Fundamental Understanding of Superplasticity in Nanocrystalline Metals

9903321 MukherjeeExploring生产纳米晶材料的方法的组合，这个实验程序检查纳米结构金属和金属间合金的超塑性变形。 先前的实验研究已经揭示了高流动应力，广泛的应变硬化，以及纳米结构材料的微观结构不稳定性和超塑性之间的相关性。 一个专门设计和仪器化的拉伸试验装置已经建成进行精密的机械试验。 采用透射电子显微镜、X射线衍射、差示扫描量热法和内耗法研究了变形前后纳米结构的变化。 科学目标是将微观结构信息与获得的力学数据相关联，以改进超塑性的理论模型。 特别强调的是，在纳米尺度的金属和它的含义滑动住宿过程中，在当前的超塑性模型的矩阵内的晶内位错生成的困难。 晶界结构的状态和晶界位错在界面滑动过程中的作用被检查为晶粒尺寸减小到纳米尺度。 一个相关的方面被认为是，是否观察到的超塑性应变速率的增加和/或超塑性温度随晶粒尺寸减小而降低是一种普遍现象。这是一个更新的建议，旨在获得纳米晶材料的超塑性的基本理解。 超塑性流变学中的主要流变特征之一是晶粒间的沿着滑动现象。 超细晶粒纳米晶材料包含大密度的界面，因此，为研究晶界的结构状态及其在超塑性中的作用提供了独特的机会。

9903321MukherjeeExploring a combination of methods for producing nanocrystalline materials, this experimental program examines superplastic deformation of nanostructured metals and intermetallic alloys. Previous experimental investigations have revealed high flow stress, extensive strain hardening, and correlation between microstructural instability and superplasticity in nanostructured materials. A specially designed and instrumented tensile testing device has been constructed for conducting precision mechanical tests. The nanostructure existing before and after deformation is investigated using transmission electron microscopy, x-ray diffraction, differential scanning calorimetry and internal friction methods. The scientific goal is to correlate the microstructural information with the mechanical data obtained to improve theoretical models of superplasticity. Special emphasis is given to the difficulty of intragranular dislocation generation within the matrix of nanoscaled metals and its implication to slip accommodation processes in current models of superplasticity. The state of grain boundary structure and the role of grain boundary dislocations in the interface sliding process are examined as the grain size decreases to nanoscale dimensions. A related aspect is considered, whether an observed increase in superplastic strain rate and/or decrease in superplastic temperature with decreasing grain size are a general phenomenon.%%%This is a renewal proposal designed to gain fundamental understanding of superplasticity in nanocrystalline materials. The phenomenon of sliding along the grain interfaces is one of the dominant rheological characteristics in superplasticity. Ultrafine grained nanocrystalline materials contain a large density of interfaces and, therefore, offer a unique opportunity to study the structural state of grain boundaries and their role in superplasticity.***