STUDY OF POINT DEFECTS AND NITROGEN INSERTION IN TITANIUM OXIDES

Work package:
Titanium-based alloys are used in the aerospace industry because of their excellent strength-to-weight ratio, which significantly reduces aircraft weight. In environments subject to high thermo-mechanical stresses, such as aircraft engines, these alloys can sometimes replace denser materials such as refractory steels or nickel- or cobalt-based alloys. However, titanium has a particular affinity for oxygen, dissolving up to 33% in atomic fraction in its pure form. Above a certain concentration (approximately 0.4% at. for industrial titanium alloys), this oxygen solubility causes significant embrittlement of the material. The use of titanium alloys is therefore limited in terms of temperature, generally not exceeding 600 °C for the most efficient alloys. Within the MEMO team, we are looking for ways to slow down the penetration of O into Ti and thus either increase the lifespan of titanium alloys or use them at higher temperatures. In the presence of air, titanium reacts with oxygen, and a layer of oxide (TiO ) quickly forms on the surface₂ of the metal. Under certain conditions, after this layer has formed, layers of titanium nitride (Ti N) may also₂ appear at the interface between the surface oxide and the titanium. This nitride layer is thought to act as a diffusion barrier for various species (particularly oxygen), thereby slowing down the penetration of these elements into the metal. To form this Ti2N layer, the nitrogen present in the air must pass through the oxide layer. In order to model the growth of this Ti2N layer as accurately as possible, it is important to estimate the mobility of nitrogen through TiO2. The main objective of this internship is therefore to focus on the atomic- scale modelling of nitrogen in TiO2. To achieve this objective, we will perform atomistic calculations using density functional theory (DFT) via the VASP software. These calculations will enable us to estimate insertion energies and diffusion barriers using the Activation-Relaxation Technique (pARTn) method recently coupled to the VASP code. To calculate the transport coefficient, we will draw on previous work carried out at the laboratory using the KineCluE code, which establishes the link between atomic-scale processes and macroscopic quantities. The ADPI code will also be used to calculate point defect concentrations.