Fundamental Understanding of Superplasticity in Nanocrystalline Metals

9903321慕克吉探索一种生产纳米晶材料的方法组合，该实验计划研究纳米结构金属和金属间化合物合金的超塑性变形。以往的实验研究已经揭示了纳米结构材料的高流动应力、广泛的应变硬化以及微结构不稳定性和超塑性之间的相关性。为进行精密机械试验，研制了一种特殊设计和仪器的拉力试验装置。用透射电子显微镜、X射线衍射仪、差示扫描量热仪和内耗法研究了变形前后的纳米结构。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。特别强调了纳米金属基质中晶内位错产生的困难，以及它对现有超塑性模型中滑移调节过程的影响。考察了晶界结构状态和晶界位错在晶界滑移过程中的作用。一个相关的方面被考虑到，观察到的超塑性应变速率的增加和/或超塑性温度随着晶粒度的减小而降低是否是普遍现象。%这是一个更新的建议，旨在获得对纳米材料超塑性的基本理解。沿晶界滑移现象是超塑性的主要流变特征之一。超细晶纳米晶材料含有大量的界面，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.***