Session M05: Simulations and phase transitions I

Tensorial stress-plastic strain fields in α - ω Zr mixture, transformation kinetics, and friction in diamond anvil cell

Various phenomena (phase transformations, chemical reactions, microstructure evolution, strength, and friction) under high pressures in diamond anvil cell are strongly affected by fields of all components of stress and plastic strain tensors. However, they could not be measured. Even measured pressure distribution contains significant error because axial strain does not contribute to x-ray patterns, and the equation of state determined under hydrostatic conditions is used for nonhydrostatic loading. We suggest coupled experimental-analytical and experimental-analytical-computational approaches utilizing synchrotron x-ray diffraction to solve an inverse problem and find fields of all components of stress and plastic strain tensors in each phase and mixture and friction rules before, during, and after α-ω phase transformation in strongly plastically predeformed Zr. The results of both approaches are in good correspondence with each other and experiments. Due to advanced characterization, the minimum pressure for the strain-induced α-ω phase transformation is changed from 1.36 to 2.7 GPa. It is independent of the plastic strain before phase transformations and compression-shear path. The theoretically predicted plastic strain-controlled kinetic equation is verified and quantified. Obtained results open opportunities for developing quantitative high-pressure/stress science, including mechanochemistry, synthesis of new nanostructured materials, geophysics, and tribology.   

High-Pressure, High-Temperature Single-Crystal X-ray Diffraction Measurements of Energetic Materials

Understanding the properties (e.g., equation of state, EOS) of unreacted energetic materials at high-pressure and temperature (HPHT) conditions plays an important role in understanding their behavior under shock conditions. Specifically, determining the HPHT EOS and crystal structure of these materials is important due to the potential existence of HPHT polymorphs, pressure-induced amorphization, and the need of high-fidelity EOS parameters for the unreacted materials for continuum based simulations. Studies measuring the room-temperature EOS of energetic materials are numerous, but few studies probe the unreacted EOS at HPHT conditions. Additionally, due to the low symmetry phases of some of these materials, single-crystal X-ray diffraction (SC-XRD) is required to provide EOS parameters. As such, we have performed SC-XRD in a resistively heated DAC up to 300 °C and 30 GPa to determine the HPHT EOS of these materials.   

High Entropy Borides under Extreme of Pressure and Temperatures

High-entropy materials represent a paradigm shift in materials science where five or more constituent elements are incorporated in a variety of alloys, oxides, nitrides, and borides. The high-entropy materials are thermodynamically stable at high temperatures and provide tunability in physical and mechanical properties that is only possible in compositionally complex systems. We have synthesized a series of high-entropy transition metal borides, e.g., (HfMoNbTaZr)B 10 at high-pressures and high-temperatures starting from a ball-milled metal oxide precursors and boron powder. A single hexagonal AlB 2 -type phase of (HfMoNbTaZr)B 10 has been synthesized and studied to 10 GPa and 2273 K in a Paris-Edinburgh press. The synthesized materials are recovered and studied in a diamond anvil cell by both axial and radial diffraction techniques. The hexagonal AlB 2 -type phase of (HfMoNbTaZr)B 10 is stable to 220 GPa pressure (30% compressions). The hardness, shear strength, and thermal oxidation resistance data will be presented for use of these materials in hypersonic applications.