The ordering behavior of Co3Al-based ?? phase with L12 structure predicted by the thermodynamic model with support of first-principles calculations

The fundamental aspects of temperature- and composition-dependent site preference of alloying elements on sublattice of Co3Al-based gamma ' phase with L12 structure have not been well understood, which hinders the design of advanced Co-based high-temperature alloys. In this work, the temperature- and composition-dependent site preferences of the binary, ternary, and quaternary Co3Al-based gamma ' phase alloyed with Mi were studied using a two-sublattice thermodynamic involving site occupying fractions (SOFs). The SOFs were calculated using a thermodynamic software package based on a thermodynamic database established in this work, where the temperature-dependent thermodynamic data were obtained using first-principles calculations based on densityfunctional theory (DFT) and density-functional perturbation theory (DFPT). The additional alloying elements Mi added represent the transitional metals Cr, Cu, Fe, Mn, Mo, Ni, Re, Ta, Ti, V, or W (arranged in alphabetical order). The results show that there is an obvious order-disorder transition for the stoichiometric and nonstoichiometric binary Co3Al as well as in ternary and quaternary Co3Al-based systems alloyed by adding additional transition metals around the solvus temperature. For the stoichiometric Co3Al gamma ' phase, the critical temperature is about 1073 K, accompanied with the site configuration changes from (Al)1a(Co)3c at room temperature to (Al0.2590Co0.7409)1a (Al0.2469Co0.7530)3c at 1273 K, while this order-disorder transition was not found in the Ni3Al-based gamma ' phase, a counterpart of Co3Al-based gamma ' phase. When xCo/xAl is over 3:1, for example, the non-stoichiometric Co3Al-based binary gamma ' phases 78Co-22Al show continuously order-disorder transition characteristics with a broadened transition temperature range. For the gamma ' phase with the composition 78Co-26Al4 Mi which is normalized as 72.22Co-24.08Al-3.7 Mi (at%), where we kept xCo/xAl = 3:1 to study the ordering behaviors of the additional alloying transition metals Mi conveniently. It is revealed that Cu, Fe, Mn, and Ni atoms always prefer the 3c sublattice (Co site), and Mo, Ta, Ti, and W atoms always prefer the 1a sublattice (Al site) in the whole temperature range, while the site preferences of Cr, Re, and V atoms are affected by the heat treatment temperature. For example, when the heat treatment temperature is lower than 700 K, Cr and V atoms randomly occupy 1a and 3c sublattice, and Re atoms prefer to 3c sublattice, while when the heat treatment temperature is higher than 1273 K, Cr atoms prefer 3c sublattice, V atoms prefer to 1a sublattice, and all Re atoms occupy 3c sublattice exclusively. The site preference of the quaternary system with a selective composition 78Co-26Al-2 M1-2 M2, which is normalized as 72.22Co-24.08Al-1.85 M1-1.85 M2 (at%) has been also investigated. The additional alloying transition metals Mi added modulate the order-disorder transition characteristics of Co3Al-based gamma '-precipitate with different amplitudes. The order-disorder transition temperatures for some selected alloys were verified experimentally using DSC. Based on the causal relationship of structure and