A general approach to simulate the atom distribution, lattice distortion, and mechanical properties of multi-principal element alloys based on site preference: Using FCC_CoNiV and CoCrNi to demonstrate and compare

Abstract

The multi-principal element alloys (MPEAs), which are also called high-entropy alloys (HEAs) or medium-entropy alloys (MEAs) based on the multi-principal element number, received intensive attention due to their unusual phase structures and potential application in diverse aspects. However, quantitative and graphical characterization of the atom distribution, lattice distortion, and mechanical properties are considerably insufficient, which hinders the exploration and development of the rich ore of MPEAs. In this work, a general and comprehensive approach was proposed to simulate the atom distribution, lattice distortion, and mechanical properties of MPEAs based on site preference, where FCC_CoNiV and CoCrNi MPEAs were demonstrated and compared quantitatively and graphically. For this purpose, the temperature- and composition-dependent site occupying fractions (SOFs) of MPEAs were predicted using a two-sublattice model based on the crystallographic information of the prototype of MPEAs, where the corresponding thermodynamic database concerning the temperature-dependent Gibbs free energy of formation of the involved end-member compounds was established in this work using first-principles calculations based on density-functional theory (DFT) and density-functional perturbation theory (DFPT). For FCC_CoNiV MPEA，the site occupying configuration is (V1.0000)1a(Co0.4444Ni0.4445V0.1111)3c, which is not affected by the heat treatment temperature, while for FCC_CoCrNi MPEA, the site preferences change continuously and slightly with the increase of the heat treatment temperature, for example, the site occupying configuration changes from (Co0.3371Cr0.0002Ni0.6627)1a(Co0.3320Cr0.4444Ni0.2236)3c at 673 K to (Co0.3690Cr0.0007Ni0.6303)1a(Co0.3214Cr0.4442Ni0.2344)3c at 1273 K. From the long-range ordered structures, the locally ordered structures were explored further by statistically analyzing the coordination number of the atom coordinated with the same type of atoms in a 30×30×30 FCC supercell based on the prototype of L12_AuCu3 which contains 108,000 atoms. It was found that all the maximum numbers n for Co*‐nCo, Ni*‐nNi, and V*‐nV are only 8 in FCC_CoNiV MPEA, respectively, while the maximum numbers n for Co*‐nCo, Cr*‐nCr, and Ni*‐nNi are 10, 8, and 10 in FCC_CoCrNi MPEA, respectively. Thus, all coordination numbers in the studied MPEAs are less than 12, i.e., the coordination number of the pure metal with a FCC unit cell. The calculated relative lattice distortion of FCC_CoNiV MPEA is 2.54%, which is larger than that of FCC_CoCrNi MPEA (2.25%) by comparing the corresponding structure with and without distortion based on the predicted site configuration at 973 K. Besides the difference in the total energy, the resultant force acting on each atom of the FCC_CoNiV MPEA with and without distortion was graphically characterized to explore the mechanism of lattice distortion. For the mechanical properties, both alloys show ductile, while the elastic anisotropic of FCC_CoNiV MPEA is larger than that of FCC_CoCrNi MPEA, the strong ordering behaviors of V atoms may contribute to these behaviors. Current results agree well with the limited experimental and simulated reports concerning the ordering behavior of V atom and ductile characters. Thus, a general approach was established, which can be applied to simulate the atom distribution, lattice distortion, and mechanical properties of MPEAs based on site preference. We expect that the current approach may act as a standard simulating strategy on MPEAs beyond the traditional random mixing consideration of the different constituent atoms.