Microstructural Effects on Phase Transformations in Metals at High Pressures

技术：该项目的重点是在高压下过渡和稀土金属相变的微观结构的影响。在半导体和过渡金属氧化物中，相变压力的增加伴随着晶粒尺寸的减小已经被充分证明。然而，类似的高压下的金属系统的系统研究是缺乏的。在过渡金属中，PI将研究不同晶粒尺寸（50 nm至1000 nm）对钛，锆和铪在高压下hcp至omega和omega至bcc相变的影响。PI还将研究在高压下纯钛中报道的其他斜方晶和单斜晶相。在稀土金属中，PI将研究不同粒度对规则稀土结构和高压下向α-U相的相变的影响。重点将放在稀土金属铈和镨，显示“晶粒生长”的现象，在高压下的α-U相和相变序列的强烈影响的微观结构效应。PI将通过在同步加速器源的高压和高温下进行随时间变化的X射线衍射和电阻测量，建立对平衡相变压力和相变动力学的微观结构影响。所有的实验研究都将使用高纯度样品，并在准流体静压介质中的受控剪切应力下进行。相变将通过同时成像板X射线衍射技术以及使用设计师金刚石砧在高压下的四探针电阻测量来监测。该研究项目得到了最近在设计师钻石制造及其在高压下的电气测量和欧姆加热应用方面的进展的有力补充。非技术性：对高压相变中微观结构在控制平衡相界和动力学中的作用的实验研究将导致对过渡金属和稀土金属高压相图的基本理解。这增加了对高压相变中微观结构效应的理解，可能会在压力-温度处理的样品中产生新的亚稳相，具有增强的物理性能。由该补助金支持的高压材料研究设施将在夏季期间在代表性不足的少数民族本科生的积极参与下，用于本科生（REU）项目的研究经验。在过去的六年里，五个博士研究生在PI的高压研究小组接受了国家实验室，工业和学术机构的职位。PI的研究实验室仍然是美国高压科学和金属研究领域训练有素的劳动力的重要来源。特别强调从科学和工程领域代表性不足的少数群体中招收学生，这一点非常成功，因为UAB校园中41%的REU参与者是少数民族。该项目的本科生和研究生将在同行评审的期刊上发表他们的研究结果，从而广泛传播NSF支持的研究中产生的新知识。

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.