Probing fractal abnormal grain growth at the atomistic level using APT
使用 APT 在原子水平上探测分形异常晶粒生长
基本信息
- 批准号:461632490
- 负责人:
- 金额:--
- 依托单位:
- 依托单位国家:德国
- 项目类别:Research Grants
- 财政年份:
- 资助国家:德国
- 起止时间:
- 项目状态:未结题
- 来源:
- 关键词:
项目摘要
Although current models are reasonably successful at explaining normal grain growth in conventional polycrystals, our understanding of grain growth in nanocrystalline (NC) materials is still quite rudimentary. At the nanoscale, coarsening is often abnormal in nature, with a few grains growing orders of magnitude larger than neighboring grains in the surrounding NC matrix. In samples of NC Pd90Au10 produced by inert gas condensation, microstructural coarsening is doubly abnormal, as the rapidly growing grains send forth “tentacles” into the matrix, first encircling nearby grains and then consuming them. The perimeters of the resulting grains are well described by a fractal dimension of about 1.2 (rather than the expected value close to unity).The widely accepted curvature-based mechanism for grain boundary (GB) migration is incompatible with such “fractal” growth, but even “ordinary” abnormal growth—as observed in technical alloys like Fe-Si steels—remains an unsolved mystery. A recent proposal by Hwang et al., however, is the first to subsume these disparate forms of abnormal growth under the single mechanism of “solid-state wetting.” In this picture, the fractal migration of GBs is an extreme manifestation of wetting, rendering NC PdAu an ideal model system for validating the concept.Without Au, grain growth in NC Pd proceeds normally; therefore, we speculate that fractal GB migration results from thermodynamic and/or kinetic effects associated with Au segregation to GBs in NC PdAu. These effects may suppress curvature-driven growth of the matrix grains while, at the same time, promoting (or at least not hindering) solid-state wetting. The key step in testing this hypothesis is to measure the Au concentration in the vicinity of fractally growing grains, comparing the amounts of Au segregation at fractal/matrix and matrix/matrix GBs. Atom-probe tomography (APT) is predestined for this task, capable of quantifying local compositions in a 3D specimen at a spatial resolution comparable to that of the interatomic spacing.In this project, the concentration of Au atoms in NC PdAu will be varied to maximize the fractal dimension. To deconvolute thermodynamic from kinetic aspects of GB migration, a ternary element (hydrogen) having a known tendency to segregate to GBs will be added to the samples. FIB milling will be employed to extract GBs of interest, which will be characterized by APT and correlated with various forms of electron microscopy. Measured values for Au segregation will be input into a phase field model for simulating simultaneous solid-state wetting and curvature-driven growth. The morphology of the simulated grains will be validated against experiment and combined with the local studies of Au and H segregation to determine whether solid-state wetting is the origin of fractal abnormal grain growth in NC PdAu.
虽然目前的模型在解释常规多晶的正常晶粒生长方面相当成功,但我们对纳米晶(NC)材料的晶粒生长的理解仍然相当初级。在纳米尺度上,粗化通常是不正常的,少数晶粒比周围NC基体中的邻近晶粒生长得大几个数量级。在惰性气体冷凝制备的NC Pd90Au10样品中,微观组织的粗化是双重异常的,因为快速生长的晶粒向基体中伸出“触角”,首先包围附近的晶粒,然后消耗它们。所得晶粒的周长可以用约为1.2的分形维数(而不是接近于1的期望值)来很好地描述。广泛接受的基于曲率的晶界迁移机制与这种“分形”生长是不相容的,但即使是“普通”的异常生长——就像在铁硅钢等技术合金中观察到的那样——仍然是一个未解之谜。然而,Hwang等人最近提出的一项建议,首次将这些不同形式的异常生长纳入“固态润湿”的单一机制之下。在这张图中,GBs的分形迁移是润湿的一种极端表现,使NC PdAu成为验证这一概念的理想模型系统。在不含Au的情况下,NC钯的晶粒生长正常;因此,我们推测分形的GB迁移是由与Au在NC PdAu中向GB偏析相关的热力学和/或动力学效应引起的。这些效应可能抑制基体晶粒的曲率驱动生长,同时促进(或至少不阻碍)固态润湿。验证这一假设的关键步骤是测量分形生长晶粒附近的Au浓度,比较分形/基体和基体/基体GBs处的Au偏析量。原子探针断层扫描(APT)是注定要完成这项任务的,它能够以与原子间间距相当的空间分辨率量化3D样品中的局部成分。在本项目中,将改变NC PdAu中Au原子的浓度,以最大化分形维数。为了从动力学方面解决GB迁移的热力学问题,将向样品中添加一种已知倾向于向GB分离的三元元素(氢)。FIB铣削将用于提取感兴趣的gb,这些gb将被APT表征并与各种形式的电子显微镜相关联。金偏析的测量值将输入相场模型,以模拟同时发生的固态润湿和曲率驱动生长。模拟晶粒的形貌将通过实验验证,并结合局部Au和H偏析的研究来确定固态润湿是否是NC PdAu分形异常晶粒生长的原因。
项目成果
期刊论文数量(0)
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Professor Carl Emil Krill III, Ph.D.其他文献
Professor Carl Emil Krill III, Ph.D.的其他文献
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