Emergent core-shifted grain boundaries at free surfaces X. Zhang∗ CRANN Institute and School of Chemistry, Trinity College Dublin ∗ xiaopuz@tcd.ie Keywords: grain boundaries, scanning tunneling microscopy, surfaces, emergent grain boundaries, wedge disclination, molecular statics Emergent grain boundaries at free surface control material properties such as nanomaterial strength, fracture, and corrosion. Using scanning tunneling microscopy, recently we discovered the rotation of whole adjoining grains at the symmetrical tilted small angle boundaries on (111) nanocrystalline copper films [1]. Geometrical analysis shows the preference for boundaries to shift their tilt axis or boundary cores across the (110) plane towards [112] and ultimately to form low energy [112] core shifted boundaries (CSBs), which is confirmed by molecular statics calculation for all fcc metals [2]. We also find the local rotation of the adjoining grains at the high angle boundaries on nanocrystalline copper films and engineered bicrystal [3]. The local rotation of adjoining grains implied the CSB with tilt axis shifted toward to [112] at the free surface and the boundaries with [111] tilt axis deep inside. In contrast to the rotation of whole adjoining grains at small angle grain boundaries, the restructuring of emergent CSBs at large angle grain boundaries are not well understood. The observed geometry of these emergent boundaries needs to be reproduced by atomic calculations. Here, using atomic calculations that involve a methodical shift of the dislocation core, we confirmed the geometry of emergent boundaries observed in experiment and reconciled the atomic calculations with the elastic analysis through the inclusion of a straight wedge disclination at the free surface [4]. References [1] X. Zhang, J. Han, J.J. Plombon, A.P. Sutton, D.J. Srolovitz, J.J. Boland, Nanocrystalline copper films are never flat, Science 357(6349) (2017) 397-400. [2] X. Zhang, J.J. Boland, Universal preference for low-energy core-shifted grain boundaries at the surfaces of fcc metals, Physical Review Research 5(1) (2023) 013223. [3] X. Zhang, M. Wang, H. Wang, M. Upmanyu, J.J. Boland, Restructuring of emergent grain boundaries at free surfaces – An interplay between core stabilization and elastic stress generation, Acta Mater 242 (2023) 118432. [4] X. Zhang, I. Demirel, J.J. Boland, Wedge disclination description of emergent core-shifted grain boundaries at free surfaces, Scripta Mater 234 (2023) 115562. 75
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