Migration Pinning and Roughening Transition of a Ni Grain Boundary
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TRODUCTION
THE migration of grain boundaries (GBs) is influenced by a dispersion of fine particles through a polycrystalline material, which is called Zener pinning. Those particles impede the migration of GBs by exerting a pinning pressure, which counteracts the driving force for GB migration. While a GB has intersected a particle, the intersection GB area is removed from the system, which is energetically favorable. This is the origin of Zener pinning. The concept was first advanced by Smith[1] in 1948 and since then it has become the basis of any theory which attempts to account for recovery, recrystallization, and grain growth in particle-containing materials[2–6] and the original Zener pinning model has been constantly modified for the past decades.[7–12] Due to the pinning pressure exerted by pinning particles, the net driving force for migration is expected to be lowered near the particles. As is well evidenced, the change in driving force for migration can lead to a structural transition for crystalline interfaces, which is SUNG BO LEE, JINWOOK JUNG, and HEUNG NAM HAN are with the Department of Materials Science and Engineering and Research Institute of Advanced Materials (RIAM), Seoul National University, Seoul 08826, South Korea. Contact e-mail: [email protected], [email protected] SEUNG JO YOO is with the Electron Microscopy Research Center, Korea Basic Science Institute, Daejeon 34133, South Korea. Manuscript submitted September 19, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
characterized by kinetic roughening.[13–17] The concept applies to GBs, which is experimentally verified in SrTiO3, ZnO, GaN, and a-Al2O3 bicrystals.[18–22] These results show that interfaces (including GBs) advance by a lateral motion of steps along the interfaces under a low driving force for migration and that they migrate by a continuous mechanism above a critical driving force.[13–22] However, despite the related results, the effect of migration pinning on GB structural transition has not been explored. In the present study, we for the first time show an indication of kinetic roughening near migration pinning points at the atomic scale by high-resolution transmission electron microscopy (HRTEM). A Ni bicrystalline grain boundary was examined. During TEM specimen preparation by focused ion beam (FIB) method, we intentionally drilled a hole in the middle of the GB as a pinning point against GB migration. The specimen was heated to 600 C. The Ni lamella in the FIB specimen is considered to be under the isostress condition during heating to 600 C, as confirmed by finite element analysis (FEA). The driving force for GB migration originates from both the surface energy anisotropy and the total strain energy reduction. The GB is observed to be strongly bowed around the hole and occasionally GB facets with a plane orientation of {0 3 2}//{1 1 1} appear near the hole. Close to the hole the GB facets are atomically flat or interrupted by atomic steps, but off the hole they become roughened. The observations are considered to be attributed to a
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