Microstructural Evolution during Superplastic Ice Creep

超塑性冰蠕变过程中的微观结构演化

• 批准号: 2317263
• 负责人: Andrew Cross
• 金额: $ 49.42万
• 依托单位: Woods Hole Oceanographic Institution
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2023
• 资助国家: 美国
• 起止时间: 2023-08-15 至 2026-07-31
• 项目状态: 未结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=2317263&HistoricalAwards=false
• 关键词: Microstructural; Evolution; during; Superplastic; Ice

The seaward motion of ice sheets and glaciers is primarily controlled by basal sliding at the base of the ice sheet and internal viscous flow within the ice mass. The latter of these — viscous flow — is dependent on various factors, including temperature, stress, grain size, and the alignment of ice crystals during flow to produce a "crystal orientation fabric" (COF). Historically, ice flow has been modeled using an equation, termed “Glen’s law”, that describes ice-flow rate as a function of temperature and stress. Glen’s law was constrained under relatively high-stress conditions and is often attributed to the motion of crystal defects within ice grains. More recently, however, grain boundary sliding (GBS) has been invoked as the rate-controlling process under low-stress, “superplastic” conditions. The grain boundary sliding hypothesis is contentious because GBS is not thought to produce a COF, whereas geophysical measurements and polar ice cores demonstrate strong COFs in polar ice masses. However, very few COF measurements have been conducted on ice samples subjected to superplastic flow conditions in the laboratory. This project would measure the evolution of ice COF across the transition from superplastic to Glen-type creep. Results will be used to interrogate the role of superplastic GBS creep within polar ice masses, and thereby provide constraints on polar ice discharge models.Polycrystalline ice samples with grain sizes ranging from 5 µm to 1000 µm will be fabricated and deformed in a laboratory, using a 1-atm cryogenic axial-torsion apparatus. Experiments will be conducted at temperatures of -30°C to -10°C, and at a constant uniaxial strain rate. Under these conditions, 5% to 99.99% of strain should be accommodated by superplastic, GBS-limited creep, depending on the sample grain size. The deformed samples will then be imaged using cryogenic electron backscatter diffraction (cryo-EBSD) and high-angular-resolution electron backscatter diffraction (HR-EBSD) to quantify COF, grain size, grain shape, and crystal defect (dislocation) densities, among other microstructural properties. These measurements will be used to decipher the rate-controlling mechanisms operating within different thermomechanical regimes, and resolve a long-standing debate over whether superplastic creep can produce a COF in ice. In addition to the polycrystal experiments, ice bicrystals will be fabricated and deformed to investigate the micromechanical behavior of individual grain boundaries under superplastic conditions. Ultimately, these results will be used to provide a microstructural toolbox for identifying superplastic creep using geophysical (e.g., seismic, radar) and glaciological (e.g., ice core) observations. This project will support one graduate student, one or more undergraduate summer students, and an early-career researcher. In addition, this project will support a workshop aimed at bringing together experimentalists, glaciologists, and ice modelers to facilitate cross-disciplinary knowledge sharing and collaborative problem solving.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.

冰盖和冰川的海向运动主要由冰盖底部的基本滑动以及冰块内的内粘性流动。后者 - 粘性流 - 取决于各种因素，包括温度，应力，晶粒尺寸和流动过程中冰晶的比对，以产生“晶体取向织物”（COF）。从历史上看，冰流是使用称为“ Glen定律”的方程式建模的，该方程将冰流率描述为温度和压力的函数。格伦的定律受到相对较高的压力条件的约束，通常归因于冰粒内晶体缺陷的运动。然而，最近，在低压力，“超塑性”条件下，晶粒边界滑动（GBS）被称为速率控制过程。晶界滑动假设是有争议的，因为GBS不被认为会产生COF，而地球物理测量值和极性冰核在极性冰块中表现出强大的COF。但是，在实验室中经受过极端流动条件的冰样品上进行了极少的COF测量。该项目将测量冰CoF在从巨型到Glen型蠕变的转变中的演变。结果将用于询问极性冰块中质质量GBS蠕变的作用，从而在极性冰出排放模型上提供限制。晶粒尺寸的晶粒尺寸范围为5 µm至1000 µm，将在实验室中使用1- ATM Cryegenic Axial轴向轴向膜构成，并在实验室中制造并变形。实验将在-30°C至-10°C的温度下进行，并以恒定的单轴应变速率进行。在这些条件下，根据样品晶粒尺寸，应通过超代重的GBS限制蠕变来容纳5％至99.99％的应变。然后，将使用低温电子反向衍射（Cryo-EBSD）和高角度分辨率电子反向散射衍射（HR-EBSD）对变形样品进行成像，以量化COF，晶粒尺寸，晶粒形状，晶粒形状和晶体缺陷（脱位）密度，等等。这些测量结果将用于解读在不同热力学机制内运行的速率控制机制，并解决有关超代重蠕变是否可以在冰中产生COF的长期争论。除了多晶实验外，还将制造并变形冰鸟类，以研究在超塑性条件下单个晶界的微机械行为。最终，这些结果将用于提供一个微结构工具箱，用于使用地球物理（例如地震，雷达）和冰川（例如冰核）观测来识别超代重的蠕变。该项目将支持一名研究生，一名或多个本科生的夏季学生和一名早期研究员。此外，该项目将支持旨在将实验家，冰川学家和ICE模型汇集在一起​​的研讨会，以促进跨学科知识共享和协作问题解决问题。该奖项反映了NSF的法定任务，并被认为是通过基金会的知识分子优点和更广泛的影响审查审查标准来通过评估来通过评估来支持的。