Micromechanisms of the electro-plastic effect in magnesium alloys investigated by means of electron microscopy

“电塑性效应”是多种物理效应同时作用的结果，其中一些效应的量级相当小。该项目的目标是深入了解各种个体效应对整体观测的相对贡献。这种详细的了解是这种效应未来可能的技术应用的先决条件。主要的影响是由施加在试样上的强电流脉冲引起的焦耳加热。虽然由于脉冲持续时间短，宏观温升相对较小，但104 K/s量级的加热速率导致试样中存在明显的热应力。研究这一贡献将通过高应变率测试和负载路径的变化来完成，以区分由于试样的约束膨胀而产生的快速热应力的影响。现场电子显微镜观察将被用来观察特定的微观结构特征的发展，如由多个电流脉冲产生的孪晶形成。为了研究直接电子-位错相互作用是否对整体效应有相关贡献，在保持机械载荷和位错运动恒定的情况下，将改变通过试样的电流方向。这些实验很可能允许建立这种影响程度的上限。这将通过观察由耦合机电载荷引起的所有位错运动的宏观响应以及直接观察统计上显著数量的单独观察到的位错过程来实现。所有这些观察都将使用高度敏感的微观结构观察工具和通过测量更大的数据集来进行。为了改进用电子通道对比成像（ECCI）对钛进行的直接位错观察，我们将包括电子通道对比成像（ECCI）实验。对钛的研究也将有助于进一步了解晶格结构与成分的影响。所有的研究都将辅以晶体塑性模拟和基于机器学习的新图像处理工具。总的来说，这些拟议的实验，由位错动力学模拟支持，将允许区分均匀加热的影响，非均匀加热（即所谓的“热点”的形成）和剩余的非热贡献的电塑性效应。

The “electroplastic effect” is the result of a variety of concurrently acting physical effects, some of which are quite small in magnitude. The goal of this project is to gain insights into the relative contributions of the various individual effects to the overall observations. This detailed understanding is a prerequisite for possible future technological applications of this effect.The dominating effect is Joule heating caused by the intense current pulses applied to the specimen. While the macroscopic temperature rise is relatively small due to the short pulse duration, heating rates in the magnitude of 104 K/s lead to significant thermal stresses in the specimen. Investigating this contribution will be accomplished by elevated strain rate testing and variation in load paths to distinguish the effects of fast thermal stresses due to constrained expansion of the specimen.In-situ electron microscopy observations will be employed to observe the development of specific microstructural features such as twin formations resulting from multiple current pulses.To investigate whether direct electron-dislocation interactions have a relevant contribution to the overall effect, the direction of the current through the specimen will be varied while keeping the mechanical loading and therefore the dislocation motion constant. These experiments will likely allow for establishing an upper bound on the magnitude of this effect. This will be accomplished by both observing the macroscopic response of all dislocation motion resulting from the coupled electro-mechanical loading as well as the direct observation of a statistically significant number of individually observed dislocation processes.All these observations will be conducted using highly sensitive microstructure observation tools and by the measurement of significantly larger data sets. To improve direct dislocation observation using electron channelling contrast imaging (ECCI) experiments on titanium will be included. Investigating titanium will also allow further insights into the effect of lattice structure vs. composition. All investigations will be complemented by crystal plasticity simulations and new image processing tools based on machine learning. In total these proposed experiments, supported by dislocation dynamics simulations, will allow to distinguish between effects of homogeneous heating, inhomogeneous heating (i. e. the formation of so-called “hot spots”) and the remaining non-thermal contributions to the electroplastic effect.