Density functional theory study of the mechanical properties of CoCrFeNi Carmel Dansou (University of Johannesburg) David Strubbe, Helix Van Leynseele and Bram Vermeiren (Ghent University) September 14, 2019 Abstract This paper investigates two main mechanical properties (bulk modulus and Poisson’s ra- tion) of the high entropy alloy CoCrFeNi, as well as convergence testing based on density functional theory (DFT) calculations. 1 Introduction High entropy alloys (HEA) made their appearance in the last decade [1], which are alloys char- acterised by being made out of at least 4 chemical elements. Since then they have attracted a lot attention since they show a variety of novel physical properties such as excellent specific strength, high mechanical performance and superconductivity [1, 2] to name only a few. Recently the equiatomic CoCrFeNi HEA has been synthetized [3, 4, 5] and widely used as base alloy to con- struct may other HEAs [6]. The CoCrFeNi based alloys show great properties holding significant promise for industrial application. Considering the potential application, exploring the physical properties of CoCrFeNi is of great importance for both technological and fundamental aspect. In this paper, the bulk modulus and Poisson’s ratio of the CoCrFeNi HEA are calculated by the use of density functional theory (DFT). 2 Computational details The DFT calculations are performed using the plane wave formalism as implemented in the Quan- tum Espresso (QE) simulation package. All calculations are performed using the Perdew-Burke- Ernzerhof (PBE) exchange correlation functional and using the high performing computer (HPC) of the Flemish Supercomputer Centre (VSC). Firstly, convergence tests are executed for the k-mesh and energy cut-offs to assure precise results for a calculation time that was as small as possible. Based on those results, it was decided to work with a 7x7x7 k-mesh and energy cut-offs of 104 and 1270. k-mesh 3x3x3 5x5x5 7x7x7 9x9x9 11x11x11 Energy (Ry) -1526.906 -1526.906 -1526.907 -1526.906 -1526.906 Hydrostatic pressure (kbar) -89,26 -91,66 -91,16 -91,96 -92,23 Table 1: Convergence testing for the k-mesh size. Performed using an energy cut-off of 64 Ry for the wavefunction and 782 Ry for the charge density. Energy cut-offs 54 - 660 64 - 782 74 - 904 84 - 1026 94 - 1148 104 - 1270 114 - 1392 Energy (Ry) -1526.899 -1526.907 -1526.917 -1526.924 -1526.925 -1526.926 -1526.926 Hydrostatic -96,01 -91,16 -97,02 -89,77 -84,15 -85.30 -86.65 pressure (kbar) Table 2: Convergence testing for the energy cut-offs (wavefunction - charge density). Performed using a k-mesh of 7x7x7. 1