Structural Energy Differences in Al₃Ti: The Role of Tetragonal Distortion in APB and Twin Energies

Abstract

At stoichiometry, DO₂₂ is the observed ground state of Al₂Ti and has a C/A ratio of 2.23, but as a function of both concentration and temperature other ordered arrangements of APB’s are observed. These phase transitions have sparked many studies in which the energy has been treated as that of chemical rearrangement on an fcc lattice. We have found that at the ideal C/A ratio, the Ll₂ structure is lower in energy, but as the tetragonal distortion increases the DO₂₂ energy drops below that of Ll₂. The critical role played by the tetragonal distortion in the balance between Ll₂ and DO₂₂ energies precludes the use of any model based on the undistorted lattice.

The major impediment to the development of Al₃Ti as a high-temperature material is its lack of ductility. The standard approach is to make alloy additions which transform the structure to Ll₂. An alternate approach is to work toward the enhancement of ductility in the DO₂₂ phase. As a first step we have calculated the twinning energy in Al₃Ti.