In situ TEM observations and theoretical investigation of the electroplastic effect on deformation mechanisms in FCC Ni and HCP Ti-6Al

Electroplasticity (EP) is defined by pulsed electrical currents during plastic deformation that results in a flow stress reduction and elongation increase prior to failure. It was first discovered in the 1960s in the Soviet Union and since then, more and more research groups have looked at the parameters to be controlled in the test, the effect on changing materials’ mechanical properties and the underlying mechanisms by theoretical modeling. Among many theories, Joule heating (a thermal effect) and the EP effect (an athermal effect, i.e. electron wind) caused by the electrical current are the two most well accepted mechanisms. Building up from the findings with temperature tracking and ex situ / in situ material structural analysis studying the microstructures change during the test, we have looked at the major contribution of plastic deformation in microscale level, the dislocation dynamics, changed with the applied external electrical current./nThe complexity of the experimental research into electroplasticity lies both in 1) achieving a stable condition for conducting repeatable, confirmable and clear in situ experiments, and 2) developing advanced data processing techniques with contrast analysis and digital image correlation to precisely tracking all the nanoscale evolutions. This thesis details the development of new experimental techniques to address these challenges. First, preloading and a comprehensive data processing method were developed to detect the precise position of the defects and the evolution of the geometry of a sample under a steady-state plastic deformation condition. Digital image correlation (DIC) and computational analysis was optimized to precisely capture the dislocation slip traces and therefore dislocation motion by a brightness subtraction technique at each pixel over consecutive diffraction contrast images. This technique provided temporal and spatial information for the structure evolution of the sample in each in situ test with high resolution. Experiments were performed with electromechanical MEMS devices, which allowed for direct observations of dislocation motion and the correlation of both applied mechanical and electrical control. Moreover, theoretical study by molecular dynamics on the dislocation behavior under similar conditions as the experiments were also performed to extrapolate the activation parameters for individual dislocation nucleation and depinning events. As a result of the experiments reported here, we concluded that the applied electrical current could make the deformation more uniform and reduce the stress localization in Nickel. Experimental and theoretical analysis on the single dislocation depinning process in hcp Ti-6Al showed that the applied electrical current could supply an external energy, change the local structure of the crystal, and promote dislocation cross-slip onto different slip planes. Our results showed that precise and quantitative correlation of load-displacement relations and dislocation behavior within in situ TEM electro-mechanical tests can provide valuable data to further improving the understanding of nanomechanical properties related to electrical pulsing. This experimental framework described in this thesis will provide a scientific foundation for the design and optimization of alloys with enhanced electroformability for targeted manufacturing applications.

Advisor: Minor, Andrew M.