Coupled simulations of low temperature microstructural evolution in nanocrystalline metals

技术摘要：从金属加工的最早期开始，人们就知道淬火冻结了成品的理想属性。然而，纳米晶金属的出现为这种普遍化提供了重要的反例，在低至77 K的温度下，大量的晶粒生长在几天，几小时，甚至几分钟内发生。这些低温演变的共同点是反常晶粒生长（AGG），即少数晶粒以缓慢演变的基体晶粒为代价长大，但低温显微组织的演变还没有得到令人满意的解释。晶界运动被广泛认为是由热激活的原子过程发生的，因此边界运动的速率应该随着温度的降低而呈指数下降。然而，最近的计算调查晶界迁移率确定了三个新的流动性类别，允许在低温下的快速运动。在这个项目中，一个合成的驱动力分子动力学方法将被应用于探索和表征这些快速，冷晶界，以确认是否高流动性持续到低温;发现低温边界运动的原子机制;并确定这些边界在真实的微观结构中的出现。这种原子的研究将提供第一个基本的理解，这一新发现的晶界运动制度。高迁移率边界本身并不足以导致在纳米晶体金属中观察到的异常晶粒生长。AGG需要异常晶粒周围的大多数或所有边界的协同运动，因此基本上是晶界网络内的集体过程。在这个项目中，微观结构演变的中尺度模型，结合完整的晶粒和晶界结构和晶体学，将与晶界运动的原子模拟结果相结合，以探索和表征包含快速，冷边界的系统中的AGG。初始模拟将解决AGG所需的微观结构特征。随后的模拟将纳入其他因素，提高AGG的频率和速率。最后，本文的结果将被用来建立低温AGG的分析模型。纳米金属的低温晶粒生长不仅仅是一个学术问题。纳米晶材料的应用要求其微结构在使用过程中保持稳定;微结构的演变会改变这些材料的理想性能，通常会降低这些性能。了解低温演变是控制它的关键。非技术摘要：我们都看到过这样一幅画面：一个剑匠将一把烧红的剑刃投入冷水中，使其最终结构冻结。同样的原则也适用于我们日常生活中使用的许多材料。从汤匙到车轴的金属物体都是使用一系列加热和冷却操作制造的，最终的结构通过冷却到室温而冻结。一旦材料被淬火，我们希望它不会再发生变化，所以当它发生变化时，这是一个重要的科学和技术问题。纳米晶体金属是由熟悉的成分制成的-例如铜，镍或铁-加工成微小的晶粒，其大小为沙粒的百万分之一，紧密堆积在一起形成固体物质。由于这些新材料比传统金属更硬、更强，它们可能会使飞机更轻、汽车更可靠。但有一个主要问题：科学家们观察到，纳米晶体金属的结构在室温下会在数小时、数天或数年内发生变化。在这个项目中，我们研究了纳米晶金属的结构在低温下如何演变。我们首先使用原子运动的计算机模拟来了解某些微晶之间的各个界面如何即使在低温下也能移动。然后，我们模拟了大量相互连接的界面的运动，以揭示快速移动的界面如何改变材料的整体结构。了解低温结构演变是控制它的关键。该项目的目标是使用计算机模拟来发展这种理解。

Technical Abstract: From the earliest days of metalworking, it has been understood that quenching freezes in the desirable attributes of the finished object. However, the advent of nanocrystalline metals has offered significant counterexamples to this generalization, with substantial grain growth occurring in days, hours, even minutes at temperatures as low as 77K. The common denominator in these instances of low temperature evolution is abnormal grain growth (AGG), where a few grains grow very large at the expense of the more slowly evolving matrix grains.As yet, low temperature microstructural evolution has not been satisfactorily explained. Grain boundary motion is widely accepted to occur by thermally activated atomic processes, so that the rate of boundary motion should decrease exponentially as temperature decreases. However, a recent computational survey of grain boundary mobilities identified three new mobility categories that permit fast motion at low temperatures. In this project, a synthetic driving force molecular dynamics method will be applied to explore and characterize these fast, cold grain boundaries to confirm whether high mobility persists to low temperature; to discover the atomic mechanisms of low temperature boundary motion; and to determine the occurrence of these boundaries in real microstructures. This atomistic study will provide the first fundamental understanding of this newly revealed grain boundary motion regime. High mobility boundaries alone are not sufficient to cause the abnormal grain growth observed in nanocrystalline metals. AGG requires the concerted motion of most or all of the boundaries surrounding the abnormal grain, and as such is fundamentally a collective process within the grain boundary network. In this project, a mesoscale model of microstructural evolution, incorporating full grain and grain boundary structure and crystallography, will be coupled with the results of atomistic simulations of grain boundary motion to explore and characterize AGG in systems containing fast, cold boundaries. Initial simulations will address which microstructural features are required for AGG. Subsequent simulations will incorporate additional factors that enhance the frequency and rate of AGG. Finally the results will be used to develop analytical models for low temperature AGG.Low temperature grain growth in nanocrystalline metals is not just an academic problem. Implementation of nanocrystalline materials requires that the microstructure remains stable during service; microstructural evolution changes, and usually degrades, the desirable properties of these materials. Understanding low temperature evolution is the key to controlling it. Non-technical Abstract: We have all seen an image of a sword maker plunging a red-hot blade into cold water to freeze in its final structure. The same principle applies to many materials that we use in our everyday lives. Metal objects from a soup spoon to a car axle are manufactured using a series of heating and cooling operations, with the final structure frozen in by cooling to room temperature. Once the material has been quenched, we expect it not to change anymore, so it is an important scientific and technological issue when it does.Nanocrystalline metals are made from familiar ingredients - such as copper, nickel, or iron - processed into tiny crystallites one-millionth the size of a grain of sand that are packed closely together to form a solid substance. Because these new materials are harder and stronger than conventional metals, they may enable lighter airplanes and more reliable cars. But there is one major problem: scientists have observed that the structure of nanocrystalline metals changes over hours, days, or years at room temperature. In this project, we investigate how the structure of nanocrystalline metals evolves at low temperatures. We first use computer simulations of atomic motion to understand how individual interfaces between certain crystallites can move even at low temperatures. We then simulate the motion of large groups of interconnected interfaces to reveal how fast-moving interfaces can change the overall structure of the material.Understanding low temperature structural evolution is the key to controlling it. The goal of this project is to use computer simulations to develop that understanding.