Towards a Fundamental Basis for Controlling Shear Flow Instabilities in HCP Metals

当金属变形到大应变时，它们经常经历从平稳流动到不稳定流动模式的转变，这通常是不希望发生的。剪切带是导致流动不稳定的机制之一，在剪切带中，应变集中在高度局部的条带中。最近发现的另一种机制是弯曲流动，其特征是表面附近的折叠过程，类似于漩涡状流体流动。这些不稳定性会在变形过程中导致失效，也会在使用过程中留下导致失效的缺陷。因此，它们对于广泛应用（例如，生物医学、汽车和航空航天）的产品质量至关重要。不稳定现象将在模型金属系统（镁、钛、锌）中进行表征，使用控制剪切变形装置，结合微观流场的直接高速成像和图像分析。流动的补充非原位表征将使用先进的显微镜方法和低负荷压痕完成。实验将与分析和数值模拟相结合，以开发描述各种不稳定及其发生条件的流动机制图。该研究将为开发广泛适用于控制先进金属流动不稳定性的方法，推进流动分析的实验技术，并促进对自然界其他类型不稳定性现象的理解提供基础。研究成果将广泛影响吸能金属结构的合成、摩擦磨损、金属加工和离散产品制造。补充研究的是一个教育计划，包括本科生和研究生创建一个材料流动的视频画廊；在研究生学习中适度注重培养创业精神。本研究旨在促进我们对金属大应变变形中尺度塑性流动不稳定性的理解，以及这些不稳定性如何介导从层流到非定常类流体流动（如剪切带、弯曲和锯齿流）的转变。先前的工作已经建立了一套实验技术来表征高分辨率的流场，这将在本工作中建立。三个协同推力将研究中尺度的流动不稳定性，为控制非定常流场奠定基础。首先，将绘制关键的流体属性，结合高空间和时间分辨率的直接原位分析，以及通过显微镜和剖面术进行补充的非原位表征。其次，将发展建模方法来描述流动不稳定性和非定常流动动力学的发展。第三，通过将实验结果与模型分析相结合，构建流动相图，根据定量变形参数划分不稳定状态和流动转变。该研究将专门针对模型HCP合金（Mg， Ti和Zn）进行，选择它们的实验适用性和变形响应范围。所得的流相图将为裁剪和控制简单剪切流提供基础，从抑制流动不稳定性到增强非定常流以消除能量。这项研究将促进一项教育计划，让研究生和本科生参与制作视频库，以说明不同的流动和不稳定现象；学生在研究实验室实习；还有研究生创业。

Non-Technical Abstract When metals are deformed to large strains they often undergo a transition from smooth, steady flow to unstable flow modes, which are usually undesirable. Shear banding, in which the strain becomes concentrated in highly localized bands is one mechanism for the flow instability. Another mechanism, recently discovered, is sinuous flow, which is characterized by a folding process near surfaces, analogous to vortex-like fluid flow. These instabilities can cause failure during the deformation itself, as well as leave behind defects that initiate failure in service. They are thus of critical importance in product quality for a wide range of applications (e.g., biomedical, automotive and aerospace). The instability phenomena will be characterized in model metal systems (magnesium, titanium, zinc) using a controlled shear deformation apparatus, in concert with direct, high-speed imaging of the microscopic flow fields and image analysis. Complementary ex situ characterization of the flow will be done using advanced microscopy methods, and low-load indentation. The experiments will be coupled with analytical and numerical modeling to develop flow mechanism maps that depict the various instabilities and the conditions of their occurrence. The research will provide a fundamental basis for developing methods of broad applicability for controlling flow instabilities in advanced metals, advance experimental techniques for flow analysis, and facilitate understanding of other types of instability phenomena in nature. The research results will broadly impact synthesis of metal structures for energy absorption, friction and wear, metals processing and discrete products manufacturing. Complementing the research is an education program involving undergraduate and graduate students in creating a video gallery of flows in materials; and a modest focus on fostering entrepreneurship in graduate study. Technical Abstract The proposed research seeks to advance our understanding of meso-scale plastic flow instabilities in large-strain deformation of metals, and how these instabilities mediate transitions from laminar to unsteady fluid-like flows, e.g., shear band, sinuous and serrated flows. Prior work has established a suite of experimental techniques to characterize flow fields at high resolution that will be built upon in this work. Three coordinated thrusts will study flow instabilities at the meso-scale to establish a fundamental basis for controlling unsteady flows. First, key flow attributes will be mapped, combining direct in situ analysis at high spatial and temporal resolution, with complementary ex situ characterization by microscopy and profilometry. Second, modeling approaches will be developed to describe the flow instabilities and development of unsteady flow dynamics. Third, by integrating the experimental results with model analyses, a phase diagram will be constructed for flows, demarcating unstable regimes and flow transitions in terms of quantitative deformation parameters. The study will be conducted specifically on model HCP alloys (Mg, Ti and Zn) selected for their experimental suitability and range of deformation responses. The resulting flow phase diagram will provide a basis for tailoring and controlling simple-shear flows, from suppressing flow instabilities to enhancing unsteady flows for energy dissipation. The research will foster an education program involving graduate and undergraduate students in developing video galleries to illustrate diverse flow and instability phenomena; student internships in research labs; and graduate student entrepreneurship.