Electronic structure calculation of Cr content effect on corrosion resistance of Ti-Nb-Cr alloy

Abstract

Ti-based alloys are widely used in aerospace and medical engineering because of their excellent properties, such as good fracture toughness, high strength, good corrosion resistance, etc. However, the corrosion resistance performance of the alloys is not adequate to meet the requirements in many cases. The Ti-Cr-Nb ternary alloy system exhibits many excellent characteristics, especially the anti-corrosion ability, making it a very promising candidate for the applications in aerospace and medical engineering. The alloying element Cr can improve the corrosion resistance of Ti-based alloys as reported by many experiments. In order to understand and then predict the effect of Cr content on Ti-Nb-Cr alloy, the electronic structures, such as the cohesive energies, the formation energies, the Fermi levels and the densities of states (DOSs) of the Ti-Nb-Cr alloys with different Cr content of the alloys, are calculated by first-principles method. The calculations in this paper are carried out by VASP (Vienna ab-initio simulation package) software package, which is based on the density functional theory. The generalized gradient approximation is selected to deal with the exchange correlation energy of electrons. And the special k-point sample of the Monkhorst-Pack type is used in the Brillouin-zone integration. The effects of Cr content on the electronic stability and corrosion resistance of the alloy are discussed. In this paper, the Ti-25 at.%Nb alloy with the stable β-phase is a matrix material, and Ti12Nb4 supercell model is adopted, in which 1 to 4 Ti atoms are replaced by the Cr atoms, respectively. In energetics, the sequence of the cohesive capacity of the system is as follows:Ti12Nb4 11Nb4Cr1 10Nb4Cr2 9Nb4Cr3 8Nb4Cr4, showing that the stability of the structure decreases with Cr content increasing. While the formation energy of the system energy shows a gradual increase trend with the increase of Cr, indicating that the formation of the system becomes gradually difficult when adding more Cr atoms. The Fermi level of the ternary alloy system containing Cr element is much lower than that of Ti12Nb4 alloy and tends to decrease slightly with the increase of Cr content. That means that with increasing the Cr content, the alloy system is not easy to lose electrons, and thus the corrosion resistance is improved. And when the Cr content is around 18.75 at.%, there should be an optimal Cr concentration for corrosion resistance. The differential charge density diagrams show that with the increase of Cr content, the covalent bonding of the system is weakened, while the metal bonding is strengthened, which makes the electronic structure of the system more stable and thus the corrosion resistance is improved. The DOS shows that the Fermi level is not zero, indicating the metallic behavior of the alloy. With the increase of Cr content in the alloy system, the pseudo-energy gap gradually disappears, indicating that the structural stability of the system decreases accordingly, which is consistent with the calculation result of the density of states. The maximum value of the DOS diagram is shifted toward the lower energy level area, showing that the stability of the electronic structure of the system is improved so that the corrosion resistance of the alloy is enhanced. And the maximum value of the DOS also shows that when the Cr content is around 18.75 at.%, there is an optimal Cr concentration for corrosion resistance.