Accounting for Climb and Cross-slip in the Crystal Plasticity of Non-Cubic Metals

非技术描述：具有非典型晶体结构(原子排列)的金属合金具有关键特性，使其能够用于各种先进技术，有助于环境保护(无铅锡基焊料)；通过运输实现节能(镁(镁)和钛合金)；以及通过核技术实现能源生产(锆合金)。目前的研究重点是轻量化的镁金属变形，这是汽车、航空航天和国防工业感兴趣的。这项研究(包括实验活动和计算建模)为这些应用的改进设计和制造提供了见解。参与这项研究的学生在材料、汽车或航空航天制造部门找到工作，一些人继续从事为国家在能源、国防或教育领域的利益服务的研究事业。一项外展计划旨在培养夏洛茨维尔和周边农村地区当地小学生对材料科学以及科学和工程的热情。在这个项目中，从莱特兄弟的首次飞行到特斯拉电动汽车和无人机等现代交通工具，坚固而轻便的材料在高效运输中的作用得到了展示。技术细节：拟议研究的目标是确定在非立方金属的多晶塑性过程中位错攀移和交叉滑移适应应变的程度。选择轻质镁及其合金进行这项研究，是因为全世界都对提高运输效率感兴趣，而且镁晶体的近热弹性各向同性简化了塑性各向异性的分析，这一点也是科学上的便利。纯镁和商业合金的织构样品将在蠕变和应力松弛条件下进行测试，测试条件与初始织构的方向不同。虽然已经公布了这些材料的数据，但关于交叉滑移和攀登的作用仍存在争议。定量考虑初始织构、各种位错类型(具有a、c和c+a Burgers矢量)以及位错亚结构的演化(在非立方材料中基本未被研究)将解决这一问题。方法是多尺度的，涉及宏观、中观和微观三个尺度。对织构多晶样品进行了宏观蠕变和应力松弛实验，并在蠕变到显著应变水平的样品上测量了织构演化和应变各向异性。宏观实验的结果指导了特定应力和温度条件的选择，以进行细观和微观表征。一种名为原位高能X射线衍射(HEXD)的新技术揭示了多晶单晶体中全应力张量和位错含量(包括不同类型的密度、Burgers矢量和线方向的密度)的介观分布。以前只限于单晶实验或多晶中非常少量的颗粒的信息最近才对多晶中具有统计意义的大量(数百)颗粒可用，一个新的多晶塑性模型，它解释了位错攀升，被用来解释HEXD数据。最后，X射线衍射线轮廓分析和透射电子显微镜(包括原位应变)实验阐明了微观尺度的位错密度，揭示了特定的热激活单元过程中的临界位错组态及其动力学描述。结果的解释被离散位错动力学(DDD)模拟所辅助，用于探索这些位错系综效应以及它们是如何恢复的。研究生接受高级实验表征和建模技术方面的培训。在指导下，本科生还进行一些实验和分析，特别是在宏观层面上。这一奖项反映了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。

NON-TECHNICAL DESCRIPTION: Metal alloys with atypical crystal structures (atomic arrangements) exhibit key properties that enable them to be employed in a wide range of advanced technologies that contribute to environmental protection (lead-free tin-based solders); energy conservation through transportation weight reduction (magnesium (Mg) and titanium alloys); and energy production via nuclear technology (zirconium alloys). The current research focuses on lightweight Mg metal deformation, which is of interest to the automotive, aerospace, and defense industries. The research (including both experimental activities and computational modeling) provides insights into improved design and manufacture for these applications. Students involved in this research find employment within the materials, automotive or aerospace manufacturing sectors, and some go on to research careers which serve the nation's interests in energy, defense, or education. An outreach program seeks to develop enthusiasm for materials science, and science and engineering more generally in local elementary school students in Charlottesville and the surrounding rural area. Within this program, the role of strong, yet light-weight materials is demonstrated in efficient transportation - from the Wright brothers' first flight to modern vehicles, like Tesla electric cars and drones. TECHNICAL DETAILS: The objective of the proposed research is to determine the degree to which dislocation climb and cross-slip accommodate strain during the polycrystal plasticity of non-cubic metals. Lightweight Mg and its alloys are selected for this study because of the world-wide interest in increasing the efficiency of transportation as well as the scientific convenience that the near-thermo-elastic isotropy of Mg crystals simplifies the analysis of plastic anisotropy. Textured samples of pure Mg and commercial alloys will be tested under creep and stress-relaxation conditions along different directions with respect to the initial texture. While there is already published data for these materials, a controversy regarding the roles of cross-slip and climb remains. Quantitative accounting for the effect of initial texture, various dislocation types (with a, c, and c+a Burgers vectors), as well as the evolution of the dislocation substructure (largely unstudied in non-cubic materials) will settle the issue. The approach is multi-scale, addressing the macroscale, mesoscale and microscale. MACROSCALE creep and stress relaxation experiments are performed on textured polycrystalline samples, and texture evolution and strain anisotropy are measured on samples crept to significant strain levels. The results of macroscale experiments guide selection of specific stress and temperature conditions to explore with meso- and micro-scale characterization. A new technique known as in situ high energy X-ray diffraction (HEXD) reveals the MESOSCALE distribution of full stress tensors and dislocation contents (including the densities of different types, Burgers vector and line direction) within individual grains of a polycrystal. Information that was previously relegated to single crystal experiments or very small numbers of grains in a polycrystal has only recently become available for statistically significant numbers (hundreds) of grains within polycrystals, and a new polycrystal plasticity model, which accounts for dislocation climb, is used to interpret the HEXD data. Finally, X-ray diffraction line profile analysis and transmission electron microscopy (including in situ straining) experiments elucidate the MICROSCALE dislocation densities, reveal critical dislocation configurations and their kinetic descriptions in terms of specific thermally activated unit processes. The interpretation of the results is aided by discrete dislocation dynamics (DDD) simulation, used to explore the effects of these dislocation ensembles and how they recover. Graduate students are trained in advanced experimental characterization and modeling techniques. With guidance, undergraduate students also perform some of the experiments and analyses, especially at the macroscale.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.