利用放电等离子体烧结设计新型微结构以研究弹塑性转变和应力分配

Research will be carried out to explore the use of spark plasma sintering for the preparation of metallic samples with tailored microstructural characteristics, with a focus on the synthesis of microstructures designed as model systems for fundamental investigations of plastic yielding and deformation in metals. Three specific directions will be followed in the project. In one set of experiments, the influence of grain size distribution on yielding at the elastic-plastic transition will be investigated using samples of Al, where the grain size is controlled by the size of the powders used for sintering, as a result of strong oxide pinning. Based on previous work research will focus on variation in grain size distribution in the near-micrometer regime, where the mechanical properties are particularly sensitive to variation in grain size. In-situ deformation experiments in the scanning electron microscope will be carried out using a specially built tensile stage giving a strain resolution of 0.002%. Digital image correlation and high resolution electron backscatter diffraction will be used to measure the local plastic deformation on a sub-micrometer scale. The work will be supplemented by high-energy X-ray synchrotron experiments to allow the collection of 3D maps showing the spreading of plastic strain through a grain structure. In a second set of experiments, samples with a gradient in grain size across the thickness will be synthesized by controlled mixing and layering of Al powders. Research efforts here will focus on the measurement of and stress and strain gradients that develop across the specimen thickness during loading, and how these gradients vary as a function of the gradient in grain size. A third set of experiments will examine the use of spark plasma sintering of mixtures of copper and steel powders to prepare model dual-phase microstructures, for the study of stress and strain partitioning in such systems. This approach will uniquely allow an independent control of both the size, volume fraction, and strength of each phase, as well as the spatial distribution of the two phases. Experimental measurements of the relationship between microstructure and spatial distribution of plastic strain will be supplemented by neutron diffraction measurements to examine stress partitioning between the hard and soft phases. As the hard and soft phases possess different crystal structures and lattice parameters full separation of the diffraction peaks from each will be possible, allowing the stress and strain in each phase to be measured without the need for peak deconvolution methods and related assumptions about diffraction peak shapes.

本研究利用放电等离子体烧结（SPS）技术制备新型微结构金属材料，研究金属塑性屈服和形变机理。第一、研究近微米尺度下铝的晶粒尺寸分布对弹–塑性转变阶段屈服过程的影响。采用一套特制的应变分辨率0.02 %的拉伸装置在扫描电镜中进行原位拉伸实验。使用数字图像相关法和高分辨电子背散射衍射技术来测定局部塑性变形。辅以同步辐射高能X射线实验，获得塑性应变扩展的三维图像。第二，通过控制铝粉的混粉与铺叠过程，制备沿厚度方向晶粒尺寸梯度变化的样品，测量形变中应力与应变沿厚度方向的梯度变化。第三，铜粉和钢粉混合后利用SPS技术制备双相结构材料，研究形变中应力和应变分配。这种方法可对各相晶粒尺寸、体积分数、强度及空间分布等因素进行独立控制。利用中子衍射方法测定应力在软硬相中的分配，研究塑性应变空间分布与微结构的关系。由于软硬相具有不同的晶体结构和晶格常数，彼此衍射峰完全分离，避免了峰位重叠所需的复杂处理。

本研究证明了使用放电等离子烧结技术设计和合成具有新型微观结构金属的可行性，并且研究了所得样品的力学性能和变形行为。合成的单相纯铝样品具有单峰或双峰晶粒尺寸分布，或者具有晶粒尺寸在样品尺度变化的层状结构。单峰粒度样品已用于研究塑性屈服的开始阶段，其中先进的X射线同步加速器方法首次在样品内部揭示了塑性屈服期间塑性应变的穿透行为。双峰样品已用于研究晶粒尺寸不均匀性对塑性变形应变协调的影响，其重点是研究力学性能发生明显转变的近微米尺度晶粒尺寸的影响。对于这些研究，基于胶状二氧化硅纳米颗粒的使用，开发了一种新的表面散斑制备方法，从而可以在扫描电镜下实现在原位加载过程中对相同区域进行重复的电子背散射衍射（EBSD）和数字图像相关（DIC）测量。这些测量提供了先前无法获得的有关局部塑性应变不均匀性与晶格旋转和梯度之间关系的信息，并揭示了双峰粒度样品中粗细晶粒所承担的塑性应变分布的不均匀性。具有层状结构的样品被用于探索实现强度和塑形之间的平衡，特别是在宏观应变局部化的延迟开始方面。本项目还研究了由铜（软相）和马氏体不锈钢（硬相）粉末的混合物合成的双相钢类似物。制备了低孔隙率样品，可以实现对双相系统的体积分数，平均晶粒尺寸和两相硬度差的独立控制。使用X射线同步加速器以及常规EBSD和DIC实验探索了这些双相样品在拉伸过程的力学响应。与标准双相钢不同，铜铁两相具有明显分离的衍射峰，可以在屈服期间研究应力分配，而无需进行衍射峰的反卷积处理，从而清楚地揭示了塑性屈服过程中载荷从软相转移到硬相的过程。这些结果通过原位加载过程中较大塑性变形的研究得到了补充，其中DIC和EBSD揭示了由于双相微结构的空间变化而引起的塑性应变不均匀性。