Fundamental Understanding of Deformation Mechanisms in Nanocrystalline Superplasticity

本研究旨在对纳米晶材料的塑性变形机制有一个基本的了解。纳米结构的金属材料将通过几种加工方法生产，包括铸造材料的高压扭转、脉冲电沉积和金属玻璃的结晶。沿晶粒界面滑动现象是高温塑性流变学的主要特征之一。超细晶纳米晶材料具有非常大的界面密度，为研究高温塑性中晶界的结构状态及其对晶界滑动的影响提供了独特的机会。智力上的优点在于试图将微观结构信息与从这些纳米级材料中获得的力学数据联系起来，这些数据是在真正缩小长度尺度的塑性背景下获得的。特别强调的是，在真正的纳米尺度结构中，在基体内部产生晶粒内位错的困难及其对当前纳米晶体高温塑性模型中的滑移调节过程的影响。研究了观察到的超塑性随晶粒尺寸的减小而增加是否是一种普遍现象。提高超塑性应变率将缩短成形时间，使之成为一种经济可行的工艺。降低超塑性成形温度将使一些现有的成形技术应用于工业上重要的金属间结构材料的车间实践。该结果有望在具有复杂形状的传感器和设备的成型技术上具有重要意义，这些传感器和设备可以受益于具有高环境温度强度和硬度的纳米晶基体。

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.