Probing fractal abnormal grain growth at the nanoscale: a percolation scenario with microstructurally based selection rules

探测纳米尺度的分形异常晶粒生长：具有基于微观结构的选择规则的渗透场景

• 批准号: 262772036
• 负责人: Professor Dr. Rainer Birringer
• 金额: --
• 依托单位: Department of Physics
• 依托单位国家: 德国
• 项目类别: Research Grants
• 财政年份: 2015
• 资助国家: 德国
• 起止时间: 2014-12-31 至 2020-12-31
• 项目状态: 已结题
• 来源: https://gepris.dfg.de/gepris/projekt/262772036?language=en
• 关键词: Probing; fractal; abnormal; grain; growth

Thanks to their large grain boundary area per unit volume, nanocrystalline materials find themselves quite far removed from thermodynamic equilibrium, as the excess energy stored in grain boundaries provides a huge driving force for coarsening of the nanoscale microstructure. The resulting grain growth typically proceeds in an abnormal manner, with a small fraction of grains growing to extremely large sizes at the expense of the nanometer-sized grains still present in the surrounding matrix. Such behavior is also observed in conventional, coarser-grained polycrystalline metals and ceramics, although it is not their usual mode of coarsening. Surprisingly, neither at the microscale nor at the nanoscale do we have an adequate grasp of the circumstances enabling abnormally growing grains to establish and maintain a remarkable growth advantage over their neighbors. This mystery is compounded at the nanoscale, where recent studies discovered that abnormally growing grains in nanocrystalline model systems (Pd and Pd-Au alloys) develop highly irregular, almost tumor-like shapes! The corresponding grain perimeters are found to be fractal in nature, much like those of structures formed upon the forced migration of domain walls through a randomly distributed field of pinning sites. Moreover, these abnormally growing grains exhibit fractal dimensionalities that closely match those of domains generated by known percolation processes.Inspired by this observation, we hypothesize that abnormal grain growth can be understood at the nanoscale as a manifestation of a percolation phenomenon occurring on a "network" defined by the initial arrangement of nanocrystalline matrix grains. This concept will be scrutinized by a combination of state-of-the-art electron microscopy and large-scale phase field simulation of microstructural evolution. The great advantage of the percolation scenario is its amenability to, on the one hand, exploratory testing of various "selection rules" for accelerated boundary migration (evaluating their impact on the development of fractality) and, on the other hand, providing a framework for narrowing down the experimental search for microscopic factors (such as grain boundary misorientations or concentration gradients) of possible relevance to the physical mechanism(s) governing fractal abnormal grain growth. Ultimately, the experimental findings will be translated into a minimal set of selection rules and inserted into a modified phase field model allowing for simultaneous abnormal and curvature-driven grain growth. The simulation results will be validated with respect to statistically averaged and local measures for growth kinetics and grain morphologies.

由于单位体积的晶界面积较大，纳米晶材料发现自己远离热力学平衡，因为晶界中储存的多余能量为纳米级微观结构的粗化提供了巨大的驱动力。所产生的晶粒生长通常以异常方式进行，一小部分晶粒生长到极大尺寸，但牺牲了仍然存在于周围基质中的纳米尺寸晶粒。在传统的粗晶多晶金属和陶瓷中也观察到这种行为，尽管这不是它们通常的粗化模式。令人惊讶的是，无论是在微米尺度还是在纳米尺度，我们都没有充分掌握使异常生长的晶粒建立并保持相对于其邻居的显着生长优势的环境。这个谜团在纳米尺度上更加复杂，最近的研究发现，纳米晶体模型系统（Pd 和 Pd-Au 合金）中异常生长的晶粒会形成高度不规则、几乎类似肿瘤的形状！发现相应的晶粒周长本质上是分形的，很像畴壁通过随机分布的钉扎位点场强制迁移时形成的结构。此外，这些异常生长的晶粒表现出的分形维数与已知渗滤过程产生的域的分形维数密切匹配。受这一观察的启发，我们假设异常晶粒生长可以在纳米尺度上理解为“网络”上发生的渗滤现象的表现“由纳米晶基质颗粒的初始排列定义。这一概念将通过最先进的电子显微镜和微观结构演化的大规模相场模拟的结合来仔细研究。渗透场景的巨大优势在于，一方面，它能够对加速边界迁移的各种“选择规则”进行探索性测试（评估它们对分形发展的影响），另一方面，为加速边界迁移提供了一个框架缩小与控制分形异常晶粒生长的物理机制可能相关的微观因素（例如晶界定向错误或浓度梯度）的实验搜索范围。最终，实验结果将转化为最小的选择规则集，并插入到修改后的相场模型中，从而允许同时异常和曲率驱动的晶粒生长。模拟结果将根据生长动力学和晶粒形态的统计平均和局部测量进行验证。

项目成果

Influence of rapid annealing on the evolution of fractal abnormal grains in nanocrystalline Pd–10 at% Au

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• DOI: 10.1088/1757-899x/580/1/012055
• 发表时间: 2019
• 期刊: IOP Conference Series: Materials Science and Engineering
• 作者: Raphael A. Zeller;Harms J. Fey;Christian Braun;Rainer Birringer;Carl E. Krill III
• 通讯作者: Carl E. Krill III

Orientation mapping linked to fractal analysis: A method for studying abnormal grain growth in nanocrystalline PdAu

与分形分析相关的取向映射：一种研究纳米晶 PdAu 中异常晶粒生长的方法

• DOI: 10.1063/5.0029832
• 发表时间: 2020
• 期刊: Journal of Applied Physics
• 影响因子: 3.2
• 作者: Christian Braun;Raphael A. Zeller;Hanadi Menzel;Jörg Schmauch;Carl E. Krill III;Rainer Birringer
• 通讯作者: Rainer Birringer