Grain Size Stability and Consolidation of Nanostructured Particulates

技术：由于工艺的限制，纳米晶材料尚未实现其作为工程材料的潜力。挑战是开发能够生产无加工缺陷的块状纳米结构金属和合金的大规模加工方法。这项研究的目的是开发在高温下粉末颗粒固结过程中稳定纳米级微结构的策略。这是因为，可以扩大规模以大量生产纳米材料的加工方法通常是从粉末颗粒开始的，这些粉末颗粒必须被固化成块状。这些方法包括机械磨损产生内部具有纳米晶结构的大尺寸颗粒的粉末，或者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方面具有很大的通用性。它们的主要缺点是需要将粉末固化成块状，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的微结构。本研究采用的实验方法着重于系统地研究机械研磨制备的精选金属合金中的晶粒长大及其影响的动力学和热力学因素。将选择两个基于体心立方Fe和体心立方Ni的模型体系进行研究。晶界扩散(Ni)和晶格扩散(Fe)的激活能接近于晶格扩散(Fe)或晶界扩散(Ni)。探索了通过添加合金来降低晶界能(热力学基础)或通过钉扎限制晶界迁移率(动力学基础)来稳定纳米晶组织的可能性。在母材中加入平衡的可溶元素和不混溶元素以及第二相氧化物弥散添加剂，以研究它们稳定组织的能力的有效性。在晶粒长大研究的指导下，粉末的固结将通过烧结锻造进行。固结过程的有效性是通过与实验室规模的样品大小相适应的力学性能测试来研究的。延性和断口应显示加工缺陷的存在。进行了分析和模拟建模，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立大学的凯南课程和领导力发展研究员项目。首席调查员担任当地K-12教师的导师，该教师提供为期两年的奖学金，在该奖学金中，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。本科生将通过参与由NSF赞助的REU项目来分享研究经验。

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.