Insights on formation of oxide layers, corrosion, and hydrogen embrittlement on the Ti2AlNb (1 1 0) surface: Density functional theory study

Ti₂AlNb alloys offer good mechanical qualities and promise for use in various applications, such as aero-engines and other industries. However, corrosion and hydrogen embrittlement remain important concerns and limitations for their use. In this work, first-principle density functional theory is used to investigate the adsorption of hydrogen, fluorine and oxygen on the surface of Ti₂AlNb (1 1 0). The effects of the considered adsorbates on the surface were compared by analysing the adsorption energy, charge density differences, density of states, and work function. The current findings revealed that the adsorption behaviour of all the adsorbates is exothermic and spontaneous due to the negative adsorption energy. More importantly, the effect of Van der Waals forces and dispersion correction was considered, it was found that for all adsorbates the dispersion correction approach exhibited the most stable adsorption energies ($E_{\mathrm{ads}}^{\mathrm{DFT}-D}$) than the standard density functional theory ($E_{\mathrm{ads}}^{\mathrm{DFT}}$). Thus, the standard DFT underestimates the adsorption energy. Furthermore, it was shown that the adsorption energy strength is dependent on the surface adsorption site, with the Ti-Nb and Al-Nb bridge sites being the most preferred sites for hydrogen, fluorine and oxygen adsorption. Subsequently, it was discovered that oxygen adsorption on the surface of Ti₂AlNb (1 1 0) was more thermodynamically stable than hydrogen and fluorine. This suggests that the Ti₂AlNb surface will likely suffer from oxidation rather than corrosion and hydrogen embrittlement. In addition, surface atoms showed electron-charge depletion, while adsorbates showed charge accumulation. The adsorption caused charge density redistribution and altered the surface work function.