Temporal Evolution of Microstructures on a Nanoscale: Experiments and Simulations

该基金从实验和模拟的角度探讨了纳米尺度上多组分合金中分解微观结构的时间演变的基本问题。对分解过程的基本理解，包括亚稳或平衡相沉淀物的成核、生长和粗化，在技术上和科学上都是有趣的。 一个主要目标是通过原子探针（3DAP）显微镜和模拟技术的结合来分析离散原子水平上的纳米结构变化：特别是3DAP显微镜根据收集的数百万个原子生成3D纳米结构;与动力学蒙特卡罗（KMC）相结合模拟可以获得几乎完整的原子图像。为了在所有相关的长度尺度上获得对不断发展的纳米结构的完整分析，使用透射电子显微镜。主要关注的合金元素Ta，Nb，W，Re或Ru对g'（有序L12结构）沉淀物在g？矩阵（无序FCC）在十二个不同的模型镍基高温合金。 通过在81.5Ni-8.5Cr-10.0Al（at.%）合金中添加一种或几种这些元素来制备三元、四元、五元和六元合金。基础组成模型Ni-Al-Cr基合金的分解动力学也研究了使用KMC模拟，其中原子在刚性晶格上的扩散由单空位机制介导。KMC模拟允许人们在直接晶格空间中研究从随机固溶体开始然后在真实的时间和直接空间中在选定的老化温度下分解的微观结构的时间演化。一部分合金将在相关温度范围（980至1150摄氏度）和应变速率（10-8 s-1至10-4 s-1）内在空气中进行恒定压缩或拉伸应力下的蠕变测试。利用3DAP显微镜来确定：（1）时间演变的所有元素的分布与亚纳米级的空间分辨率;（2）分配的合金元素之间的g？（3）在g？/ g“异相界面;（4）作为时间的函数的g'沉淀物的数密度;（5）作为时间的函数的每种元素的过饱和度;（6）作为时间的函数的g'沉淀物的平均直径。这项研究计划涉及通过REU计划的本科生沿着与埃文斯顿镇高中（ETHS）的推广，这导致了在ETHS的大三和大四学生的就业开发和维护功能齐全的数据分析应用程序，ADAM 1.5（原子探针数据操作）。这项研究的一个重要目标是提供一个实验和定量验证的性质和性能的合金充分表征的纳米结构。 ***

This grant explores the fundamental problem of the temporal evolution of decomposing microstructures in multicomponent alloys on a nanoscale, from both the experimental and simulation points-of-view. A basic understanding of the decomposition processes, involving nucleation, growth, and coarsening of precipitates of metastable or equilibrium phase(s) is technologically and scientifically interesting. A major goal is to analyze nanostructural changes at the discrete atomic level by a combination of atom-probe (3DAP) microscopy and simulation techniques: specifically 3DAP microscopy yields the nanostructure in 3D, based on the millions of atoms collected; in concert with kinetic Monte Carlo (KMC) simulations a nearly complete atomistic picture can be obtained. To obtain a complete analysis of the evolving nanostructures on all relevant length scales, transmission electron microscopies are used. The major focus concerns the effects of the alloying elements Ta, Nb, W, Re or Ru on the evolution of g' (ordered L12 structure) precipitates in a g?matrix (disordered FCC) in twelve different model nickel-base superalloys. Ternary, quaternary, quinary and sexinary alloys are fabricated by adding one or several of these elements to an 81.5 Ni-8.5 Cr-10.0 Al (at.%) base composition. The kinetics of decomposition in model Ni-Al-Cr base alloys are also studied using KMC simulations, where diffusion of atoms on a rigid lattice is mediated by a monovacancy mechanism. KMC simulations allow one to study in direct lattice space the temporal evolution of a microstructure starting from a random solid solution and then following the decomposition at a selected aging temperature in real time and direct space. A subset of the alloys will be subjected to creep testing under constant compressive or tensile stress in air over the relevant temperature range (980 to 1150 degrees C) and strain rates (10-8 s-1 to 10-4 s-1).%%%The 3DAP microscope is utilized to determine: (1) temporal evolution of the distribution of all elements with subnanoscale spatial resolution; (2) partitioning of the alloying elements between the g? and g' phases; (3) segre-ga-tion at the g?/g' heterophase interfaces; (4) number density of the g' precipitates as a function of time; (5) super-saturation of each element as a function of time; (6) mean diameter of g' precipitates as a function of time. This research program involves undergraduate students through the REU program along with outreach at Evanston Township High School (ETHS), which has resulted in the employment of both juniors and seniors at ETHS for developing and maintaining full-featured data analysis application, ADAM 1.5 (Atom Probe Data Manipulation). An important goal of this research is to provide an experimental and quantitative verification of the properties and performance of alloys with fully characterized nanostructures. ***