First-principle study of the solubility and diffusion of oxygen and boron in γ-TiAl

Abstract

Solubility and diffusion of boron and oxygen in the γ-TiAl intermetallic compound are herein investigated by firstprinciple calculations. On the basis of the space group of the γ phase, the accommodation of the light atoms in the various interstitial positions is studied by density functional theory. Diffusion coefficients are also obtained from ab initio calculations. Barrier energies for the boron and oxygen diffusion among the different stable interstitial sites are computed using the Nudged Elastic Band method and atomic jumping rates are obtained from the Transitional State Theory. Diffusion coefficients are obtained from the solution of the transport equation in the infinite time limit, using the analytical Multi-State Diffusion method. The methodology here applied is validated by the good agreement between the computed diffusion coefficient of boron and the experimental data available in the literature for this species.

Interactions of oxygen with intrinsic defects in L10 γ-TiAl in presence of substitutional solutes: Influence on diffusion kinetics

“…As reported in previous works [24][25][26], in the TiAl system, O atoms prefer to occupy 2h sites among many possible interstitial positions, see Fig. 1.…”

“…O atoms are thus located (along the c axis of the tetragonal structure) above an Al atom at an interstitial position, but not at the octahedral position. These earlier studies indicated [24][25][26] that O has a strong affinity with Ti atoms, which explains for instance its high solubility in Ti systems. In these works, it was also shown that other interstitial positions have higher energies than the 2h positions.…”

“…To the authors' knowledge, these are the only experimental data that can be found in the literature. However, O diffusion coefficients were determined theoretically in three separate studies and results were comparable [23][24][25][26]. Surprisingly, these works all predict values that are quite high (in the order of 10 −11 m 2 /s at 700°C, for a diffusion perpendicular to the [001] crystalline axis).…”

“…The effect of intrinsic point defects and solutes on oxygen diffusivity will be discussed hereinafter. Without traps (as solute substitutional or anti-sites interacting with the O atoms diffusing in interstitial sites), the diffusion coefficients of oxygen according to the crystallographic directions were studied and expressed in literature [24][25][26]. In these works, the atomic process of oxygen diffusivity were investigated.…”

“…where D ideal is the ideal diffusion coefficient of oxygen in interstitial positions [24,26], and C i and E i seg the trap content in the system and the segregation energy of an O atom at trap of kind i, respectively. It was used successfully in the case of hydrogen diffusivity in the Ni system [52].…”