Forced Mixing and Nanoscale Self-Organization During Severe Plastic Deformation of Complex Metal Alloys

通过球磨、累积滚压或等通道转角加工对金属进行大塑性变形(SPD)的高能加工正成为制备高强度材料的一种有吸引力的手段。目前的提案寻求发展这一处理方案的科学，重点是两个关键特征，即强制化学混合和自组织。强制化学混合是通过位错运动使合金成分发生非热重排。目前对这一过程的了解还很少，这在很大程度上是由于过去在控制和表征SPD过程中的总应变、局部温度和应力状态方面的实验困难。本研究将采用高压扭转实验，其中的过程变量被很好地定义。这项工作将检验理论模型，这些模型预测SPD过程中的原子混合是超扩散的，而不是费克的，混合行为应该揭示合金成分的热化学和热机械性质的强烈影响。这项研究的第二个关键部分涉及SPD过程中合金的介观自组织。模型预测，许多相分离合金在高温下受到SPD处理时，将在纳米尺度上自组织成成分图案。这些模型进一步表明了一个令人惊讶的结果，即当点缺陷介导的扩散过程被抑制时，这种图案化甚至可能在低温下发生。这项研究将首先考虑建立模型二进制系统来测试模型预测，但随后将通过考虑三进制甚至四进制系统来建立复杂性。纳米结构材料的微结构表征将包括透射电子显微镜方法、原子探针断层扫描和X射线衍射。非技术摘要：最近人们认识到，经过SP处理的材料通常具有在极端环境中使用的优异性能；它们具有非常高的强度，并且往往能够抵抗高能粒子辐射的破坏，例如在核反应堆中。这项研究的目的是为加工纳米复合材料结构以及如何设计具有特定应用特性的纳米复合材料结构提供科学依据。原子在SPD过程中混合和随机化的机制类似于摇动一瓶通常不相容的油和水--漂浮在密度更高的水上的油。当小瓶摇动时，两者之间的界面首先变得粗糙，随着摇动强度的增加，油中会形成小水滴，反之亦然。随着振荡强度的增大，乳状液的尺寸逐渐减小，最终形成均匀的乳状液。两种不相容的金属在固相沉积过程中的混合以及新形成的纳米复合材料中固相“球状”的长度、尺度和图案化是主要感兴趣的。拟议的研究对开发新型纳米复合材料的设计战略具有广泛的科学影响，这些材料对许多先进材料的应用至关重要：氢存储、电池、抗辐射核材料等。除了通过出版物和科学会议传播我们的研究外，还将为UIUC的“材料流动”开发这些新材料的教育演示，该活动向伊利诺伊州各地的高中生介绍材料科学和工程的概念。

TECHNICAL SUMMARY: High energy processing of metals using severe plastic deformation (SPD) by ball milling, accumulative roll bonding, or equal channel angular processing is becoming an attractive means to fabricate high-strength materials. The current proposal seeks to develop the science of this processing scheme, focusing on two critical features, forced chemical mixing and self-organization. Forced chemical mixing is the athermal rearrangement of alloy components by dislocation motion. Presently this process is only poorly understood, owing in large part to past experimental difficulties in controlling and characterizing the total strain, local temperature, and stress state during SPD. The present research will employ high pressure torsion experiments, for which the process variables are well defined. The work will test theoretical models, which predict that atomic mixing during SPD is superdiffusive rather than Fickian and that the mixing behavior should reveal strong influences of the thermochemical and thermomechanical properties of the alloy components. The second key component of the research concerns mesoscale self-organization in alloys during SPD. The models predict that many phase separating alloys should self-organize into compositional patterns on a nanometer length scale when subjected to SPD at high temperatures. These models further indicate the surprising result that such patterning may even occur at low temperatures, when diffusion processes mediated by point defects are suppressed. The research will consider at first model binary systems to test model predictions, but will then build complexity by considering ternary and even quaternary systems. Characterization of the microstructures of the nanostructured materials will include transmission electron microscopy methods, atom probe tomography and x-ray diffraction.NON-TECHNICAL SUMMARY: It has been recognized recently that materials subjected to SP often possess excellent properties for use in extreme environments; they have very high strengths and they tend to be resistant to damage by energetic particle irradiation, such as in a nuclear reactor. The goal of this research is to provide the scientific basis for processing nanocomposite structures and how to design them with properties tailored to specific applications. The mechanism by which atoms intermix and randomize during SPD is analogous to shaking a vial of oil and water, which are normally immiscible - oil floating on top of the more dense water. When the vial is shaken, the interface between the two at first roughens, and as the intensity of the shaking increases, small globules of water form in the oil and vice-versa. The sizes of the globules decrease with the intensity of shaking until finally a homogeneous emulsion is obtained. The intermixing of two immiscible metals during SPD and the length scales and patterning of the "globules" of solid phases in the newly formed nanocomposite materials are of primary interest. The proposed research has broad scientific impact for developing design strategies for new nanocomposite materials that are critical to a number of advanced materials applications: hydrogen storage, batteries, radiation-resistant nuclear materials, etc. In addition to disseminating our research through publications and scientific meetings, educational demonstrations of these new materials will be developed for the "Materials Mobile" at UIUC which introduces concepts of materials science and engineering to high school students around the State of Illinois.