Preferential site occupancy of alloying elements in TiAl-based phases

Abstract

First principles calculations are used to study the preferential occupation of ternary alloying additions into the binary Ti-Al phases, namely γ-TiAl, α 2 -Ti 3 Al, β o -TiAl, and B19-TiAl. While the early transition metals (TMs, group IVB, VB , and VIB elements) prefer to substitute for Ti atoms in the γ-, α 2 -, and B19-phases, they preferentially occupy Al sites in the β o -TiAl. Si is in this context an anomaly, as it prefers to sit on the Al sublattice for all four phases. B and C are shown to prefer octahedral Ti-rich interstitial positions instead of substitutional incorporation.The site preference energy is linked with the alloying-induced changes of energy of formation, hence alloying-related (de)stabilisation of the phases. We further show that the phase-stabilisation effect of early TMs on β o -phase has a different origin depending on their valency. Finally, an extensive comparison of our predictions with available theoretical and experimental data (which is, however, limited mostly to the γ-phase) shows a consistent picture.

Intermetallic β‐Solidifying γ‐TiAl Based Alloys − From Fundamental Research to Application

“…[92] The low C-solubility in g stems from the presence of Ti 2 Al 4and Ti 4 Al 2 -type octahedral sites within the L1 0 -lattice, as described in refs. [94][95][96] Thereby, Nb occupies Ti-sites within the Ti-sublattice [97,98] and forces the occupation of sites in the Al-sublattice of the g-phase by Ti. [94][95][96] Thereby, Nb occupies Ti-sites within the Ti-sublattice [97,98] and forces the occupation of sites in the Al-sublattice of the g-phase by Ti.…”

“…[74] In particular for crystalline solid solutions, the question arises whether an alloying element will be incorporated substitutionally into the crystal lattice or whether it will occupy an interstitial position. [97] In the following, the focus is laid on the ternary additions Nb, Mo, and B along with C and Si into the binary phases g-TiAl, a 2 -Ti 3 Al, and b o -TiAl. While the experimental determination of this information is non-trivial, because it requires high-resolution characterization techniques such as APT (see section 4), it can be obtained using first-principles methods, most commonly within the framework of Density Functional Theory (DFT), [105,106] as implemented, for example, in the Vienna Ab initio Simulation Package (VASP), [107] further employing projector-augmented wave method pseudopotentials [108] and the generalized gradient approximation [109] to describe the electron-ion and electron-electron interactions, respectively.…”

Atomistic Modeling‐Based Design of Novel Materials

“…With an increasing amount x of Mo substituting for Al in TiAl 1Àx Mo x (since Mo preferentially occupies Al the sublattice, [114] ) the ordered phase is also stabilized. [113] This is in agreement with a well known fact, that Mo acts as socalled b-stabilizer, [115] and the ordered phase appears also in the equilibrium ternary Ti-Al-Mo phase diagram.…”

Impact of Alloying on Stacking Fault Energies in γ-TiAl

“…Experimentally, the most relevant are Ti-rich compositions [1,3,25]. Moreover, the early TMs preferably occupy the Ti sublattice in γ-TiAl [26,27]. Therefore, a scenario with a Ti anti-site (Ti atom on the Al sublattice) and a TM substitutional atom on the Ti sublattice was considered.…”