Effect of transition metal element additions on the mechanical and electronic properties of L10 CoNi alloys

Abstract

This paper systematically investigates the site preference and mechanical and electronic properties of L10 CoNi alloys doped with transition metal elements by first-principles calculations. The goal is to determine the elements that can improve the properties of nickel–cobalt alloys. The results of the formation enthalpy ΔHf and substitution enthalpy Esite illustrate that a large portion of the doping elements tends to substitute at the Co sites and form Co7MNi8, except Cr, Os, Ir and Pt. According to the mechanical properties results, only the Cr, Os and Ir additions form Co8Ni7Cr, Co8Ni7Os and Co8Ni7Ir, respectively, which remarkably improve the mechanical properties. The anisotropy of the directional Young’s modulus and acoustic velocities predict that these alloys are all anisotropic materials. In addition, the thermal properties are studied, and the Debye temperature ΘD and minimum thermal conductivity kmin reveal that only the Cr doping results in the formation of Co8Ni7Cr and improves the thermal conductivity in all directions. The density of states (DOS), crystal orbital Hamilton population (COHP) and electron location function (ELF) are calculated to elucidate the origin of the mechanical properties of the CoNi alloys. The results indicate that the strengthening effects of doping elements, such as Cr, are attributed to the enhanced covalent characteristic between the Co (or Ni) and Cr atoms. In summary, it is found that Cr, Os and Ir have good solid solution effects in the CoNi alloys, which is worthy of further study.