Grain Size Stability and Consolidation of Nanostructured Particulates

技术：由于加工限制，纳米晶体材料尚未实现其作为工程材料的潜力。挑战在于开发大规模的加工方法，可以生产无加工缺陷的大块纳米结构金属和合金。该研究的目的是开发策略，以稳定纳米级的微观结构，在高温下的粉末颗粒的固结。这是由以下事实推动的：可以按比例放大以大批量生产纳米级材料的加工方法通常从必须固结成块状形式的粉末颗粒开始。这些方法包括粉末的机械研磨，其制造具有内部纳米晶晶粒结构的大尺寸颗粒或产生纳米级粉末颗粒的化学反应。这些方法在生产各种合金和多相系统中具有很大的通用性。它们的主要缺点是需要将粉末固结成块体形式，达到理论密度和完全的颗粒间结合，而不会显著粗化纳米级微观结构。在这项研究中使用的实验方法强调了系统的研究晶粒生长和动力学和热力学因素，影响它在选定的金属合金制备的机械研磨。本论文选取了bcc结构的Fe和fcc结构的Ni两个模型体系进行研究。晶粒生长研究表明，这两种金属的纳米晶样品的行为根本不同，晶粒生长的激活能接近晶格扩散（Fe）或晶界扩散（Ni）。通过使用合金添加剂来降低晶界能（热力学基础）或通过钉扎（动力学基础）来限制晶界迁移率来探索稳定纳米晶微结构的可能性。将平衡可溶性和不混溶元素以及第二相氧化物分散添加剂添加到基底金属中，以研究它们稳定微观结构的能力的有效性。在晶粒生长研究的指导下，粉末的固结将通过烧结锻造进行。使用适合实验室规模样本大小的力学性能测试来研究固结过程的有效性。延展性和断裂面应显示加工缺陷的存在。进行分析和模拟建模，以确定和优化抑制晶粒生长的热力学和动力学机制。非技术性：拟议研究的教育影响包括参加北卡罗来纳州州立大学的凯南研究员课程和领导力发展计划。一名首席研究员担任当地K-12教师的导师，该教师提供为期两年的奖学金，他/她将开展研究并将最新的科学，工程和技术知识带入课堂。本科生将通过参加由NSF赞助的REU计划分享研究经验。

TECHNICAL: Nanocrystalline materials have yet to realize their potential as engineering materials because of processing limitations. The challenge is to develop large-scale processing methods that can produce bulk nanostructured metals and alloys free of processing defects. The objective of the research is to develop strategies to stabilize nanoscale microstructures during consolidation of powder particulates at elevated temperatures. This is motivated by the fact that processing methods which can be scaled up for large volume production of nanoscale materials typically start out with powder particulates that must be consolidated into bulk form. These methods include mechanical attrition of powders that make large-size particulates with an internal nanocrystalline grain structure or chemical reactions that produce nanoscale powder particles. These methods have great versatility in producing a variety of alloy and multiphase systems. Their major drawback is the need to consolidate the powders into bulk form, attaining theoretical density and complete interparticle bonding, without significantly coarsening the nanoscale microstructure. The experimental approach to be used in this research emphasizes a systematic study of grain growth and the kinetic and thermodynamic factors that influence it in selected metals alloy prepared by mechanical attrition. Two model systems based on bcc Fe and fcc Ni will be selected for the research. Grain growth studies have shown fundamentally different behavior for nanocrystalline samples of these two metals, with the activation energies for grain growth being close to either lattice diffusion (Fe) or grain boundary diffusion (Ni). The possibility to stabilize nanocrystalline microstructures is explored by using alloy additions to reduce the grain boundary energy (thermodynamic basis) or limit the grain boundary mobility by pinning (kinetic basis). Equilibrium soluble and immiscible elements and second-phase oxide dispersion additives are added to the base metals to study the effectiveness of their ability to stabilize the microstructure. Guided by the grain growth studies, consolidation of powders will be carried out by sinter forging. The effectiveness of the consolidation processes is investigated using mechanical property tests suited to laboratory-scale sample sizes. The ductility and fracture surfaces should reveal the presence of processing defects. Analysis and simulation modeling is conducted to identify and optimize the thermodynamic and kinetic mechanisms that inhibit grain growth. NONTECHNICAL: The educational impact of the proposed research includes participation in the Kenan Fellows for Curriculum and Leadership Development program at North Carolina State University. A principal investigator acts as a mentor for a local K-12 teacher who serves a two-year fellowship in which he/she will carry out research and bring up-to-date knowledge of science, engineering, and technology into the classroom. Undergraduate students will share in research experience through participation in the REU program sponsored by the NSF.