Microstructural Effects on Phase Transformations in Metals at High Pressures

技术：本项目主要研究高压下相变和稀土金属的微观组织效应。在半导体和过渡金属氧化物中，随着晶粒尺寸的减小，转变压力的增加已经得到了很好的证明。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将研究不同晶粒尺寸（50 nm到1000 nm）对钛、锆和铪在高压下hcp到omega和omega到bcc相变的影响。PI还将研究在高压下纯钛中报道的其他正交和单斜相。在稀土金属中，PI将研究不同晶粒尺寸对规则稀土结构和高压下α - u相转变的影响。重点将放在稀土金属铈和镨，在α - u相的高压下显示出“晶粒生长”现象，其中相变顺序受微观结构效应的强烈影响。PI将通过在同步加速器源的高压和高温下进行随时间的x射线衍射和电阻测量，建立微观结构对平衡转变压力和转变动力学的影响。所有实验研究将使用高纯度样品，并在准静流体压力介质中控制剪切应力。相变将通过同时成像板x射线衍射技术以及在高压下使用设计的金刚石砧进行四个探头电阻测量来监测。该研究项目得到了最近在设计钻石制造方面的进展及其在高压电测量和欧姆加热方面的应用的有力补充。非技术：对高压转变中微观结构在控制平衡相边界和动力学中的作用的实验研究将导致对转变和稀土金属高压相图的基本理解。这种对高压相变中微观结构效应的理解的增加，可能会在经过压力-温度处理的样品中产生具有增强物理性质的新型亚稳相。本基金资助的高压材料研究设施将在夏季期间用于本科生研究经验（REU）项目，并由未被充分代表的少数民族本科生积极参与。在过去的六年中，PI高压研究小组培养的五名博士研究生在国家实验室，工业和学术机构任职。PI的研究实验室继续作为高压科学和金属研究训练有素的美国劳动力的重要来源。在科学和工程领域，特别强调从代表性不足的少数群体中招收学生的做法非常成功，因为UAB校园中41%的REU参与者是少数群体。该项目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，从而广泛传播这项由美国国家科学基金会支持的研究所产生的新知识。

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.