In-situ transmission electron microscopy of microstructure formation during laser irradiation induced irreversible transformations in metals and alloys

非技术摘要本活动发展了对多组分金属材料在远离平衡条件下的凝固过程中微观组织形成的科学理解。将使用独特的电子显微镜结合微观结构的表征来研究二元和三元模型合金。凝固组织的局部特征与其形成条件的直接关联将提供其他方法所无法获得的实验数据集，这些方法适用于验证来自竞争的合金快速凝固理论模型的预测。这项研究加强了对复杂合金凝固过程中微观组织形成的科学理解，并有助于材料纳米级分辨研究技术的发展。了解工程材料加工过程中微观结构的形成是材料科学与工程(MSE)领域的基本挑战。它能够确定在技术应用中为实现最佳材料性能而进行性能调整的策略。在金属材料的制造中，凝固是普遍存在的，这对能源的产生和传输、先进的运输、生物医学和信息技术尤为重要。研究成果将通过期刊发表和在会议上发表。与教学资源和外展模块开发的整合将加强MSE本科生课程。将与学生团队合作开发有针对性的外展模块，并在匹兹堡大学保持改进的MSE外展努力。美国科学、技术、工程和数学劳动力的未来成员将获得研究培训、高级科学和工程教育、领导力和指导机会。与劳伦斯·利弗莫尔国家实验室团队的合作为项目资源和学术界以外的多方面专业准备提供了协同效应。这项活动将对工程教育产生积极的影响，促进终身学习，扩大未被充分代表的群体参与研究的范围，并促进关于金属合金在技术相关的非平衡条件下相变的科学知识。技术摘要凝固是材料制造中普遍存在的基本过程。在快速凝固过程中产生的极端条件下，固/液界面的迁移远离平衡，在决定最终组织的过程中，动力学因素可能成为决定最终组织的主导因素。我们使用电影模式动态电子显微镜(MM-DTEM)对合金薄膜的快速凝固转变进行了纳米尺度的时空分辨率的原位成像观测和衍射测量。结合原位研究和死后的定量微观表征，可以直接将凝固组织中的局部特征与其在远离平衡的快速凝固下的不可逆相变过程中的形成条件相关联。研究的重点是浓缩的二元Al-Cu和Al-Ag合金，对于三元Al-Cu-Ag合金，这项研究将提供关于相变界面的准确的全局和局部分辨信息，包括平均和局部速度，这些速度的变化，以及与晶体生长模式变化相关的形貌。对凝固微观结构的尸检分析提供了成分梯度、晶体结构以及组成相的局部排列、大小、形状和组成，这些可能不同于在平衡条件下或接近平衡条件下形成的相。使用薄膜合金样品可以研究未探索的成分区域(如Al-Cu中的过共晶)的快速凝固转变组织形成和非常大的转变速率，适合于解释向带状形态转变的细节和无分割合金晶体生长。以Al-Cu和Al-Ag合金Al-Cu-Ag三元系为研究对象，研究了原子尺寸失配、刻面倾向、化学有序性和界面粘性以及添加Ag对两相凝固组织形成的影响。建议的研究将提供独特的实验数据和见解，适用于评估现有的快速凝固模型，并将加强对多相合金系统凝固过程中微观组织形成的科学理解。

Non-Technical AbstractThis activity develops scientific understanding of the formation of microstructures in multi-component metallic materials during solidification under far-from-equilibrium conditions. Binary and ternary model alloys will be studied using unique electron microscopy in conjunction with characterization of the microstructures. Direct correlation of local features in the solidification microstructure with the conditions of their formation will deliver experimental data sets unobtainable with other approaches that are suitable for validation of predictions from competing theoretical models of alloy rapid solidification. The research enhances scientific understanding of microstructure formation during solidification in complex alloys and contributes to the development of techniques for nano-scale resolved studies of materials. Understanding of microstructure formation during processing of engineering materials is a fundamental challenge of the field of materials science and engineering (MSE). It enables identification of strategies for property tailoring for optimal material performance in a technological application. Solidification is ubiquitous in fabrication of metallic materials, which are particularly critical to energy generation and transmission, advanced transportation, biomedical and information technologies. Research results will be emanated by journal publication and presentations at conferences. Integration with instructional resources and outreach module development will enhance the MSE undergraduate curriculum. Modules for targeted outreach will be developed collaboratively with student teams and sustain improved MSE outreach efforts at the University of Pittsburgh. Future members of the US science, technology, engineering and mathematics workforce will receive research training, advanced science and engineering education, leadership and mentoring opportunity. Collaboration with the Lawrence Livermore National Laboratory team provides synergy for project resources and multi-faceted professional preparation outside of academia. The activity will positively impact engineering education, promote lifelong learning, broaden participation of underrepresented groups in research and advance the scientific knowledge of transformations in metallic alloys under technologically relevant non-equilibrium conditions.Technical Abstract Solidification is a ubiquitous and fundamental process in materials fabrication. Under extreme conditions arising in rapid solidification processing the migration of solid/liquid interfaces is driven far-away from equilibrium and kinetic factors can become dominant over thermodynamic factors in determining the final microstructure. We use the movie-mode dynamic transmission electron microscope (MM-DTEM) for nano-scale spatio-temporal resolution in situ imaging observations and diffraction measurements of rapid solidification transformations in alloy thin films. Complementing the in situ studies with quantitative post-mortem micro-characterization delivers direct correlation of local features in the solidification microstructure with the conditions of their formation during the irreversible transformation under-far-from-equilibrium rapid solidification. Focusing on concentrated binary Al-Cu and Al-Ag alloys, and for ternary Al-Cu-Ag alloys the research will deliver accurate global and locally resolved information on the transformation interface, including average and local velocity, changes in these velocities, and the morphology associated with changes in crystal growth modes. Post-mortem analyses of solidification microstructures provide compositional gradients, crystal structures, as well as the local arrangements, size, shape and composition of the constituent phases, which may differ from those that would form at or near equilibrium conditions. The use of thin film alloy specimens enables study of rapid solidification transformation microstructure formation for unexplored regimes of composition (e.g. hypereutectics in Al-Cu) and very large transformation rate, suitable to elucidate details of transitions to banded morphology and partitionless alloy crystal growth for instance. Effects of atomic size misfit, faceting tendencies, chemical ordering and interfacial coherency, as well as Ag addition effects on two-phase solidification microstructure formation will be determined using Al-Cu and Al-Ag alloys Al-Cu-Ag ternaries. The proposed research will deliver unique experimental data sets and insights suitable to evaluate current rapid solidification models and will enhance scientific understanding of microstructure formation in solidification of multi-phase alloy systems.