Fundamental Understanding of Deformation Mechanisms in Nanocrystalline Superplasticity

这一建议旨在从根本上了解纳米晶体材料的塑性变形机制。纳米金属材料将通过几种加工方法来生产，包括铸造材料的高压扭转、脉冲电沉积和金属玻璃的晶化。沿晶界滑移现象是高温塑性的主要流变特征之一。超细晶纳米晶材料含有非常大的界面密度，为研究晶界结构状态及其在高温塑性中晶界滑移的作用提供了难得的机会。智能的优点在于试图将微观结构信息与从这种纳米材料获得的力学数据联系起来，在真正减小的长度尺度下的塑性。将特别强调在真正的纳米结构中在基质内产生晶内位错的困难，以及它对当前纳米晶体高温塑性模型中的滑移调节过程的影响。提高超塑性应变速率将缩短成形时间，并使其成为经济可行的工艺。降低超塑性成形温度将允许将一些现有的成形技术用于工业上重要的金属间化合物结构材料的车间实践。这一结果有望在制造复杂形状的传感器和器件方面具有重要的技术意义，这些传感器和器件可以受益于具有较大常温强度和硬度的纳米晶基质。

This proposal is designed to gain a fundamental understanding of the deformation mechanisms of plasticity in nanocrystalline materials. The nanostructured metallic materials will be produced by several processing methods that include high-pressure torsion of cast material, pulsed electrodeposition, and crystallization from metallic glass. The phenomenon of sliding along the grain interfaces is one of the dominant rheological characteristics in elevated temperature plasticity. The ultrafine-grained nanocrystalline materials contain a very large density of interfaces and they offer a unique opportunity to study the structural state of grain boundary and its role on grain boundary sliding in elevated temperature plasticity. The intellectual merit lies in attempting to correlate the microstructural information with the mechanical data obtained from such nanoscale materials in the context of plasticity in really diminished length scales. Special emphasis will be given to the difficulty of intragranular dislocation generation inside the matrix in truly nanoscale structure and its implication to slip accommodation processes in current models of nanocrystalline plasticity at elevated temperatures.The grant explores whether the observed increase in superplasticity with decreasing grain size is a general phenomenon or not. Increasing superplastic strain rate will decrease forming time and will make it an economically viable process. Lowering superplastic forming temperature will allow utilization of some of the existing forming technology for shop-floor practice in industrially significant intermetallic structural materials. The results are expected to be technologically significant in forming of sensors and devices with complex shapes that can benefit from the nanocrystalline matrix with large ambient temperature strength and hardness.