First-principles study on effect of Re on micro structure and mechanical properties of Ni<sub>3</sub>Al intermetallics

Abstract

The effects of Re on the microstructure and mechanical properties of <inline-formula><tex-math id="M2">\begin{document}$ {\gamma '}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="10-20200097_M2.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="10-20200097_M2.png"/></alternatives></inline-formula> phase Ni<sub>3</sub>Al intermetallics are investigated by using first-principles method based on density functional theory and generalized gradient approximation. It shows that in most stoichiometric ranges, the dissolution energy of Re replacing Al site is smaller than that of Re replacing Ni site. That means the energetically-favorable site of Re in Ni<sub>3</sub>Al is Al replace site. We further investigate the interaction between two Re atoms in Ni<sub>3</sub>Al. The larger the distance between two Re atoms, the more stable the system becomes, showing that Re atoms are dispersed in Ni<sub>3</sub>Al instead of being aggregated. Re doping causes a small increase in the lattice constant of Ni<sub>3</sub>Al intermetallics without causing serious lattice deformation. Analyses on differential charge density and state density show that Re atom bonds with neighboring atoms, especially with Ni atoms, and reduces the surrounding Ni—Al bond energy. Analyses on local state density show that Re atom has orbital interaction with the neighboring Ni and Al atoms, and the interaction with Ni is larger, which concerns the 5d orbit of Re and the 3d orbit of Ni. The effect of Re on the mechanical properties of Ni<sub>3</sub>Al intermetallics is also investigated. The elastic constants calculating results together with empirical criteria indicate the presence of Re atom (corresponding concentration is 0.93%) can cause increase of the stiffness and hardness of Ni<sub>3</sub>Al. The Cauchy pressure value shows a slight improvement in toughness. The increase of Re doping concentration (the concentration of Re in Ni<sub>3</sub>Al is 1.85%) can cause increase of the lattice constant, stiffness and hardness and decrease of the ductility of Ni<sub>3</sub>Al. In order to correct the temperature of the results obtained by first-principles method, the influence of temperature on the mechanical properties of Ni<sub>3</sub>Al has been further investigated through Phonopy calculation. The influence of temperature on the coefficient of thermal expansion and bulk modulus of elasticity is obtained by quasi harmonic approximation. The results show that the addition of Re slightly enhances the entropy of Ni<sub>3</sub>Al, but causes decrease of its Helmholtz free energy on a small extent. At high temperature, doping of Re greatly strengthens the bulk modulus of Ni<sub>3</sub>Al but decreases the thermal expansion coefficient of which. Results of the current research can provide theoretical data for improving the mechanical properties of single crystal turbine blades of aero-engines.