Dislocations in Grain Boundary Regions: The Origin of Heterogeneous Microstrains in Nanocrystalline Materials
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THE role of dislocations in nanocrystalline solids has been an issue of discussion ever since nanocrystalline solids have become a hot topic for application and research.[1–6] There is almost general consensus that when the average grain size is around 20 nm or smaller ZHENBO ZHANG and MICHAEL PREUSS are with the School of Materials, University of Manchester, Oxford Road, Manchester M13 9PL, UK. Contact e-mail: [email protected] E´VA O´DOR, BERTALAN JO´NI, GA´BOR RIBA´RIK, and GE´ZA TICHY are with the Department of Materials Physics, Eo¨tvo¨s Lora´nd University Budapest, PO Box 32, 1518 Budapest, Hungary. DIANA FARKAS is with the Department of Materials Science and Engineering, Virginia Tech, Blacksburg, VA 24060. SREE-HARSHA NANDAM and JULIA IVANISENKO are with the Institute for Nanotechnology, Karlsruhe Institute for Technology, 76021 Karlsruhe, Germany. TAMA´S UNGA´R is with the School of Materials, University of Manchester and also with the Department of Materials Science and Engineering, Virginia Tech. Manuscript submitted April 15, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
the grain interiors are free of dislocations.[2–4,6–10] Obtaining evidence of grain interior structures with a grain size in the range of 20 nm or smaller is very difficult to obtain by Transmission Electron Microscopy (TEM), see for example the following figures from the literature: Figs. 3a and 3b in Reference 11, Fig. 1 in Reference 12, and Fig. 3a in Reference 13. Though the TEM images in the work by Zhang et al.[13] do indicate strain, the resolution is not sufficient to conclude on its origin. Dislocation activity in nanocrystalline materials has been widely investigated by molecular dynamic (MD) simulations.[2–4,7–10,14] In many cases, MD simulations predict that grain boundaries (GBs) emit partial dislocations pulling stacking faults or twin boundaries decorating grain interiors at the end of straining.[2,3,8,10,12] Line profile analysis of X-ray diffraction patterns reveals large microstrains in nanocrystalline materials.[4,5,7–9,15–18] Markmann et al.[4,8] evaluated the full width at half maxima (FWHM) of diffraction patterns of nanocrystalline Pd[19] along with the FWHM of a computer-generated diffraction pattern of an
MD-simulated specimen of the same material. Figure 1 in Reference 4 shows that the measured and computer-generated diffraction patterns are identical. The modified Williamson–Hall plots[20] of the FWHM and the integral breadths of the computer-generated diffraction patterns have large positive slopes indicating the presence of significant microstrains. Both the real and the MD-simulated nanocrystalline Pd specimens have grain sizes of about 10 nm. It was concluded that the microstrains in both the MD-simulated and experimentally measured nanocrystalline specimens are similar to those found in plastically deformed fcc metals, even though no specific lattice defects were introduced during the simulation process.[4] It was further concluded in Reference 6 that the presence of microstrains, i.e., the positive slope i
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