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

Abstract

The fundamental aspects of temperature- and composition-dependent site preference of alloying elements on sublattice of Co 3 Al-based γ′ phase with L1 2 structure have not been well understood, which hinders the design of advanced Co-based high-temperature alloys. In this work, the temperature-dependent and composition-dependent site preferences of the binary, ternary, and quaternary Co 3 Al-based γ′ phase alloyed with M i 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 density-functional theory (DFT) and density-functional perturbation theory (DFPT). The additional alloying elements M i 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 non-stoichiometric binary Co 3 Al as well as in ternary and quaternary Co 3 Al-based systems alloyed by adding additional transition metals around the solvus temperature. For the stoichiometric Co 3 Al γ′ phase, the critical temperature is about 1073 K, accompanied with the site configuration changes from (Al) 1a (Co) 3c at room temperature to (Al 0.2590 Co 0.7409 ) 1a (Al 0.2469 Co 0.7530 ) 3c at 1273 K, while this order-disorder transition was not found in the Ni 3 Al-based γ′ phase, a counterpart of Co 3 Al-based γ′ phase. When x Co / x Al is over 3:1, for example, the non-stoichiometric Co 3 Al–based binary γ′ phases 78Co-22Al show continuously order-disorder transition characteristics with a broadened transition temperature range. For the γ′ phase with the composition 78Co-26Al-4M i which is normalized as 72.22Co-24.08Al-3.7M i (at.%), where we kept x Co / x Al = 3:1 to study the ordering behaviors of the additional alloying transition metals M i 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-2M 1 -2M 2 , which is normalized as 72.22Co-24.08Al-1.85M 1 -1.85M 2 (at. %) has been also investigated. The additional alloying transition metals M i added modulate the order-disorder transition characteristics of Co 3 Al-based γ′-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 mechanical properties, according to the modulating amplitude of the order-disorder transition behavior, we recommend that the additional alloying elements added such as Cu, Mn, and Mo atoms make the mechanical properties degrade gradually, and thus the structural components avoid sudden damage, which is beneficial for the safety of the engine component at high operating temperature. Fig. The temperature and composition-dependent site occupying fractions (SOFs) of the alloying element in Co 3 Al-based γ′ phase, where stoichiometric, non-stoichiometric binary Co 3 Al, ternary and quaternary systems by adding additional alloying elements were predicted, meanwhile, the atom distributions of the full ordered configuration and the disordered configuration at high temperature are presented graphically. It is revealed that there is a sudden order-disorder transition (also called first-ordered phase transition) in the Co 3 Al-based γ′ phase with L1 2 structure, meanwhile, Cu, Fe, Mn, and Ni atoms always prefer 3c sublattice (Co site), and Mo, Ta, Ti, W atoms always prefer 1a sublattice (Al site) in the whole temperature range, while the site preference of the alloying elements such as Al, Co, Cr, Re, or V is affected by the heat treatment temperature. The additional alloying elements M i added modulate the order-disorder transition characteristics of Co 3 Al-based γ′-precipitate with different amplitudes. • Co 3 Al-based γ′ phases show strong site preference and order-disorder transition, which were verified using DSC. • The transition temperature is about1000 K, while is modulated and broadened by additional elements. • Cu, Fe, Mn, and Ni always prefer 3c sublattice, while Mo, Ta, Ti, and W always prefer 1a sublattice in the whole temperature range. • The site preferences of Al, Co, Cr, Re, and V are temperature-dependent.