TECHNICAL: This project is focused on the microstructural effects on phase transformations in transition and rare earth metals under high pressures. In semiconductors and transition metal oxides, an increase in transformation pressure with a decrease in crystalline grain size has been well documented. However, similar systematic studies on metallic systems under high pressures are lacking. In transition metals, PI will study the effect of varying grain size (50 nm to 1000 nm) on the hcp to omega and omega to bcc phase transitions in titanium, zirconium, and hafnium at high pressures. PI will also investigate additional orthorhombic and monoclinic phases that have been reported in pure titanium under high pressures. In rare earth metals, PI will investigate the effect of varying grain size on the regular rare earth structures and on the phase transformation to the alpha-U phase under high pressures. The focus will be on rare earth metals cerium and praseodymium that show "crystal grain growth" phenomenon under high pressures in the alpha-U phase and where the phase transformation sequence is strongly influenced by microstructural effects. PI will establish the microstructural effects on the equilibrium transformation pressure as well as transformation kinetics by performing time dependent x-ray diffraction and electrical resistance measurements under high pressures and high temperatures at a synchrotron source. All experimental studies will be carried out using high purity samples and under controlled shear stresses in a quasi-hydrostatic pressure medium. The phase transitions will be monitored by simultaneous image plate x-ray diffraction technique as well as four probe electrical resistance measurements under high pressures using designer diamond anvils. This research project is strongly complemented by the recent advances in the fabrication of designer diamonds and their applications in electrical measurements and ohmic heating at high pressures. NON-TECHNICAL: The experimental studies on the role of microstructure in controlling the equilibrium phase boundary and kinetics in high-pressure transformations will lead to fundamental understanding of high-pressure phase diagrams of transition and rare earth metals. This increased understanding of microstructural effects in high-pressure phase transformations is likely to yield novel metastable phases in pressure-temperature treated samples with enhanced physical properties. High pressure materials research facilities supported by this grant will be employed in the Research Experiences for Undergraduates (REU) projects during the summer period with active participation of underrepresented minority undergraduate students. In the last six years, five PhD graduate students trained in PI's high pressure research group have accepted positions at national laboratories, industry, and academic institutions. PI's research lab continues to serve as an important source of trained US workforce in high-pressure science and metals research. A special emphasis on recruiting students from underrepresented minority groups in the sciences and engineering has been highly successful as 41% of REU participants on UAB campus have been minorities. The undergraduate and graduate students in this project will publish their findings in peer-judged journals leading to a broad dissemination of new knowledge generated in this NSF supported research.
