Ti-Ti Bonding in γ-TiAl and f.c.c. Ti

Effects of O in a binary-phase TiAl-Ti3Al alloy: from site occupancy to interfacial energetics

We have investigated site occupancy and interfacial energetics of a TiAl-Ti(3)Al binary-phase system with O using a first-principles method. Oxygen is shown to energetically occupy the Ti-rich octahedral interstitial site, because O prefers to bond with Ti rather than Al. The occupancy tendency of O in TiAl alloy from high to low is α(2)-Ti(3)Al to the γ-α(2) interface and γ-TiAl. We demonstrate that O can largely affect the mechanical properties of the TiAl-Ti(3)Al system. Oxygen at the TiAl-Ti(3)Al interface reduces both the cleavage energy and the interface energy, and thus weakens the interface strength but strongly stabilizes the TiAl/Ti(3)Al interface with the O(2) molecule as a reference. Consequently, the mechanical property variation of TiAl alloy due to the presence of O not only depends on the number of TiAl/Ti(3)Al interfaces but also is related to the O concentration in the alloy.

First-principles characterization of the anisotropy of theoretical strength and the stress-strain relation for a TiAl intermetallic compound

We perform first-principles computational tensile and compressive tests (FPCTT and FPCCT) to investigate the intrinsic bonding and mechanical properties of a γ-TiAl intermetallic compound (L 1(0) structure) using a first-principles total energy method. We found that the stress-strain relations and the corresponding theoretical tensile strengths exhibit strong anisotropy in the [001], [100] and [110] crystalline directions, originating from the structural anisotropy of γ-TiAl. Thus, γ-TiAl is a representative intermetallic compound that includes three totally different stress-strain modes. We demonstrate that all the structure transitions in the FPCTT and FPCCT result from the breakage or formation of bonds, and this can be generalized to all the structural transitions. Furthermore, based on the calculations we qualitatively show that the Ti-Al bond should be stronger than the Ti-Ti bond in γ-TiAl. Our results provide a useful reference for understanding the intrinsic bonding and mechanical properties of γ-TiAl as a high-temperature structural material.

Use of quantitative convergent-beam electron diffraction in materials science

Methods for quantitative convergent-beam electron diffraction are outlined and some results of our applications of convergent-beam electron diffraction are shown, with emphasis on quantitative analysis of crystal structures in materials science. Examples of thickness measurements and determination of lattice parameters are presented. Measurements of low-order structure factors to obtain information on bonding charge-density distributions are reviewed, with examples from TiAl intermetallics. For non-centrosymmetric crystals, a method to determine three-phase structure invariants is given. Determination of polarity is also discussed.