Microstructural Evolution during Superplastic Ice Creep

冰盖和冰川的向海运动主要由冰盖底部的底部滑动和冰块内部的粘性流动控制。后者--粘性流动--取决于各种因素，包括温度、应力、颗粒大小以及流动过程中冰晶的排列情况，从而形成“晶体取向结构”(COF)。从历史上看，冰流是用一个称为“格伦定律”的方程来模拟的，该方程将冰流速率描述为温度和应力的函数。格伦定律在相对较高的应力条件下受到限制，通常被归因于冰粒中晶体缺陷的运动。然而，最近，晶界滑动(GBS)被认为是低应力“超塑性”条件下的速率控制过程。晶界滑动假说是有争议的，因为GBS被认为不会产生COF，而地球物理测量和极地冰芯表明，极地冰块中有很强的COF。然而，在实验室中对超塑性流动条件下的冰样进行的COF测量很少。该项目将测量从超塑性到格伦型蠕变转变过程中冰COF的演变。结果将被用来询问极地冰团中超塑性GBS蠕变的作用，从而为极地冰流模型提供约束。多晶冰样品的粒度从5微米到1000微米将在实验室中使用1-ATM低温轴向扭转装置进行制造和变形。实验将在-30°C到-10°C的温度和恒定的单轴应变速率下进行。在这些条件下，5%到99.99%的应变应由超塑性、GBS限制的蠕变来适应，具体取决于样品的颗粒尺寸。然后，将使用低温电子背散射衍射(Cryo-EBSD)和高角分辨率电子背散射衍射(HR-EBSD)对变形的样品进行成像，以量化COF、晶粒度、晶形和晶体缺陷(位错)密度等微结构特性。这些测量将被用来破译在不同热力机制下运行的速率控制机制，并解决关于超塑性蠕变是否会在冰中产生COF的长期争论。除了多晶实验，还将制作和变形冰双晶，以研究超塑性条件下单个晶界的微观力学行为。最终，这些结果将被用来提供一个微观结构工具箱，用于利用地球物理(例如地震、雷达)和冰川(例如冰芯)观测来识别超塑性蠕变。这个项目将支持一名研究生，一名或多名本科生暑期学生，以及一名职业生涯早期的研究人员。此外，该项目还将支持一个研讨会，旨在将实验学家、冰川学家和冰川模型师聚集在一起，以促进跨学科的知识共享和协作解决问题。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

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.