Ab-Initio Calculation of Mechanically Unstable High-Temperature Phases

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Title: Ab-Initio Calculation of Mechanically Unstable High-Temperature Phases
Creators: Antolin, Nikolas
Advisor: Windl, Wolfgang
Issue Date: 2011-06
Abstract: Ab-initio calculations have been hugely successful in the past decade in providing the necessary free energies to predict phase transformations and phase stability for elemental and alloyed materials. The application of ab-initio methods, however, is problematic when it comes to model metals and their alloys with mechanically unstable high-temperature phases, such as the bcc high-temperature phases of Ti, Zr, Hf, Sc, or U. Combining novel large-displacement phonon methods with strain calculations and a self-consistent phonon approach, the free energies and elastic constants of high-temperature phases of several transition and rare earth metals were calculated. These data were then used to calculate coefficients of thermal expansion and transition temperatures for a few model systems. With a few exceptions, calculated data agreed well with that obtained through experimental methods. Calculated elastic constants met the stability criterion for the cubic crystal system. Coefficients of thermal expansion matched experimental values where they existed, and calculated phase transition temperatures were within 100K of their experimental values. For future development of this method, research into a determination of the appropriate large displacement is necessary; without such a determination, separating data that matches experimental values from that which results from computational intricacies is not possible, and the methodology cannot be applied to unexplored material systems with any confidence.
Embargo: No embargo
Series/Report no.: The Ohio State University. Department of Materials Science and Engineering Honors Theses; 2011
Keywords: computation ab-initio DFT phase high-temperature
Sponsors: OSU Center for Emergent Materials
NSF grant 0606417
OSU Lumley Engineering Fund
Ohio Supercomputer Center
URI: http://hdl.handle.net/1811/48772
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