IMX Seminar Series - Grain Boundaries: An “old” Defect Gaining New Momentum in Materials Science

Grain boundaries (GBs) are omnipresent in most materials and often decisive for the resulting material properties. Electrical resistivity in metals increases by the additional scattering of electrons at GBs, while for ceramic varistors the electrical activity of GBs leads to highly non-linear electrical conductivity. Many metals fail by segregation induced embrittlement of GBs – examples are S in Ni or Zn in Fe. However, GB segregation may also “stabilize” nanocrystalline metals by reducing GB energy (and often also kinetics) to prevent spontaneous grain growth, which leads to high strength materials due to the numerous GBs acting as obstacles for dislocation motion.
Recently, it has been resolved that GB can undergo structural transitions triggered by temperature, pressure and chemical composition, and thus properties may alter as a function of these variables. In ceramics like alpha Al₂O₃ it is debated if such transitions are the cause of abnormal grain growth. In the present talk newest results obtained by atomic resolved transmission electron microscopy for metallic materials are provided, shedding light on the GB phase transitions also termed “complexion” transitions in literature to discriminate it from bulk phases. Examples include Al and Cu in pure and alloyed form as well as Fe based materials with multiple alloying additions. Techniques to address material properties for single GBs are briefly discussed.
Bio: Gerhard Dehm is director at the Max-Planck-Institut für Eisenforschung in Düsseldorf and professor at the Ruhr-Universität-Bochum. From 2005 to 2012 he was head of the department Materials Physics at the Montanuniversität Leoben (Austria), and managing director of the Erich Schmid Institute of the Austrian Academy of Sciences. His research focusses on advanced S/TEM and in-situ microscopy of interfaces, their structure, chemistry, stability, and impact on material properties. This research directions was recently awarded with an ERC Advanced Grant. A further cornerstone of his research is small scale mechanical testing to provide fundamental understanding of local mechanical properties of materials.