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

当金属变形到大应变时，它们经常经历从平滑、稳定流动到不稳定流动模式的转变，这通常是不希望的。剪切带是流动不稳定性的一种机制，其中应变集中在高度局部化的带中。 最近发现的另一种机制是弯曲流动，其特征在于表面附近的折叠过程，类似于漩涡状流体流动。 这些不稳定性可在变形本身期间引起失效，以及留下在使用中引发失效的缺陷。因此，它们在广泛应用的产品质量方面至关重要（例如，生物医学、汽车和航空航天）。 不稳定现象的特点是在模型金属系统（镁，钛，锌）使用受控的剪切变形装置，在音乐会上的微观流场的直接，高速成像和图像分析。将使用先进的显微镜方法和低载荷压痕对流动进行补充非原位表征。 实验将与分析和数值模拟相结合，以绘制流动机制图，描绘各种不稳定性及其发生条件。 这项研究将为开发控制先进金属中流动不稳定性的广泛适用性方法提供基本基础，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现象的理解。研究成果将广泛影响金属结构的能量吸收，摩擦和磨损，金属加工和离散产品制造的合成。补充研究的是一个教育计划，涉及本科生和研究生在创建一个视频画廊的材料流;和一个适度的重点是培养创业精神的研究生学习。技术摘要拟议的研究旨在促进我们对金属大应变变形中的介观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不稳定流体状流动的转变，例如，剪切带、弯曲流和锯齿流。先前的工作已经建立了一套实验技术，以高分辨率表征流场，将建立在这项工作。 三个协调推进器将研究中尺度流动不稳定性，为控制非定常流动奠定基础。首先，将绘制关键的流动属性，结合高空间和时间分辨率的直接原位分析，与显微镜和轮廓测量的辅助非原位表征。其次，将开发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，将构建流动的相图，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，这些合金是根据其实验适用性和变形响应范围选择的。 由此产生的流动相图将提供一个基础，剪裁和控制简单剪切流，从抑制流动不稳定性，以提高非定常流的能量耗散。 这项研究将促进一个教育计划，涉及研究生和本科生开发视频画廊，以说明不同的流动和不稳定现象;学生在研究实验室实习;和研究生创业。

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.