Son, Woongrak (2018-07). First Principles Investigation of Structural, Mechanical and Thermodynamic Properties of MAX Phases. Doctoral Dissertation.
Thesis
MAX phases are layered carbides or nitrides with general formula of Mvn+1AXvn. In this work we present a first principles investigation of structural, mechanical, and thermodynamic properties of MAX phases. Up to date approximately 70 pure MAX phases are synthesized and characterized. But the possible number of MAX phases is large when we consider different chemicals in M, A, and X sublattices as well as the possible stacking numbers, n. First, we studied Tiv3AlCv2, Tvi3SiCv2, and their solid solutions to understand the composition-properties relationship. Among the pure MAX phases, the Al-containing MAX phases are some of the most important as they are considered to be promising high-temperature applicable materials. They are known to form continuous alumina layer when exposed to high temperature oxidizing environment, and have excellent oxidation properties. While their overall strength is low compared to other MAX phases. In contrast, the Si-containing MAX phases have excellent mechanical properties. Finally solid solution MAX phases offer the opportunity to tune the thermodynamic, and mechanical properties of MAX phases. Solid solution MAX phases were modeled using special quasirandom structures (SQS), and calculated thermodynamic and mechanical properties using Density Functional Theory (DFT), which is implemented in the Vienna Ab initio Simulation Package (VASP). Second, we studied Tivn+1AlCvn and Tavn+1AlCvn systems to understand structure-properties relationship, and to address the effect of stacking layers, and the effect of different M chemicals on deformation behavior. Since, many MAX phases with n = 1-3 have studied, but higher order MAX phases have not been studied in detail. Third, we studied the cleavage and shear behavior of TiC, Tiv2AlC, Ti, and graphite to understand what is MAX phases in terms of the deformation behavior. MAX phases have a unique combination of properties, which are both of metals and ceramics, since MAX phases have ceramic like MX layers and metal like A layers. By comparing deformation behaviors of different types of layers materials, we studied whether the deformation behavior of MAX phases is similar to ceramics or metals. Lastly, we studied structural and elastic properties of (Mv1Mv2)AlC systems, and the deformation behaviors of M2AlC systems. The critical stress and USFE ofM2AlC have a good trend in the periodic table, and the analysis suggest that someM2AlC MAX phases have stable or metastable state in the sheared structure.
