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

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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.