Jaafar El-Awady, Johns Hopkins University, USA
"Acoustic Emission Measurements of Damage Accumulation and Crack Initiation in Metals at the Micron-scale"
Recent advances in small scale experiments have significantly improved the ability to quantify the mechanisms that control deformation and failure in metals. In this talk a new methodology coupling acoustic emission measurements and high frequency in-situ scanning electron microscopy experiments to quantify the evolution of damage and crack initiation in nickel microcrystals will be presented. Mechanical properties are continuously monitored during the cyclic loading through analysis of the different instrument signals. Through the in-situ observations the nucleation and propagation speeds of persistent slip bands (PSBs) was quantified, while postmortem characterizations have led to an unprecedented understanding of the origin of these PSBs. Additionally, quantification of crack initiation and propagation was also analyzed from acoustic emission measurements, and the statistics of these events at different stages of crack propagation (i.e. short crack and long crack regimes) are quantified. Challenges and limitations of the methodology will also be discussed.

Satoshi Hata, Kyushu University, Japan
"Toward Dynamic 3D Visualization of Dislocations by Electron Tomography"
Observation of dislocation dynamics in three-dimensions (3D) is still challenging in the 3DMS research field. The authors’ group recently obtained a preliminary result of subsequently repeating in-situ specimen straining and electron tomography (ET) observation for a steel specimen in which a dislocation was interacting with a spheroidized cementite [1]. Although the observed magnitude of the dislocation movement was small, a few tens nm, the observation demonstrated the importance of 3D observation: the dislocation movement was recognized from some directions while not from the other directions. This different visibility of the dislocation movement is due to the geometrical relationship between the active slip plane and the viewing directions. In other words, a 3D imaging technique is indispensable for visualizing arbitral dislocation movements in such an in-situ straining and observation experiment. There are technical issues to be resolved toward a complete establishment of a 3D dislocation dynamics imaging method using ET. For example, diffraction alignment is an essential part of visualizing dislocations in ET. If we can predict how dislocations move in a specimen, we could keep the diffraction condition during the dislocation movement by precisely setting the crystallographic orientation regarding the loading direction. However, the aligned diffraction condition actually can change during the deformation of the specimen with applied stress. Therefore, the authors propose an alternative way to visualize 3D dislocation dynamics “without” keeping particular diffraction conditions during ET data acquisition. A procedure of the alternative way will be discussed.

Helena Van Swygenhoven, Paul Scherrer Institute & Swiss Federal Institute of Technology Lausanne, Switzerland
"Operando and Insitu Synchrotron Experiments Following Microstructural Evolutions"
Thanks to the high brilliance and the fast detectors available at synchrotrons, operando and insitu experiments have become possible, adding the timescale to microstructural characterization techniques. Such experiments allow identification of operating mechanisms that are responsible for microstructural evolutions. This talk will demonstrate the potential of such experiments. Addressing microstructures of superelastic NiTi alloys, insitu xray diffraction experiments provide understanding why some microstructures can accommodate for changing strain paths, and why degradation during fatigue loading depends strongly on the strain path (Acta Materialia 144(2018)68 and 167(2019)149). Addressing 3D printing, operando selected laser melting of Ti6Al4V during xray diffraction, allows to track with a time resolution of 50µs the dynamics of the á and â phases during fast heating and solidification, providing the cooling rates of each phase and the duration the â phase exists during processing. Such an experiment reveals for instance the link between the cooling rate, the scan vector length and the resulting microstructure (Materials Today, November 2019). This research is performed within the ERC Advanced Grant MULTIAX (339245), PREAMPA and the CCMX-AM3 challenge projects financed by the ETH board and the Swiss watch and precious metal industry.