Microstructural Effects on Phase Transformations in Metals at High Pressures

技术：本项目重点研究高压下稀土金属和过渡金属相变的微观结构效应。在半导体和过渡金属氧化物中，随着晶粒度的减小，相变压力的增加已经被很好地记录下来。然而，对高压下金属体系的类似系统研究还很缺乏。在过渡金属方面，PI将研究不同的颗粒尺寸(50 nm到1000 nm)对高压下钛、锆和Hf的hcp到omega和omega到bcc相变的影响。PI还将研究在高压下已报道的纯钛中的其他正交相和单斜相。在稀土金属中，PI将研究不同的晶粒度对规则稀土结构的影响以及高压下向α-U相的相变。重点将放在稀土金属Ce和Pr上，它们在α-U相中表现出高压下的“晶粒生长”现象，其相变序列受到微结构效应的强烈影响。PI将通过在同步加速器源上进行高压和高温下随时间变化的X射线衍射和电阻测量来建立对平衡相变压力和相变动力学的微观结构效应。所有的实验研究都将使用高纯度的样品，并在准静水压力介质中控制剪应力。相变将通过同时成像平板X射线衍射技术以及使用设计的金刚石锤在高压下进行四探头电阻测量来监测。对这一研究项目的有力补充是在人造钻石的制造及其在高压下的电测量和欧姆加热方面的应用的最新进展。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有助于对过渡金属和稀土金属的高压相图有基本的了解。这增加了对高压相变中微观结构效应的理解，很可能在压力-温度处理的样品中产生具有增强物理性能的新的亚稳相。这笔助学金支持的高压材料研究设施将在暑期期间被用于本科生研究体验(REU)项目，未被充分代表的少数族裔本科生将积极参与。在过去的六年里，在派的高压研究组接受培训的五名博士研究生接受了国家实验室、工业和学术机构的职位。派的研究实验室继续作为高压科学和金属研究方面训练有素的美国劳动力的重要来源。特别重视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参与者是少数族裔。该项目中的本科生和研究生将在同行评议的期刊上发表他们的发现，从而使这项由NSF支持的研究产生的新知识得到广泛传播。

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.