Micromechanics Model for Nanovoid Growth in Nanocrystalline Materials

2013 ◽  
Vol 364 ◽  
pp. 754-759 ◽  
Author(s):  
Jian Qiu Zhou ◽  
Lu Wang ◽  
Zhi Xiong Ye
Keyword(s):  
Grain Size ◽  
Theoretical Model ◽  
Stress Field ◽  
Grain Boundary ◽  
Surface Stress ◽  
Nanocrystalline Materials ◽  
Critical Stress ◽  
Dislocation Emission ◽  
Nanovoid Growth ◽  
Remote Stress

A theoretical model to describe the nanovoid growth by emission dislocation shear loop in nanocrystalline metal under equal biaxial remote stress was developed. The critical stress for emission of dislocation was derived by considering the effects of surface stress. Within our description, dislocations emitted from surface of nanovoid were piled up at grain boundaries and the stress field generated by arrested dislocations can prevent further dislocation emission. The effect of grain boundary of nanocrystalline materials on nanovoid growth was investigated, and the results showed that the smaller of the grain size, the harder for the nanovoid growth.

Dislocation emission criterion: Grain boundary effect

1993 ◽  
Vol 8 (8) ◽  
pp. 1853-1857 ◽  
Author(s):  
Sham-Tsong Shiue ◽  
Tong-Yi Zhang ◽  
Sanboh Lee
Keyword(s):  
Grain Size ◽  
Stress Intensity ◽  
Grain Boundary ◽  
Plastic Zone ◽  
Critical Stress ◽  
Boundary Effect ◽  
Critical Stress Intensity Factor ◽  
Dislocation Emission ◽  
Brittle Behavior ◽  
Free Zone

Based on the results of Shiue and Lee [J. Appl. Phys. 70, 2947 (1991)], the effect of plastic zone and grain boundary on the dislocation emission criterion was investigated. The emission criterion is based on the concept of spontaneous emission. The critical stress intensity factor for dislocation emission increases with the increasing size of dislocation-free zone and the number of piled-up dislocations in the plastic zone, but decreases with increasing grain size. The ductile versus brittle behavior of material was determined by the competition of critical stress intensity factors for dislocation emission and crack propagation. A material with larger grain size is easier to emit dislocation and allows more dislocations to be piled up, so that it behaves more ductile.

Coupled effect of grain boundary sliding and dislocation emission on fracture toughness of nanocrystalline materials

2016 ◽  
Vol 01 (02) ◽  
pp. 1650008 ◽  
Author(s):  
Q. H. Fang ◽  
L. C. Zhang
Keyword(s):  
Grain Size ◽  
Grain Boundary ◽  
Crack Growth ◽  
Nanocrystalline Materials ◽  
Crack Tip ◽  
Grain Boundary Sliding ◽  
Dislocation Emission ◽  
Disclination Dipole ◽  
Coupled Effect ◽  
Wedge Disclination

This paper establishes a theoretical model to explore the coupled effect of grain boundary (GB) sliding deformation and crack tip dislocation emission on the critical stress intensity factor (SIF) for crack growth in ultrafine-grained and nanocrystalline materials (NCMs). The model postulates that the stress concentration near a crack tip initiates GB sliding. It is found that GB sliding leads to the formation of wedge disclination dipole at the triple junctions of grain boundaries. Under the external load and stress fields produced by wedge disclinations, dislocations are emitted from crack tips but will stop at the opposite GBs. The influence of the wedge disclination dipole and the dislocation emitted from crack tip on the critical SIF for crack growth is investigated. The model prediction shows that the critical SIF varies with the decrement of grain size, and that there is a critical grain size corresponding to a minimum value of SIF. Compared with the pure brittle fracture in NCMs at the grain sizes of tens of nanometers, the combined deformation mechanisms can bring an increase of the critical SIF for crack growth.

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

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 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.

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.

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.

scholarly journals Numerical Study of Thermal Shock Damage Mechanism of Polycrystalline Ceramics

2021 ◽  
Vol 8 ◽  
Author(s):  
Zhenyuan Gong ◽  
Kang Guan ◽  
Pinggen Rao ◽  
Qingfeng Zeng ◽  
Jiantao Liu ◽  
...  
Keyword(s):  
Grain Size ◽  
Grain Boundary ◽  
Thermal Shock ◽  
Thermal Shock Resistance ◽  
Critical Stress ◽  
Crack Nucleation ◽  
Damage Mechanism ◽  
Shock Resistance ◽  
Grain Boundary Fracture ◽  
Boundary Fracture

A dual-scale model is proposed to study the effect of microstructure parameters (grain size and grain boundary fracture energy) on the thermal shock damage mechanism on an example of alumina. At microscale, representative volume element (RVE) models generated by Voronoi tessellation are simulated to obtain the mechanical parameters for macro models. At macroscale, a coupled thermomechanical model based on the finite–discrete element method (FDEM) is applied to simulate the crack nucleation and propagation. Energy dissipation (ALLDMD) is introduced to investigate the thermal shock cracking mechanism by combining crack patterns and crack density, which indicates that decreasing grain size and increasing grain boundary fracture energy have a positive effect on thermal shock resistance. The proposed models not only predict the critical stress temperature which is well consistent to the theoretical thermal shock resistance factor, but also quantify the two previously unconsidered stages (crack nucleation and crack instability stage). Our models suggest the crack nucleation and instability will not occur immediately when the model reaches critical stress, but the models can sustain for higher temperature difference. The thermal shock damage mechanism and the influence of microstructural parameters on thermal shock resistance have also been discussed in detail.

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