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Strain-Induced Phase Transformations in Ceramics under High Pressure

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Iowa State University of Science and Technology Ames United States

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The objectives included theoretical, computational, and experimental investigations of phase transformations PTs in ceramics under high pressure and large plastic shear. Main results Algorithm for contact sliding between diamond, gasket, and sample is developed. Strain-induced PTs under compression, compression and torsion, unloading, and reloading in a rotational diamond anvil cell RDAC are studied in detail. Various experimental phenomena are reproduced. Possible misinterpretations of experiments are demonstrated. Ways to extract kinetic information from heterogeneous experimental fields are suggested. New mechanism of plastic deformation and stress relaxation at high strain-rates 109-1012 s-1 via virtual melting 4000 K below the melting temperature is predicted using new thermodynamic theory and confirmed by molecular dynamic simulations. PT from disordered nanocrystalline hexagonal hBN to superhard wurtzitic wBN was found at 6.7 GPa under plastic shear in RDAC. Under hydrostatic compression to 52.8GPa, hBN did not transform. Developments in the phase field approacha General theory for multivariant martensitic PTs including interface stresses and anisotropic interface energy, dislocations, and interaction of dislocations and PTs and explicit models are formulated at large strains. b Finite-element approach is developed and numerous problems are solved, including interaction between PT and plasticity under compression and shear in nanograin material.

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Technical Report,20 Aug 2012,19 Aug 2016



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Approved For Public Release;

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