Abstract: Atom probe tomography (APT) is a three-dimensional characterization technique that ideally can resolve both positions and chemical identities of the atoms in a material. Unlike “focused-beam” microscopy techniques which rely on X-rays or electron beams for imaging, in APT, atoms in the sample are imaged by themselves. Individual atoms or molecules are field-evaporated from the surface of a needle-shape specimen under an intense electric field and fly towards a two-dimensional detector where their impact positions and sequence are recorded. From that, along with the chemical identities revealed by a mass spectrometer, a three-dimensional distribution of the atoms in the specimen can be reconstructed. However, since field evaporation is a destructive process, it is impossible to verify reconstruction results and quantify uncertainties in experiments. In this case, atomic-scale forward modeling becomes the only viable way to produce verifiable virtual data to test reconstruction where each single atom is traceable. A number of atomic modeling approaches have been developed during the past 25 years, however, all of them are implicitly based on harmonic transition state theory which can only predict the rate of transition from one state to another but not describe any dynamics between the two states. As an alternative, we propose to simulate field evaporation with full dynamics using molecular dynamics (MD) simulations. For that, we have integrated field evaporation events as part of the MD simulation by combining the electrostatics from the finite element field evaporation code TAPSim with the MD simulator LAMMPS. With full dynamics, atoms in the specimen are evaporated in an “ab-initio” way as a result of the competition between the interatomic forces and the electrostatic forces. To demonstrate our full-dynamics approach, we will show results that explain for the first time the enhanced zone lines in field evaporation maps, “ab-initio” prediction of the evaporation sequence in [001]-oriented γ-TiAl intermetallic compounds explaining the observed artifact of mixed layers, and simulations of GP-zones in Al-Cu alloys that demonstrate the inherent inaccuracies in resolving atomic positions. At the end, we discuss if there are ways to take the quantification capability of the APT technique to the next level and what they may be.

Bio: Wolfgang Windl is Professor in the Departments of Materials Science and Engineering and Physics at The Ohio State University where he was also responsible for the graduate studies program from 2015-19. Outside of OSU, Dr. Windl is co-founder and vice president of the device company GonioTech. Before joining OSU in 2001, he spent four years with Motorola, ending his tenure as Principal Staff Scientist in the Digital DNA Laboratories in Austin, TX. Previously, he held postdoctoral positions at Arizona State University and Los Alamos National Laboratory. He received his diploma and doctoral degree in physics from the University of Regensburg, Germany. 
Among others, Wolfgang received the inaugural Fraunhofer-Bessel Research Award jointly from the German Humboldt Foundation and Fraunhofer Society in 2006; two Patent and Licensing Awards from Los Alamos National Laboratory in 1998 and 1999, where he was also named 2023 Institute of Materials Science Distinguished Faculty Scholar; 2019 Best Paper in Graduate Studies and Best Diversity Paper Awards from ASEE; the 2020 Faculty Diversity Excellence Award, four Lumley Research Awards, and the 2015 Boyer Award for Excellence in Undergraduate Teaching from the College of Engineering; and 2006 and 2015 Mars Fontana Best Teacher Awards from his department.