Novel Metastable Phases and Kinetics Studies in Transition Metals and Alloys under Terapascal Pressures

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Non-technical AbstractSeveral elements in the periodic table when subjected to external high-pressure and high-temperature adapt novel structural modifications leading to improvements in their physical and mechanical properties. In certain cases, these novel crystal modifications with enhanced physical and mechanical properties can be retained after removal of pressure resulting in potential industrial applications. The conversion of graphitic carbon into diamond by application of high-pressure and high-temperature is a prime example of this research. This research program will carry out high-pressure high-temperature studies on transition metals and alloys that are widely used in aerospace, biomedical, and nuclear industry. This research is enabled by advances in the generation of ultra high pressure up to ten million atmospheres in the laboratory using an innovative design of diamond anvils in high pressure devices. This advance in high pressure technique coupled with very bright sources of x-rays allows us to probe changes in crystal structures so we can construct stability maps for various crystal structures at elevated pressures and temperatures. These stability maps will allow researchers to design new strategies to synthesize these materials in bulk form for industrial applications. The project participants will receive extensive research training at the premier national facilities employing x-ray synchrotron radiation in materials research leading to a pipeline of trained scientific workforce in high pressure science. Our long-term partnership with the Historically Black Colleges and Universities in the southeastern region will ensure participation of underrepresented minority groups in this research program. Technical AbstractThe study of transition metals under extreme conditions has seen rapid progress in the last few years with the attainment of near Terapascal (1 TPa = 1000 GPa) static pressures using two-stage diamond micro-anvils along with the development of millisecond x-ray diffraction techniques using fast detectors at the synchrotron facilities. At the University of Alabama at Birmingham, a novel method for the fabrication of two-stage diamond micro-anvils using a chemical vapor deposition technique has been developed. In addition, rapid pressure and temperature changes can be applied to samples using piezoelectric drivers (dynamic diamond anvil cells) and boron-doped heating anvils, thus providing ideal tools to study phase transformations and transformation kinetics in transition metals and alloys under extreme conditions. In this study, a focus is on transformation kinetics of hexagonal close-packed (hcp-phase) to simple hexagonal (omega-phase) transformation and from omega-phase to body-centered cubic (bcc-phase) in Titanium-Vanadium (Ti-V) and Hafnium-Tantalum (Hf-Ta) alloys. The goal is to establish the occurrence of other novel phases like the orthorhombic modifications (gamma, delta, and eta phases) in early transition metals and alloys under hydrostatic (neon pressure medium) and non-hydrostatic (no pressure medium) conditions. In Osmium (Os) metal, structural anomalies have been observed at 150 GPa and 440 GPa at ambient temperature that warrant further investigations at elevated temperatures. It is anticipated that most of the early members of the transition metal series are converted in to nearly pure d-band metals as s-electron levels move above the Fermi-level at extreme pressures. This study would provide crystal structure data and pressure-volume data for this unique electronic state of transition metals and alloys under extreme conditions for direct comparison with the theoretical models.