, can also be added for solid-solution strengthening or to improve properties such as corrosion resistance and lattice misfit. Moreover, B and C can be added trace amount in order to control grain size or strengthen the grain boundaries. In this study, the effect of alloying element, Mo and Co (2 and 4 at. %), additions on solidification behavior (solvus, solidus and liquidus temperatures), density, partitioning behavior (partition in either in γ matrix or γ' phase), precipitate radius, precipitate volume fraction and precipitate size distribution of Ni-11Al-9Cr superalloys is investigated in detail with computational materials science. Computational design was first performed using a CALPHAD (Calculation of Phase Diagrams) approach with Thermo-Calc 2022a and the TCNi11 thermodynamic database for phase diagrams. It was shown that Mo addition increased the solvus temperature, decreased the γ' precipitate radius and increased the volume fraction of γ' precipitates. On the other hand, Co addition slightly increased solvus temperature, slightly decreased precipitate radius and decreased the γ' volume fraction. In addition, Mo and Co atoms tended to occupy γ-Ni matrix phase.