Molecular Dynamics Based Observations of Grain Boundaries and Lattice Defects Functions in Fine Grained Metal

2010 ◽  
Vol 654-656 ◽  
pp. 1582-1585 ◽  
Author(s):  
Toshihiro Kameda ◽  
Bao Rong Zhang
Keyword(s):  
Molecular Dynamics ◽  
Grain Boundaries ◽  
Deformation Process ◽  
Crystal Defects ◽  
View Point ◽  
Dislocation Source ◽  
Fine Grained ◽  
Grain Sizes ◽  
Polycrystalline Metals ◽  
Source And Sink

In order to study the characteristics of fine grained polycrystalline metals, it is important to recognize the function of grain boundaries (GB), crystal defects such as dislocation and/or nanoscale voids, since the fraction of GB increases as grain sizes decreases, the deformation process of these metals could be different from those in larger size grains. In this study, we first evaluate the hypothesis that GB behaves as dislocation source and sink during the deformation of fine grained metal, then compare the behavior between GB and a tiny defect from the view point of dislocation source and sink phenomena. Since continuous dislocation supplies could be considered as the key issue to improve the toughness of fine grained metals, this concept could be helpful to design next generation polycrystalline metals.

A Computational Model for Intergranular Fracture in Nanocrystalline and Ultra-Fine Polycrystalline Metals

2009 ◽  
Vol 633-634 ◽  
pp. 39-53 ◽  
Author(s):  
Bo Wu ◽  
Yue Guang Wei
Keyword(s):  
Computational Model ◽  
Grain Boundaries ◽  
Intergranular Fracture ◽  
Failure Behavior ◽  
Gradient Plasticity ◽  
Deformation And Failure ◽  
Grain Sizes ◽  
Polycrystalline Metals ◽  
Polycrystalline Aggregates ◽  
Strength And Ductility

By means of finite element method which is based on the conventional theory of mechanism-based strain gradient plasticity, cohesive interface model is used to study the intergranular fracture in polycrystalline metals with nanoscale and ultra-fine grains. A systematical study on the overall strength and ductility of polycrystalline aggregates which depend on both grain interiors and grain boundaries for different grain sizes is performed. The results show that the overall strength and ductility of polycrystalline aggregates with nanoscale and ultra-fine grains are strongly related to the competition of grain boundaries deformation with that in grain interiors. Finally, the deformation and failure behavior of nanocrystalline nickel are described by using the computational model.

Effect of Preoxidation and Grain Size on Ductility of a Boron-Doped Ni3Al at Elevated Temperatures

1988 ◽  
Vol 133 ◽  
Author(s):  
M. Takeyama ◽  
C. T. Liu
Keyword(s):  
Grain Boundaries ◽  
Elevated Temperatures ◽  
Short Circuit ◽  
Tensile Tests ◽  
Oxide Formation ◽  
Fine Grained ◽  
Oxygen Penetration ◽  
Grain Sizes ◽  
Boron Doped ◽  
The Difference

ABSTRACTThe ductility of preoxidized Ni3Al (Ni-23Al-0.5Hf-0.2B, at.%) specimens with various grain sizes (17∼193 μm) was evaluated by means of tensile tests at 600 and 760°C in vacuum. It was found that the preoxidation does not affect the ductility of the finest-grained material at either temperature, whereas it causes severe embrittlement in the largest-grained material, especially at 760°C. A continuous, thin Al-rich oxide layer, which forms on the fine-grained samples, protects the underlying alloy from oxygen penetration, preventing any loss of ductility, whereas the nickel-rich oxide which forms on the large-grained samples allows oxygen to penetrate along grain boundaries, causing severe embrittlement. The grain boundaries act as short-circuit paths for rapid diffusion of aluminum atoms from the bulk to the surfaces, and this is responsible for the difference in oxidation behavior between fine- and large-grained materials. The embrittlement of large-grained samples can be eliminated through control of oxide formation on Ni3Al surfaces.

Atomistic Studies of Plasticity in Nanophase Metals

2000 ◽  
Vol 634 ◽  
Author(s):  
H. Van Swygenhoven ◽  
P. Derlet ◽  
A. Caro ◽  
D. Farkas ◽  
M. Caturla ◽  
...  
Keyword(s):  
Molecular Dynamics ◽  
Computer Simulation ◽  
Grain Size ◽  
Grain Boundaries ◽  
Excess Enthalpy ◽  
Uniaxial Stress ◽  
Polycrystalline Materials ◽  
Triple Junctions ◽  
Grain Sizes ◽  
20 Nm

ABSTRACTMolecular dynamics computer simulation of nanocrystalline Ni and Cu with mean grain sizes ranging from 5 to 20 nm show that grain boundaries in nanocrystalline metals have structures similar to most grain boundaries found in conventional polycrystalline materials. Moreover, the excess enthalpy density in grain boundaries and triple junctions appears to be independent of grain in both, computer generated and experimental measured samples. Simulations of deformation under constant uniaxial stress demonstrate a change in deformation mechanism as function of grain size: at the smallest grain sizes all deformation is accommodated in the grain boundaries, at higher grain sizes, intragrain deformation is observed

Constrained minimal-interface structures in polycrystalline copper with extremely fine grains

Science ◽  
2020 ◽  
Vol 370 (6518) ◽  
pp. 831-836
Author(s):  
X. Y. Li ◽  
Z. H. Jin ◽  
X. Zhou ◽  
K. Lu
Keyword(s):  
Grain Boundaries ◽  
Pure Copper ◽  
Three Dimensional ◽  
Fine Grained ◽  
Grain Sizes ◽  
Equilibrium Melting Point ◽  
Polycrystalline Solids ◽  
Dynamics Simulations ◽  
Amorphous States ◽  
Interface Structures

Metals usually exist in the form of polycrystalline solids, which are thermodynamically unstable because of the presence of disordered grain boundaries. Grain boundaries tend to be eliminated through coarsening when heated or by transforming into metastable amorphous states when the grains are small enough. Through experiments and molecular dynamics simulations, we discovered a different type of metastable state for extremely fine-grained polycrystalline pure copper. After we reduced grain sizes to a few nanometers with straining, the grain boundaries in the polycrystals evolved into three-dimensional minimal-interface structures constrained by twin boundary networks. This polycrystalline structure that underlies what we call a Schwarz crystal is stable against grain coarsening, even when close to the equilibrium melting point. The polycrystalline samples also exhibit a strength in the vicinity of the theoretical value.

scholarly journals Structure and Mechanical Behavior of Bulk Nanocrystalline Materials

MRS Bulletin ◽  
1999 ◽  
Vol 24 (2) ◽  
pp. 44-53 ◽  
Author(s):  
J.R. Weertman ◽  
D. Farkas ◽  
K. Hemker ◽  
H. Kung ◽  
M. Mayo ◽  
...  
Keyword(s):  
Mechanical Properties ◽  
Molecular Dynamics ◽  
Grain Size ◽  
Mechanical Behavior ◽  
Nanocrystalline Materials ◽  
Volume Fraction ◽  
Grain Boundary Sliding ◽  
Uniaxial Tensile ◽  
Fine Grained ◽  
Grain Sizes

The reduction of grain size to the nanometer range (˜2-100 nm) has led to many interesting materials properties, including those involving mechanical behavior. In the case of metals, the Hall-Petch equation, which relates the yield stress to the inverse square root of the grain size, predicts great increases in strength with grain refinement. On the other hand, theory indicates that the high volume fraction of interfacial regions leads to increased deformation by grain-boundary sliding in metals with grain size in the low end of the nanocrystalline range. Nanocrystalline ceramics also have desirable properties. Chief among these are lower sintering temperatures and enhanced strain to failure. These two properties acting in combination allow for some unique applications, such as low-temperature diffusion bonding (the direct joining of ceramics to each other using moderate temperatures and pressures). Mechanical properties sometimes are affected by the fact that ceramics in a fine-grained form are stable in a different (usually higher pressure) phase than that which is considered “normal” for the ceramic. To the extent that the mechanical properties of a ceramic are dependent on its crystal-lographic structure, these differences will become evident at the smaller size scales.It is uncertain how deformation takes place in very fine-grained nanocrystalline materials. It has been recognized for some time that the Hall-Petch relationship, which usually is explained on the basis of dislocation pileups at grain boundaries, must break down at grain sizes such that a grain cannot support a pileup. Even some of the basic assumptions of dislocation theory may no longer be appropriate in this size regime. Recently considerable progress has been made in simulating the behavior of extremely fine-grained metals under stress using molecular-dynamics techniques. Molecular-dynamics (MD) simulations of deformation in nanophase Ni and Cu were carried out in the temperature range of 300–500 K, at constant applied uniaxial tensile stresses between 0.05 GPa and 1.5 GPa, on samples with average grain sizes ranging from 3.4 nm to 12 nm.

Nanograin size effects on deformation mechanisms and mechanical properties of nickel: A molecular dynamics study

2021 ◽  
Vol 11 (11) ◽  
pp. 1841-1855
Author(s):  
Alexandre Melhorance Barboza ◽  
Ivan Napoleão Bastos ◽  
Luis César Rodríguez Aliaga
Keyword(s):  
Mechanical Properties ◽  
Molecular Dynamics ◽  
Grain Size ◽  
Stress Relaxation ◽  
Strain Rate ◽  
Grain Boundaries ◽  
Rate Sensitivity ◽  
Deformation Mechanisms ◽  
Triple Junctions ◽  
Grain Sizes

The grain size refinement of metallic materials to the nanometer scale produces interesting properties compared to the coarse-grained counterparts. Their mechanical behavior, however, cannot be explained by the classical deformation mechanisms. Using molecular dynamics simulations, the present work examines the influence of grain size on the deformation mechanisms and mechanical properties of nanocrystalline nickel. Samples with grain sizes from 3.2 to 24.1 nm were created using the Voronoi tessellation method and simulated in tensile and relaxation tests. The yield and ultimate tensile stresses follow an inverse Hall-Petch relationship for grain sizes below ca. 20 nm. For samples within the conventional Hall-Petch regime, no perfect dislocations were observed. Nonetheless, a few extended dislocations were nucleated from triple junctions, suggesting that the suppression of conventional slip mechanism is not uniquely responsible for the inverse Hall-Petch behavior. For samples respecting the inverse Hall-Petch regime, the high number of triple junctions and grain boundaries allowed grain rotation, grain boundary sliding, and diffusion-like behavior that act as competitive deformation mechanisms. For all samples, the atomic configuration analysis showed that Shockley partial dislocations are nucleated at grain boundaries, crossing the grain before being absorbed in opposite grain boundaries, leaving behind stacking faults. Interestingly, the stress relaxation tests showed that the strain rate sensitivity decreases with grain size for a specific grain size range, whereas for grains below approximately 10 nm, the strain rate sensitivity increases as observed experimentally. Repeated stress relaxation tests were also performed to obtain the effective activation volume parameter. However, the expected linear trend in pertinent plots required to obtain this parameter was not found.

A Triple-Scale Crystal Plasticity Simulation on Yield Behavior of Annealed FCC Fine-Grained Metals

2008 ◽  
Vol 584-586 ◽  
pp. 1027-1032 ◽  
Author(s):  
Eisuke Kurosawa ◽  
Yoshiteru Aoyagi ◽  
Yuichi Tadano ◽  
Kazuyuki Shizawa
Keyword(s):  
Crystal Plasticity ◽  
Homogenization Method ◽  
Crystal Defects ◽  
Yield Behavior ◽  
Fine Grained ◽  
Grain Sizes ◽  
Crystal Plasticity Model ◽  
Proposed Model ◽  
Initial Dislocation Density ◽  
Critical Resolved Shear Stress

In this study, the conventional Bailey-Hirsch’s relationship is extended in order to express the increase of critical resolved shear stress due to the lack of dislocation lines in a grain. This model is introduced into a triple-scale crystal plasticity model based on geometrically necessary crystal defects and the homogenization method. A FE simulation is carried out based on the proposed model for FCC polycrystals with different grain sizes. It is numerically predicted that yield behavior of fine-grained metals depends on the initial dislocation density and the initial grain size. Furthermore, yield point drop that is observed in annealed FCC fine-grained metal can be reproduced.

Molecular dynamics study on the diffusion behavior of Li in the grain boundaries of α-Fe

2016 ◽  
Vol 109-111 ◽  
pp. 678-683 ◽  
Author(s):  
Xingang Yu ◽  
Chengrui Liu ◽  
Tiansi Han ◽  
Xianglai Gan
Keyword(s):  
Molecular Dynamics ◽  
Grain Boundaries ◽  
Diffusion Behavior
Sign in / Sign up

Export Citation Format

Share Document