Theoretical study of oxygen insertion and diffusivity in theg-TiAlL1₀system

Abstract

This work is a first-principles study of the insertion and diffusivity of oxygen in the γ-TiAl L1 0 system. Five interstitial positions were identified as stable. One, however, the 2h site a pyramid composed of a Ti square topped by an Al atom, was found more stable than the others. The oxygen interactions with the TiAl system were thus studied and analyzed in detail using vibrational, elastic and electronic properties. The results show that the O atom prefers to be surrounded by Ti atoms and tries to minimize the number of bonds with aluminum. The diffusion mechanism is subsequently studied at the atomic scale, by analyzing displacements between stable interstitial sites. The oxygen diffusivity is found to be anisotropic and the components in the x and z direction, D x and D z , are then calculated and compared with those of O diffusion into other Ti-Al alloys. The analysis of results shows two effects. First, the stability of sites is related to the number of O-Al bonds, the fewer there are, the more stable the site is, and second, the diffusion is faster when the content of interstitial sites composed of many Ti atoms is low.

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].…”