Structural Energy Differences in Al 3 Ti: The Role of Tetragonal Distortion in APB and Twin Energies
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STRUCTURAL ENERGY DIFFERENCES IN A13Ti: IN APB AND TWIN ENERGIES
THE ROLE OF TETRAGONAL DISTORTION
0. M. NICHOLSON,* G. M. STOCKS,* W. M. TEMMERMANJ P. STERNE,ยง D. G. PETTIFOR' *Oak Ridge National Laboratory, P. 0. Box 2008, Oak Ridge, TN 37831-6114 tDaresbury Laboratory, SERC, Daresbury, Warrington WA4 4AD, England lUniversity of Maryland, College Park, MD20742 #Imperial College, Queen's Gate, London SW7 2BZ, England
ABSTRACT At stoichiometry, D022 is the observed ground state of Al2 Ti 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 L12 structure is lower in energy, but as the tetragonal distortion increases the D022 energy drops below that of L12 . The critical role played by the tetragonal distortion in the balance between L12 and D022 energies precludes the use of any model based on the undistorted lattice. The major impediment to the development of A13 Ti as a high-temperature material is its lack of ductility. The standard approach is to make alloy additions which transform the structure to L12 . An alternate approach is to work toward the enhancement of ductility in the D02 2 phase. As a first step we have calculated the twinning energy in A13Ti.
INTRODUCTION Al3Ti is a lightweight, high-melting point, oxidation resistant intermetallic compound of the D022 structure. The major impediment to its development as a high-temperature material is its lack of ductility. The standard approach is to make alloy additions which transform the structure to L12 . L12 alloys typically have reasonable ductility which is attributable to the slip modes available in cubic structures. The linear muffin-tin orbital (LMTO) method is used to investigate the role of tetragonal distortion in stabilizing Al3 Ti in the D022 structure. There are implications for current models of phase transitions in A12.xTii+x. Near the 3:1 stoichiometric compound Al3 Ti, the phase diagram is complicated by the competition between different ordered arrangements of antiphase boundaries' (APB). There have been many attempts to explain this behavior in terms of chemical effects on an fcc lattice. 2 Using the LMTO, it has been shown that the experimentally observed ground state, which is D02 2, can only lower itself below the L12 structure as a result of considerable tetragonal distortion. Thus, earlier attempts to explain the free energy of Al3Ti in terms of chemical interchange on an fcc lattice are doomed to failure because they have neglected the critical element, the tetragonal distortion. Al3Ti can be stabilized in L12 by the addition of Fe, Ni, or Cu. However, it remains brittle; the failure mode is cleavage along (110) planes. We are therefore motivated to attempt to enhance ductility while remaining in the DO22 phase. The primary deformation mode in D022 is twinning. As a f
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