Temporal Evolution of Microstructures on a Nanoscale: Experiments and Simulations

这项资助从实验和模拟的角度探索了纳米尺度上多组分合金分解微结构的时间演化的基本问题。对分解过程的基本理解，包括亚稳或平衡相沉淀的形核、生长和粗化(S)，在技术和科学上都很有趣。一个主要的目标是通过结合原子探针(3DAP)显微镜和模拟技术来分析离散原子水平上的纳米结构变化：具体地说，3DAP显微镜基于收集的数百万个原子产生3D纳米结构；与动力学蒙特卡罗(KMC)模拟相结合，可以获得几乎完整的原子学图像。为了获得在所有相关长度尺度上演变的纳米结构的完整分析，使用了透射电子显微镜。重点研究了Ta、Nb、W、Re、Ru等合金元素对12种不同型号镍基高温合金中g‘(有序L12结构)析出物在g？基体(无序fcc)中演化的影响。三元、四元、五元和六元合金是通过将这些元素中的一种或几种添加到81.5Ni-8.5Cr-10.0Al(at.%)基组分中来制备的。用KMC模拟研究了Ni-Al-Cr基合金模型的分解动力学，其中原子在刚性晶格上的扩散是由单空位机制引起的。KMC模拟允许人们在直接晶格空间中研究微结构的时间演化，从随机固溶体开始，然后在选定的时效温度下在实时和直接空间中进行分解。一组合金将在相应的温度范围(980-115 0℃)和应变率(10-8 S-1到10-4 S-1)的空气中进行恒定压应力或拉应力的蠕变试验。%3DAP显微镜用于确定：(1)所有元素的空间分辨率的时间演化；(2)合金元素在g？(4)g‘相的数密度随时间变化；(5)各元素的过饱和度随时间变化；(6)g’相的平均直径随时间变化。这项研究计划通过REU计划让本科生参与，并在埃文斯顿乡镇高中(ETHS)进行推广，导致ETHS聘请大三和大四学生开发和维护功能齐全的数据分析应用程序ADAM 1.5(Atom Probe Data Manipment)。这项研究的一个重要目标是为具有完全表征的纳米结构的合金的性质和性能提供实验和定量的验证。***

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. ***