Thermal vacancy formation and positron–vacancy interaction in Ti3Al at high temperatures

Abstract

In order to study the formation of thermal vacancies in the Ti–Al alloy system, high-temperature positron lifetime measurements together with a modeling of defect formation in the framework of nearest-neighbor pair bonds were performed for α2Ti3Al and compared to recent results on γTiAl [U. Brossmann, R. Würschum, K. Badura, and H.-E. Schaefer, Phys. Rev. B 49, 6457 (1994)]. Substantial increases of the positron lifetime τ were observed for Ti65.6Al34.4 and Ti77.1Al22.9 in the temperature range T≳1200 K where thermal vacancy concentrations above the detection limit of positron annihilation are expected from the model calculations for the α2 phase. Within the high-temperature increase of the positron lifetime in the α2 and the β phase single-component positron lifetime spectra were observed. This behavior is in contrast to the two-component spectra observed conventionally at intermediate positron trapping rates and is attributed to a fast detrapping and retrapping of positrons at vacancies due to a low positron–vacancy binding energy. For this case, a vacancy formation enthalpy of HFV=(1.55±0.2) eV in α2Ti65.6Al34.4 and HFV=(1.8±0.2) eV in βTi77.1Al22.9 can be derived. These results are discussed in the context of recent 44Ti tracer diffusion studies.

Effect of disorder on the dilute equilibrium vacancy concentrations of multicomponent crystalline solids

“…While the vacancy concentration increases above x=0.25, Al anti-site defects on the Ti sublattice dominate to accommodate off stoichiometry in DO 19 . The increase in vacancy concentration with Al concentration in α 2 between x = 0.22 and x = 0.35 agrees qualitatively with the dependence of an effective vacancy formation energy with Al concentration as determined with positron lifetime measurements 36 . An accurate description of vacancy solute interactions is a crucial input for predictions of diffusion coefficients in multi-component solids 58,59 .…”

“…The vacancy formation energy in hcp Ti as predicted with approximations to DFT, for example, are in the vicinity of 2 eV 34,35 . Effective vacancy formation energies of 1.55±0.2 eV and 1.8±0.2 eV have been reported in ordered Ti 3 Al at off-stoichiometric concentrations as determined with positron lifetime measurements 36 . A large fraction of substitutional binary alloys and multi-component solids contain thermal vacancies, which are usually substantially more dilute than structural vacancies that accommodate off-stoichiometry as occurs in some intermetallic compounds such as Al rich B2-NiAl 15 .…”

Growth kinetics of antiphase domain in Ti3Al intermetallic compound

“…In order to estimate the vacancy concentration on APDB which is required to evaluate the m; the formation enthalpy of vacancy on APDB H APDB f should be substituted for the H f : While there are no data on H APDB f unavailable in literature, the H f in the ordered matrix H order f has been obtained by positron life time study [36]. The H APDB f is considered to be closely related to that in the matrix.…”

A3B Intermetallics: Defect chemistry and nonstoichiometry

“…7 that the energies of formation of vacancies on the two sublattices would be rather high compared to those for antistructure atoms. From their positron annihilation study, Würschum et al derived a value for the vacancy formation enthalpy of 1.55 ± 0.2 eV [47]. Therefore, it was assumed that E f (V α ) = E f (V β ) = 1.5 eV would be a reasonable value for the vacancy formation energies in our calculations.…”

“…[47][48][49][50][51] it was assumed for the calculation of the theoretical activity curves in Fig. 7 that the energies of formation of vacancies on the two sublattices would be rather high compared to those for antistructure atoms.…”