Electron Microscopy of Pulsed Laser Induced Rapid Solidification and Transient Solid State Phenomena in Nano-Scale Metal and Alloy Thin Films

技术概述：本项目是研究纯金属和富铝铝(铜)薄膜的快速凝固过程。主要的技术是使用劳伦斯利弗莫尔国家实验室(LLNL)的动态透射电子显微镜(DTEM)进行超快电子衍射(UED)和成像的原位电子显微镜(TEM)。目前还没有其他测量技术能够观测到界面速度在0.1~100m/S之间的金属薄膜的快速凝固过程，其时空分辨率达到了DTEM所要求的纳米级。这项研究将揭示铝和富铝铝(铜)薄膜以及其他面心立方、体心立方和六方金属在单次脉冲激光熔化后极快的液-固转变和其他瞬时现象(如固-固转变)的定量动力学细节。通过与纯金属原位DTEM实验的定量测量进行直接比较，将开发出用于脉冲激光熔化的稳健的建模软件代码。采用事后组织和原位显微组织表征的方法，研究了不同晶体结构的纯金属凝固后固态组织的演变和缺陷的形成。富铝Al(Cu)薄膜的研究将有助于确定生长界面形态失稳的条件，它是脉冲激光功率、合金成分以及Al和Al2Cu之间的亚共晶、共晶和过共晶成分的热提取参数的函数。在决定相变前沿稳定性或不稳定性的参数上获得的数据将使理论凝固模型得以验证。除了在激光诱导快速凝固的极端条件下提供金属的基本科学知识外，还将开发最先进的原位透射电子显微镜实验技术，目的是在接近计算模型可达到的时间尺度上探索材料体积的行为。非技术摘要：凝固是材料制造中普遍存在的基本过程，特别是对金属来说，这对能源的产生和传输、运输和信息技术至关重要。在激光加工的极端条件下，生长动力学决定了工程部件和器件的最终微结构，从而决定了与性能相关的性能。理解这些现象在科学上很有趣，在技术上也很重要。这项研究的结果将通过同行评议的出版物和演示文稿进行传播。该项目包括通过宾夕法尼亚州初级科学学院向高中生推广，并为新成立的匹兹堡科学与技术学院(6-12年级)的学生开发长达一学期的研究经验。学生将接受真空和激光科学、物理冶金、薄膜科学、微制造方法、散射和衍射物理、透射和扫描电子显微镜、结晶学、热力学、传输现象和材料建模的数值方法方面的培训。对LLNL的访问和与LLNL的持续互动将提供学术环境之外的专业准备和获得最先进的仪器设备。

TECHNICAL SUMMARY: This project is an investigation of rapid solidification processes in pure metal and Al-rich Al(Cu) thin films. The main technique is in-situ transmission electron microscopy (TEM) using the dynamic TEM (DTEM) at Lawrence Livermore National Laboratory (LLNL) for ultrafast electron diffraction (UED) and imaging. There are no other measurement techniques currently capable of observing rapid solidification processes with interfacial velocities ranging from about 0.1 to 100 m/s in metal thin films with the required nano-scale spatial and temporal resolution offered by the DTEM. The research will reveal quantitative dynamic details of the extremely rapid liquid-solid transformation and other transient phenomena (e.g. solid-solid transitions) associated with Al and Al-rich Al(Cu) thin films and other FCC, BCC and HCP metals after single-shot pulsed laser melting. Robust modeling software codes, validated by direct comparison with quantitative measurements from in-situ DTEM experiments on pure metals, will be developed for the pulsed-laser melting. Post-mortem and in-situ microstructural characterization will be used to investigate microstructural evolution and defect formation in the solid state following solidification of pure metals with different crystal structures. The study of Al-rich Al(Cu) thin films is expected to help identify the conditions of morphological destabilization of the growth interface as function of pulsed laser power, alloy composition and heat-extraction parameters for hypo-eutectic, eutectic and hypereutectic compositions between Al and Al2Cu. The data obtained on parameters that determine the stability or instability of the transformation front will enable validation of theoretical solidification models. Apart from delivering basic scientific knowledge on metals under the extreme conditions of laser-induced rapid solidification, state-of-the-art experimental techniques for in-situ TEM will be developed for the purpose of probing the behavior of material volumes at time scales close to those accessible by computational modeling.NON-TECHNICAL SUMMARY: Solidification is a ubiquitous and fundamental process in materials fabrication, especially for metals, which are critical to energy generation and transmission, transportation and information technologies. Under extreme conditions of laser processing the growth dynamics determine the final microstructure and consequently performance-related properties in engineered components and devices. Understanding such phenomena is scientifically interesting and technologically important. The results of this research will be disseminated by peer-reviewed publications and presentations. The project involves outreach to high school students, via the Pennsylvania Junior Academy of Science and the development of semester-long research experiences for students enrolled in the new Pittsburgh Science & Technology Academy (grades 6-12). Students will receive training in vacuum and laser science, physical metallurgy, thin film science, micro-fabrication methods, scattering and diffraction physics, transmission and scanning electron microscopy, crystallography, thermodynamics, transport phenomena and numerical methods for materials modeling. Visits to, and continuous interaction with, LLNL will provide professional preparation outside the academic environment and access to state-of-the-art instrumentation.