Dislocations in Grain Boundary Regions: The Origin of Heterogeneous Microstrains in Nanocrystalline Materials

Abstract

Nanocrystalline materials reveal excellent mechanical properties but the mechanism by which they deform is still debated. X-ray line broadening indicates the presence of large heterogeneous strains even when the average grain size is smaller than 10 nm. Although the primary sources of heterogeneous strains are dislocations, their direct observation in nanocrystalline materials is challenging. In order to identify the source of heterogeneous strains in nanocrystalline materials, we prepared Pd-10 pct Au specimens by inert gas condensation and applied high-pressure torsion (HPT) up to c @ 21. High-resolution transmission electron microscopy (HRTEM) and molecular dynamic (MD) simulations are used to investigate the dislocation structure in the grain interiors and in the grain boundary (GB) regions in the as-prepared and HPT-deformed specimens. Our results show that most of the GBs contain lattice dislocations with high densities. The average dislocation densities determined by HRTEM and MD simulation are in good correlation with the values provided by X-ray line profile analysis. Strain distribution determined by MD simulation is shown to follow the Krivoglaz-Wilkens strain function of dislocations. Experiments, MD simulations, and theoretical analysis all prove that the sources of strain broadening in X-ray diffraction of nanocrystalline materials are lattice dislocations in the GB region. The results are discussed in terms of misfit dislocations emanating in the GB regions reducing elastic strain compatibility. The results provide fundamental new insight for understanding the role of GBs in plastic deformation in both nanograin and coarse grain materials of any grain size.

Dislocation–grain boundary interactions: recent advances on the underlying mechanisms studied via nanoindentation testing

“…The strain accumulation in the vicinity of the GBs also been studied via Electron Backscattered Diffraction (EBSD) [46][47][48]. To study the GB-dislocation interactions at the atomic scale, Transmission Electron Microscopy (TEM) has been utilized in the literature [14,[49][50][51]. A small dimension specimen's mechanical response is different as compared to the bulk material.…”

Magnetron Sputter-Deposited β-Ga2O3 Films on c-Sapphire Substrate: Effect of Rapid Thermal Annealing Temperature on Crystalline Quality

“…Detailed information on the lattice dislocation structure of the nanocrystals in the film at 800 • C was obtained by the IFFT of the TEM image. The IFFT image, obtained from the center of the TEM image in Figure 5a, shows linear defects, including an edge dislocation (T-shaped symbol) and screw dislocations (red-dashed rectangular frames) [61].…”

Study of The Electrical, Optical and Morphological Properties in Submicron and Microstructured ZnO Thin Films Obtained by Spin Coating and Chemical Bath Deposition

“…However, the di erence between D values obtained from the Scherrer formula and W-H methods is large, showing that is essential to estimate D by considering both crystallite size and lattice strain contributions in the widening of the di raction peaks. This large di erence was associated with the great contribution of heterogeneous microstrains in the widening of the di raction peaks (Zhang et al, 2020) . These microstrains are due to dislocations in grain boundaries regions (Zhang et al, 2020) .…”

“…This large di erence was associated with the great contribution of heterogeneous microstrains in the widening of the di raction peaks (Zhang et al, 2020) . These microstrains are due to dislocations in grain boundaries regions (Zhang et al, 2020) . Zhang et al (2020) conclude in their work that assuming grain boundary structures in nanocrystalline and coarse grain materials are very similar, then the dislocation density in the grain boundary (GB) regions is almost like a material constant depending mostly on the misorientation and structure of GBs.…”