Guest seminar: Unraveling complex nanoscale deformation mechanisms by quantitative in situ mechanical testing

Tijmen Vermeij¹, Philipp Kroeker¹, Amit Sharma^1,2, Johann Michler¹, Xavier Maeder¹

¹ Laboratory for Mechanics of Materials and Nanostructures, EMPA, Thun, Switzerland

² Swiss Cluster AG, Spiez, Switzerland

tijmen.vermeij empa.ch

Advancements in nano- and microscale characterization techniques are crucial for understanding the complex plastic deformation mechanisms of alloys, thin films and nanolaminates. Post-mortem characterization of deformed samples, using Scanning or Transmission Electron Microscopy (SEM/TEM), can provide valuable insights into the deformation mechanisms. However, to quantify the full evolution of plasticity and the interactions between different mechanisms, in-situ nano- and micromechanics are required. While Electron Backscatter Diffraction (EBSD), performed in a SEM, enables (in situ) measurement of crystallography, it is limited in spatial resolution. Alternatively, Transmission Electron Microscopy (TEM) can provide atomic resolution but is less accessible/convenient for in situ testing. Therefore, a compromise is required, for which Transmission Kikuchi Diffraction (TKD), which is essentially EBSD in transmission, is highly promising.

In this seminar, I will outline a novel methodology for in situ TKD tensile testing. By integrating a modified in situ SEM nanoindenter with a microscale push-to-pull device and a conventional EBSD detector, we achieve TKD measurements at high spatial resolution during mechanical deformation. A dedicated focused ion beam procedure was developed for site-specific specimen fabrication, including lift-out, thinning, and shaping into a dog-bone geometry [1]. The methodology is demonstrated on several case studies: (i) a metastable β-Ti single crystal, on which we quantify the initiation and evolution of nanoscale twinning and stress-induced martensitic transformation, (ii) an HCP Re single crystal loaded along the c-axis showing complex nanoscale twinning and twin-to-twin interactions, and (iii) a CuAl/Al₂O₃ nanolaminate, which exhibited nanoscale plasticity and twinning/detwinning in a complex microstructure. Furthermore, we explore the possibilities for performing elastic and plastic strain measurements using HR-TKD and SEM-based Digital Image Correlation (DIC), respectively, alongside regular TKD mapping. Overall, this approach provides a robust alternative to in situ EBSD and TEM testing, with the potential of providing local stress-strain data, facilitating detailed analysis of deformation mechanisms at the nanoscale.

Tijmen Vermeij – Short biography

Tijmen Vermeij is a postdoctoral researcher at EMPA's Laboratory for Mechanics of Materials and Nanostructures in Thun, Switzerland, specializing in advanced experimental methods for studying fundamental deformation mechanisms in metallic materials. He earned his PhD (cum laude) in Mechanics of Materials from Eindhoven University of Technology (TU/e) in 2023, where he worked on identification of plasticity and damage in multi-phase steels. Funded by an NWO Rubicon grant, his current interests involve high-resolution EBSD, nanoscale DIC, and in situ nano/micromechanical testing to unravel twinning and phase transformations in various materials.

E-mail: tijmen.vermeij empa.ch
Google Scholar: https://scholar.google.com/citations?user=htxp7_kAAAAJ&hl=en