EAGER: IMPRESS-U: Gradient surface nanostructuring with short laser pulses

工业对高强度、高硬度、高延展性和抗疲劳性能的高性能材料的需求迅速增长，这推动了所谓的“纳米结构材料”的发展，在这种材料中，高密度的晶体缺陷（晶界、孪晶、层错和位错）可以显著提高机械性能。然而，纳米结构材料的利用受到其对晶粒粗化和缺陷湮灭的热稳定性差，以及现有合成技术的规模扩大和与现代制造工艺集成的困难的限制。这项早期概念探索性研究（EAGER）国际多边合作伙伴关系乌克兰弹性教育和科学系统（IMPRESS-U）奖支持探索一种新的非接触式表面纳米结构方法，该方法基于通过短脉冲激光加工同时产生高密度晶体缺陷，并通过激光辅助纳米合金化稳定这些缺陷。该项目的目标是为灵活的激光辅助缺陷工程和稳定的可行性提供概念证明，推动激光加工技术的前沿。该项目得益于来自美国、立陶宛和乌克兰的国际研究小组成员之间的专业知识互补和现有的研究联系。该项目的主要目标之一是建立一种长期可持续的合作，使乌克兰研究人员充分融入全球研究界。互访、研究专业知识交流和教育活动为利沃夫国立理工大学研究实力的新领域的出现创造了肥沃的土壤。特别是，利沃夫理工学院新成立的研究中心采用了立陶宛维尔纽斯成熟的激光技术中心的最佳做法。利用弗吉尼亚大学的计算专业知识，乌克兰研究中心对短脉冲激光加工中潜在材料修改过程的坚实基础理解进行了调查。揭示和解开激光纳米结构中产生各种晶体缺陷的相互交织的过程的挑战，在本项目中通过结合激光诱导结构和相变的大规模原子建模，先进的大面积非烧蚀激光加工，使用新型纳米合金化装置，以及激光修饰表面的纳米级表征来解决。通过纳米合金化来稳定激光产生的高度非平衡缺陷结构是通过添加优先向晶界和其他缺陷偏析的合金元素来实现的，从而降低了纳米晶结构的自由能，并成为缺陷迁移的障碍。在原子模拟中系统地研究了在快速推进的结晶前沿产生晶体缺陷及其通过纳米合金化稳定的基本机制。将模拟条件映射到激光加工的实际情况，并在短脉冲激光辐照修饰的表面区域的详细实验表征中验证了计算预测。将计算预测直接映射到激光修饰表面的纳米级表征结果，指导了对激光加工参数的多维空间的探索，并使模型假设的验证和改进成为可能。探讨了几种扩大导致纳米晶化的辐照条件范围的策略，包括通过在液体环境和透明固体覆盖层的约束下进行激光加工来抑制亚表面空化和散裂。在模拟和实验中系统研究了合金/成分梯度对不同靶型凝固动力学和最终纳米结构的影响。EAGER: IMPRESS-U项目由美国国家科学基金会、立陶宛研究委员会、美国国家科学院和美国海军全球研究办公室（DoD）联合资助。该合作伙伴关系项目的美国部分由NSF国际科学与工程办公室（主任办公室）、CMMI的先进制造计划（工程理事会）和DMR的金属和金属纳米结构计划（数学和物理科学理事会）提供支持。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

The rapidly growing industrial demand for high-performance materials exhibiting a combination of high strength and hardness, substantial ductility, and fatigue resistance has been driving the development of so called “nanostructured materials,” where a high density of crystal defects (grain boundaries, twins, stacking faults and dislocations) can lead to a striking enhancement of the mechanical properties. The utilization of nanostructured materials, however, is limited by their poor thermal stability against grain coarsening and defect annihilation, as well as difficulties of the scale-up of existing synthesis techniques and their integration into modern manufacturing processes. This EArly-concept Grant for Exploratory Research (EAGER) International Multilateral Partnerships for Resilient Education and Science System in Ukraine (IMPRESS-U) award supports the exploration of a novel non-contact approach to surface nanostructuring based on a simultaneous generation of a high density of crystal defects by short pulse laser processing and stabilization of these defects through the laser-assisted nanoalloying. The goal of this project is to provide a proof of concept for the feasibility of a nimble laser-assisted defect engineering and stabilization, pushing forward the frontiers of the laser processing technologies. The project benefits from the complementary expertise and existing research links between the members of the international research team from the United States, Lithuania, and Ukraine. One of the major goals of the project is to establish a long-term sustainable collaboration fully integrating Ukrainian researchers into the global research community. Mutual visits, exchange of research expertise, and educational activities create a fertile ground for the emergence of a new area of research strength at the Lviv Polytechnic National University. In particular, the best practices of a well-established Center of Laser Technologies in Vilnius, Lithuania are adopted by the research center emerging at Lviv Polytechnic. Leveraging computational expertise at the University of Virginia grounds investigations at the Ukrainian research center on solid fundamental understanding of processes underlying material modifications from short pulse laser processing.The challenge of revealing and untangling the intertwined processes responsible for the generation of various crystal defects in laser nanostructuring is addressed in this project by combining large-scale atomistic modeling of laser-induced structural and phase transformations, advanced large-area non-ablative laser processing using a novel setup for nanoalloying, and nanoscale characterization of the laser-modified surfaces. The stabilization of laser-generated highly nonequilibrium defect structures through nanoalloying is achieved by adding alloying elements that preferentially segregate to grain boundaries and other defects, thus reducing the free energy of nanocrystalline structures and acting as obstacles for defect migration. The fundamental mechanisms responsible for the generation of crystal defects at a rapidly advancing crystallization front and their stabilization through nanoalloying is systematically investigated in atomistic simulations. The conditions of the simulations are mapped to those realized in laser processing, and the computational predictions are verified in a detailed experimental characterization of surface regions modified by short pulse laser irradiation. The direct mapping of the computational predictions to the results of nanoscale characterization of laser modified surfaces guides the exploration of the multidimensional space of laser processing parameters and enables the verification and refinement of the model assumptions. Several strategies for expanding the range of irradiation conditions leading to nanocrystallization are explored, including suppression of subsurface cavitation and spallation by performing laser processing in a liquid environment and under confinement by a transparent solid overlayer. The effect of alloying/compositional gradients on the solidification kinetics and final nanostructure are systematically investigated for different target configurations in simulations and experiments.This EAGER: IMPRESS-U project is jointly funded by NSF, Research Council of Lithuania, US National Academies of Sciences, and Office of Naval Research Global (DoD). US portion of this collaborative partnership project is supported by NSF Office of International Science and Engineering (Office of the Director), CMMI’s Advanced Manufacturing Program (Engineering Directorate), and DMR’s Metals and Metallic Nanostructures Program (Directorate for Mathematical and Physical Sciences).This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.