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

当金属变形为大应变时，它们通常经历从平稳、稳定的流动模式到不稳定流动模式的转变，这通常是不受欢迎的。剪切带是流动不稳定性的一种机制，应变集中在高度局域化的带上。最近发现的另一种机制是弯曲流动，其特征是在表面附近发生折叠过程，类似于涡流。这些不稳定性可能会在变形过程中导致故障，也会在使用过程中留下引发故障的缺陷。因此，它们对于广泛的应用(例如，生物医学、汽车和航空航天)的产品质量至关重要。模型金属系统(镁、钛、锌)中的不稳定现象将使用受控剪切变形装置、微观流场的直接高速成像和图像分析来表征。流动的互补性非原位表征将使用先进的显微镜方法和低负载压痕进行。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。这项研究将为开发具有广泛适用性的控制先进金属流动不稳定性的方法提供基本基础，为流动分析提供实验技术，并有助于理解自然界中其他类型的不稳定现象。研究成果将广泛影响用于能量吸收、摩擦磨损、金属加工和离散产品制造的金属结构的合成。与这项研究相辅相成的是一个教育项目，让本科生和研究生参与创建材料流动的视频画廊；并适度关注在研究生学习中培养企业家精神。这项研究旨在加深我们对金属大应变变形中细观塑性流动不稳定性的理解，以及这些不稳定性如何促成从层流到非定常类流体流动的转变，例如剪切带流动、弯曲流动和锯齿流动。以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，这将在本工作的基础上建立。三次协同推进将在细观尺度上研究流动不稳定性，为控制非定常流动奠定基础。首先，将绘制关键流动属性图，将高空间和时间分辨率的直接原位分析与通过显微镜和轮廓术进行补充的非原位表征相结合。其次，将发展模拟方法来描述流动的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，建立流动的相图，根据定量变形参数划分不稳定区域和流动转变。这项研究将专门针对根据实验适宜性和变形响应范围而选择的模型HCP合金(镁、钛和锌)。由此得到的流动相图将为定制和控制简单剪切流动提供基础，从抑制流动不稳定性到增强非定常流动的能量耗散。这项研究将培养一个教育项目，让研究生和本科生参与开发视频画廊，以说明不同的流动和不稳定现象；学生在研究实验室的实习；以及研究生创业。

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.