Percolative composite model for prediction of the properties of nanocrystalline materials

1997 ◽  
Vol 12 (7) ◽  
pp. 1828-1836 ◽  
Author(s):  
Rachman Chaim
Keyword(s):  
Experimental Data ◽  
Grain Size ◽  
Grain Boundary ◽  
Nanocrystalline Materials ◽  
Composite Model ◽  
Grain Sizes ◽  
Percolation Thresholds ◽  
Definition Of ◽  
Critical Grain Size ◽  
Transport Type

A physical percolating composite model is presented for description of the changes in the transport-type properties with grain size in nanocrystalline materials. The model is based on hierarchial percolation through the different microstructural components such as grain boundaries, triple lines, and quadruple nodes at grain sizes when their respective percolation thresholds are reached. The model yields critical grain sizes at which the properties may change significantly. These grain sizes depend on the grain boundary thickness. Master curves were calculated for the elastic modulus and compared to the experimental data from the literature. Better fit was found with the experimental data in comparison to Hill's approximation model. The critical grain size at grain boundary percolation threshold is suggested as a criterion for definition of materials to exhibit nanocrystalline properties.

Grain size effects in nanocrystalline materials

1992 ◽  
Vol 7 (8) ◽  
pp. 2114-2118 ◽  
Author(s):  
C. Suryanarayana ◽  
D. Mukhopadhyay ◽  
S.N. Patankar ◽  
F.H. Froes
Keyword(s):  
Grain Size ◽  
Grain Boundary ◽  
Size Effects ◽  
Nanocrystalline Materials ◽  
Volume Fraction ◽  
High Hardness ◽  
Careful Analysis ◽  
Grain Sizes ◽  
Critical Grain Size ◽  
Very High

Nanocrystalline materials have a grain size of only a few nanometers and are expected to possess very high hardness and strength values. Even though the hardness/strength is expected to increase with a decrease in grain size, recent observations have indicated that the hardness increases in some cases and decreases in other cases. A careful analysis of the available results on the basis of existing models suggests that there is a critical grain size below which the triple junction volume fraction increases considerably over the grain boundary volume fraction and this is suggested to be responsible for the observed softening at small grain sizes.

Mechanical Properties of Novel Nanocrystalline Materials

2001 ◽  
Author(s):  
J. Narayan ◽  
H. Wang ◽  
A. Kvit
Keyword(s):  
Grain Size ◽  
Grain Boundary ◽  
Nanocrystalline Materials ◽  
High Surface ◽  
Grain Boundary Sliding ◽  
Boundary Shear ◽  
Functionally Gradient ◽  
Boundary Sliding ◽  
First Time ◽  
Critical Grain Size

Abstract We have synthesized nanocrystalline thin films of Cu, Zn, TiN, and WC having uniform grain size in the range of 5 to 100 nm. This was accomplished by introducing a couple of manolayers of materials with high surface and have a weak interaction with the substrate. The hardness measurements of these well-characterized specimens with controlled microstructures show that hardness initially increases with decreasing grain size following the well-known Hall-Petch relationship (H∝d−½). However, there is a critical grain size below which the hardness decreases with decreasing grain size. The experimental evidence for this softening of nanocrystalline materials at very small grain sizes (referred as reverse Hall-Petch effect) is presented for the first time. Most of the plastic deformation in our model is envisioned to be due to a large number of small “sliding events” associated with grain boundary shear or grain boundary sliding. This grain-size dependence of hardness can be used to create functionally gradient materials for improved adhesion and wear among other improved properties.

scholarly journals Molecular Dynamics as a Means to Investigate Grain Size and Strain Rate Effect on Plastic Deformation of 316 L Nanocrystalline Stainless-Steel

Materials ◽  
2020 ◽  
Vol 13 (14) ◽  
pp. 3223 ◽  
Author(s):  
Abdelrahim Husain ◽  
Peiqing La ◽  
Yue Hongzheng ◽  
Sheng Jie
Keyword(s):  
Plastic Deformation ◽  
Grain Size ◽  
Stainless Steel ◽  
Grain Boundary ◽  
Strain Rate ◽  
Deformation Mechanism ◽  
Grain Boundary Sliding ◽  
Plastic Deformation Mechanism ◽  
Grain Sizes ◽  
Critical Grain Size

In the present study, molecular dynamics simulations were employed to investigate the effect of strain rate on the plastic deformation mechanism of nanocrystalline 316 L stainless-steel, wherein there was an average grain of 2.5–11.5 nm at room temperature. The results showed that the critical grain size was 7.7 nm. Below critical grain size, grain boundary activation was dominant (i.e., grain boundary sliding and grain rotation). Above critical grain size, dislocation activities were dominant. There was a slight effect that occurred during the plastic deformation mechanism transition from dislocation-based plasticity to grain boundaries, as a result of the stress rate on larger grain sizes. There was also a greater sensitive on the strain rate for smaller grain sizes than the larger grain sizes. We chose samples of 316 L nanocrystalline stainless-steel with mean grain sizes of 2.5, 4.1, and 9.9 nm. The values of strain rate sensitivity were 0.19, 0.22, and 0.14, respectively. These values indicated that small grain sizes in the plastic deformation mechanism, such as grain boundary sliding and grain boundary rotation, were sensitive to strain rates bigger than those of the larger grain sizes. We found that the stacking fault was formed by partial dislocation in all samples. These stacking faults were obstacles to partial dislocation emission in more sensitive stress rates. Additionally, the results showed that mechanical properties such as yield stress and flow stress increased by increasing the strain rate.

Influence of grain boundary energy on the grain size evolution in nanocrystalline materials

2009 ◽  
Vol 475 (1-2) ◽  
pp. 893-897 ◽  
Author(s):  
Zheng Chen ◽  
Feng Liu ◽  
Wei Yang ◽  
Haifeng Wang ◽  
Gencang Yang ◽  
...  
Keyword(s):  
Grain Size ◽  
Grain Boundary ◽  
Nanocrystalline Materials ◽  
Boundary Energy ◽  
Grain Boundary Energy ◽  
Size Evolution

Simulation of the interaction of plastic zone dislocations with the grain boundary at brittle-plastic transition temperatures in molybdenum

2021 ◽  
Vol 2021 (3) ◽  
pp. 77-85
Author(s):  
K. M. Borysovska ◽  
◽  
N. M. Marchenko ◽  
Yu. M. Podrezov ◽  
S. O. Firstov ◽  
...  
Keyword(s):  
Fracture Toughness ◽  
Grain Size ◽  
Grain Boundary ◽  
Plastic Zone ◽  
Grain Boundaries ◽  
Crack Tip ◽  
Dynamics Simulation ◽  
Density Of Dislocations ◽  
Grain Sizes ◽  
Pile Up

The (DD) method was used to model the formation of the plastic zone of the top of the cracks in polycrystalline molybdenum. Special attention was paid to take into account the interaction of dislocations in the plastic zone with grain boundaries. Structural sensitivity of fracture toughness was analyzed under brittle-ductile condition. Simulations were performed for a range of grain sizes from 400 to 100 μm, at which a sudden increase in fracture toughness with a decrease of grain size was experimentally shown. We calculated the value of K1c taking into account the shielding action of dislocations. The position of all dislocations in the plastic zone at fracture moment was calculated. Based on these data, we obtained the dependences of dislocation density on the distance from the crack tip thereby confirming significant influence of the grain boundaries on plastic zone formation. At large grain sizes, when the plastic zone does not touch the boundary, the distribution of dislocations remained unchanged. As grains reduce their size to size of the plastic zone, they start formating a dislocation pile – up near the boundaries. Dislocations on plastic zone move slightly toward the crack tip, but the density of dislocations in the middle of the grain remains unchanged, and fracture toughness remains almost unchanged. Further reduction of the grain size leads to the Frank-Reed source activation on the grain boundary Forming dislocation pile-up of the neighbor grains. Its stress concentration acts on dislocations of the first grain and causes redistribution of plastic zone dislocations. If the reduction in grain size is not enough to form a strong pile-up, density of dislocations on plastic zone increases slightly and crack resistance increases a few percent. Further reduction of grains promotes strong pile-up, dislocations move to crack tip, and its density on plastic zone increases. Crack is shielded and fracture toughness increases sharply. The calculation showed that the fracture toughness jump is observed at grain sizes of 100—150 μm, in good agreement with the experiment. Keywords: dislocation dynamics simulation, molybdenum, fracture toughness, grain size, plastic zone, brittle-ductile transition.

Deformation mechanisms in natural dolomite mylonites from a detachment system (Mt. Hymittos, Greece)

2021 ◽  
Author(s):  
Mark Coleman ◽  
Bernhard Grasemann ◽  
David Schneider ◽  
Konstantinos Soukis ◽  
Riccardo Graziani
Keyword(s):  
Grain Size ◽  
Grain Boundary ◽  
Boundary Migration ◽  
Size Reduction ◽  
Greenschist Facies ◽  
Normal Faults ◽  
Grain Boundary Migration ◽  
Grain Sizes ◽  
Subgrain Rotation ◽  
Middle Unit

<p>Microstructures may be used to determine the processes, conditions and kinematics under which deformation occurred. For a given set of these variables, different microstructures are observed in various materials due to the material’s physical properties. Dolomite is a major rock forming mineral, yet the mechanics of dolomite are understudied compared to other ubiquitous minerals such as quartz, feldspar, and calcite. Our new study uses petrographic, structural and electron back scatter diffraction analyses on a series of dolomitic and calcitic mylonites to document differences in deformation styles under similar metamorphic conditions. The Attic-Cycladic Crystalline Complex, Greece, comprises a series of core complexes wherein Miocene low-angle detachment systems offset and juxtapose a footwall of high-pressure metamorphosed rocks against a low-grade hanging wall. This recent tectonic history renders the region an excellent natural laboratory for studying the interplay of the processes that accommodate deformation. The bedrock of Mt. Hymittos, Attica, preserves a pair of ductile-then-brittle normal faults dividing a tripartite tectonostratigraphy. Field observations, mineral assemblages and observable microstructures suggests the tectonic packages decrease in metamorphic grade from upper greenschist facies (~470 °C at 0.8 GPa) in the stratigraphically lowest package to sub-greenschist facies in the stratigraphically highest package. Both low-angle normal faults exhibit cataclastic fault cores that grade into the schists and marbles of their respective hanging walls. The middle and lower tectonostratigraphic packages exhibit dolomitic and calcitic marbles that experienced similar geologic histories of subduction and exhumation. The mineralogically distinct units (calcite vs. dolomite) of the middle package deformed via different mechanisms under the same conditions within the same package and may be contrasted with mineralogically similar units that deformed under higher pressure and temperature conditions in the lower package. In the middle unit, dolomitic rocks are brittlely deformed. Middle unit calcitic marble are mylonitic to ultramylonitic with average grain sizes ranging from 30 to 8 μm. These mylonites evince grain-boundary migration and grain size reduction facilitated by subgrain rotation. Within the lower package, dolomitic and calcitic rocks are both mylonitic to ultramylonitic with grain sizes ranging from 28 to 5 μm and preserve clear crystallographic preferred orientation fabrics. Calcitic mylonites exhibit deformation microstructures similar to those of the middle unit. Distinctively, the dolomitic mylonites of the lower unit reveal ultramylonite bands cross-cutting and overprinting an older coarser mylonitic fabric. Correlated missorientation angles suggest these ultramylonites show evidence for grain size reduction accommodated by microfracturing and subgrain rotation. In other samples the dolomitic ultramylonite is the dominant fabric and is overprinting and causing boudinage of veins and relict coarse mylonite zones. Isolated interstitial calcite grains within dolomite ultramylonites are signatures of localized creep-cavitation processes. Following grain size reduction, grain boundary sliding dominantly accommodated further deformation in the ultramylonitic portions of the samples as indicated by randomly distributed correlated misorientation angles. This study finds that natural deformation of dolomitic rocks may occur by different mechanisms than those identified by published experiments; notably that grain-boundary migration and subgrain rotation may be active in dolomite at much lower temperatures than previously suggested.</p>

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

2019 ◽  
Vol 51 (1) ◽  
pp. 513-530 ◽  
Author(s):  
Zhenbo Zhang ◽  
Éva Ódor ◽  
Diana Farkas ◽  
Bertalan Jóni ◽  
Gábor Ribárik ◽  
...  
Keyword(s):  
Grain Size ◽  
Grain Boundary ◽  
Nanocrystalline Materials ◽  
Md Simulation ◽  
High Pressure Torsion ◽  
Md Simulations ◽  
Line Profile Analysis ◽  
Misfit Dislocations ◽  
X Ray ◽  
Average Grain Size

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 γ ≅ 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.

What is Behind the Inverse Hall-Petch Behavior in Nanocrystalline Materials?

2006 ◽  
Vol 976 ◽  
Author(s):  
Christopher Carlton ◽  
P. J. Ferreira
Keyword(s):  
Grain Size ◽  
Yield Strength ◽  
Strain Rate ◽  
Grain Boundaries ◽  
Nanocrystalline Materials ◽  
Critical Value ◽  
Petch Relationship ◽  
And Diffusion ◽  
Critical Grain Size

AbstractAn inverse Hall-Petch effect has been observed for nanocrystalline materials by a large number of researchers. This result implies that nanocrystalline materials get softer as grain size is reduced below a critical value. Postulated explanations for this behavior include dislocation based mechanisms and diffusion based mechanisms. In this paper, we report an explanation for the inverse Hall-Petch effect based on the statistical absorption of dislocations by grain boundaries, showing that the yield strength is both dependent on strain rate and temperature, and that it deviates from the Hall-Petch relationship at a critical grain size.

The Inverse Hall-Petch Effect—Fact or Artifact?

2000 ◽  
Vol 634 ◽  
Author(s):  
Carl C. Koch ◽  
J. Narayan
Keyword(s):  
Grain Size ◽  
Size Distribution ◽  
Grain Size Distribution ◽  
Computer Simulations ◽  
Nanocrystalline Materials ◽  
Grain Sizes ◽  
Average Grain Size

ABSTRACTThis paper critically reviews the data in the literature which gives softening—the inverse Hall-Petch effect—at the finest nanoscale grain sizes. The difficulties with obtaining artifactfree samples of nanocrystalline materials will be discussed along with the problems of measurement of the average grain size distribution. Computer simulations which predict the inverse Hall-Petch effect are also noted as well as the models which have been proposed for the effect. It is concluded that while only a few of the experiments which have reported the inverse Hall-Petch effect are free from obvious or possible artifacts, these few along with the predictions of computer simulations suggest it is real. However, it seems that it should only be observed for grain sizes less than about 10 nm.

Electrical Resistivity as a Characterization Tool for Nanocrystalline Metals

1999 ◽  
Vol 581 ◽  
Author(s):  
J.L. McCrea ◽  
K.T. Aust ◽  
G. Palumbo ◽  
U. Erb
Keyword(s):  
Thermal Stability ◽  
Grain Size ◽  
Electrical Resistivity ◽  
Grain Boundary ◽  
Nanocrystalline Materials ◽  
Elevated Temperatures ◽  
Polycrystalline Materials ◽  
Grain Growth Kinetics ◽  
Insight Into ◽  
Grain Boundary Resistivity

ABSTRACTThe electrical resistivity as a function of temperature (4K to 673K) of several electrodeposited nanocrystalline materials (Ni, Ni-Fe, Co) has been examined. The contribution of the grain boundaries to the electrical resistivity was quantified in terms of a specific grain boundary resistivity, which was found to be similar to previously reported values of specific grain boundary resistivity for copper and aluminum obtained from studies involving polycrystalline materials. In the high temperature range, the resistivity of the nanocrystalline samples was monitored as a function of time. The observed time dependence of the resistivity at elevated temperatures was correlated to microstructural changes in the material. The study has shown that electrical resistivity is an excellent characterization tool for nanocrystalline materials giving useful information regarding grain size and degree of thermal stability, as well as some insight into the grain growth kinetics at various temperatures.

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