Grain Boundary Triple Junctions Research Articles

INTERACTION OF HYDROGEN IMPURITY WITH NANOCRYSTALLINE PALLADIUM AND NICKEL

The interaction of hydrogen atoms with nanocrystalline palladium and nickel in the work was studied by the molecular dynamics method. The nanocrystalline structure of palladium and nickel was created in the model by crystallization from the liquid state at the presence of several specially introduced crystalline embryos. After solidification, the calculation blocks, in addition to the crystalline phase, contained grain boundaries and triple junctions of grain boundaries. The interactions of metal atoms with each other were described by the multi-particle Cleri-Rosato potential constructed in the framework of the tight-binding model. Morse potentials were used to describe the interactions of hydrogen atoms with metal atoms and with each other. The parameters of Morse potentials were calculated from the experimental data of theabsorption energy, the activation energy of the above-barrier diffusion of hydrogen in a metal (at normal and high temperatures), the binding energy with a vacancy, dilatation. According to the results obtained in the present work, at a high concentration of hydrogen (the concentration of 10% from the metal atoms was considered), the hydrogen atoms combine into aggregates, which are formed predominantly near the surface of the metal. The aggregates contained, as a rule, several dozen hydrogen atoms and had low diffusion activity. The binding energy of hydrogen atoms with these aggregates was greater than with the metal crystal lattice or grain boundaries in it. In palladium, hydrogen aggregates were formed farther from the surface than in nickel. Apparently, this is due not so much to the relatively low energy of hydrogen absorption by palladium (–0.1 eV) in comparison with nickel (0.16 eV), but rather to the difference in lattice parameters of the metals under consideration: 3.89 Å for Pd and 3.524 Å for Ni. For the same reason, conspicuously, hydrogen aggregates in a pure crystal lattice were more often observed in Pd than in Ni. In Ni, aggregates, as a rule, were formed in defect areas containing an excess free volume: near the free surface, in grain boundaries and in triple junctions.

To the Question of Forming Geometrically Necessary Disclinations in Triple Junctions of Grain Boundaries in Metals

The Bollman and King models are tested by means of molecular dynamics simulation for the formation of geometrically necessary disclinations in triple junctions of grain boundaries in metals. It is shown that the stresses arising in a triple junction due to the non-multiple lengths of low-angle tilt boundaries to the distance between grain boundary dislocations is not compensated for mainly by the formation of an additional disclination in the junction (the King model) but by the bending of one or several grain boundaries, accompanied by the displacement of grain boundary dislocations. A triple junction of the Bollman U-type (containing a geometrically necessary disclination) is not formed at the conjugation of tilt boundaries with common misorientation along the junction or at the conjugation of mixed-type boundaries.

Hardening by Annealing and Implementation of High Ductility of Ultra-Fine Grained Aluminum: Experiment and Theory

Abstract The influence of low temperature annealing and subsequent deformation on microstructure, strength and ductility was investigated for the first time for high pressure torsion (HPT) processed commercially pure Al. Extremely high increases in the conventional yield stress (up to 50%) and ultimate tensile strength (up to 30%) were obtained by annealing of the ultrafine grained (UFG) samples in the range 90-200 °C for 1 h. Such increases were accompanied by a sharp drop in ductility up to 1%. Implementation of high ductility at the level of coarse-grained Al, while maintaining high strength of the HPT-processed sample was demonstrated for the first time and achieved by repeating the low temperature annealing followed by subsequent additional HPT deformation. The key role of relaxation of non-equilibrium high-angle grain boundaries (GBs) in the strengthening effect of UFG-Al by annealing is shown. Two theoretical models are suggested to explain the hardening by annealing and the implementation of high ductility in UFG structures. Within the models, plastic deformation occurs through emission of lattice dislocations from triple junctions of GBs containing pile-ups of grain-boundary dislocations, glide of the lattice dislocations across neighboring grains, their accumulation at and climb along the opposite GBs. The energy characteristics and the critical stresses of dislocation emission are determined in two different cases, for UFG Al subjected to annealing only and to annealing with subsequent additional HPT deformation. The calculated theoretical dependences of the flow stress on the plastic deformation value well fit qualitatively and quantitatively to our experimental data.

Epifluorescence microscopy study of a quadruple node of triple junctions of grain boundaries in a Eu2+ -doped Kcl:Kbr solid solution by using the doping ion as a fluorochrome.

A typical quadruple node (QN) of triple junctions (TJs) of grain boundaries (GBs) in a Eu2+ -doped KCl0.52 Br0.48 solid solution is investigated from the geometrical point of view by epifluorescence microscopy using the doping ion as a fluorochrome. The excitation and fluorescence optical properties of the fluorochrome were previously characterised by spectrophotometry whereas the structural nature of the studied material as well as its Bravais lattice type, unit cell size and long-range translational order degree was determined by X-ray diffraction. A three-dimensional reconstruction was built from the microscopy images of different optical cross-sections of the studied arrangement of crystal defects. In the close vicinity of the QN, the studied arrangement of crystal defects adopts the geometry of a collapsed tristetrahedron which, centred at the QN, has its legs along the TJs and, hence, has its faces as collapsed in pairs into the GBs. The angles defined by different TJ couples as well as the dihedral angles defined by the different GB couples meeting in every TJ were measured at the QN site. All, the image recording and stacking as well as the measuring procedures are carefully described. The measured TJ angles (97°, 117°, 95°, 117°, 99° and 130° ± 2°) depart from the characteristic angle (109.47°) of a tetrahedron whereas the measured GB angles (101°, 119°, 140°; 125°, 127°, 108°; 133°, 109°, 119°; 129°, 99° and 132° ± 2°) depart from the angular argument (120°) of a 3-fold symmetry rotation indicating that, in the close neighbourhood of the QN, the studied arrangement of crystal defects is structurally unstable. Such an instability is associated with an observed mismatch in orientation (by angles of 20°, 15°, 33° and 30° ± 2°) between the TJs and some <111> zone axis matrix lattice crystallographic directions ([1¯1¯1¯], [111¯], [11¯1] and [1¯11]), respectively). Local variations in anionic composition, existing within the solid solution matrix, are discussed to be responsible for this mismatching and, therefore, for the observed structural instability.

Исследование влияния избыточного свободного объема на подвижность тройных стыков границ зерен

Исследование влияния избыточного свободного объема на подвижность тройных стыков границ зерен

Deformation and microstructure evolution above the B2 transus of Ti-22Al-25Nb (at%) orthorhombic alloy

Hot compression deformations above the B2 transus of Ti-22Al-25Nb (at%) orthorhombic alloy were conducted from 20°C above the B2 transus to 100°C above the transition point with 20°C intervals. Four deformation strain rates including 10s−1, 1s−1, 0.1s−1 and 0.01s−1 were employed in order to study the effect of strain rates on the deformed microstructures. The constitutive relation establishment has been attempted using Zener-Hollomon parameter. The analyses indicate that exponential law equation is not applicable for Ti-22Al-25Nb alloy in present conditions while the power law equation and the hyperbolic sine law equation are both effective. However, the power law equation is more applicable than the exponential law equation since the linear correlation coefficient between lnZ and lnσ is 0.989 while the correlation coefficient between lnZ and lnsinh(ασ) is 0.972. It is found that strain rates and deformation temperatures have remarkable effect on the deformed microstructures. Dynamic recovery (DRV) dominated the deformation mechanism at high strain rates (10s−1 and 1s−1) while dynamic recrystallization (DRX) occurred in the microstructures deformed at low strain rate (0.1s−1 and 0.01s−1). The DRX are mainly continuous DRX (CDRX) although some discontinuous DRX (DDRX) grains are observed at prior high angle grain boundaries and triple junctions of grain boundaries.

Molecular Dynamics Simulation of Crack Propagation in Nanoscale Polycrystal Nickel Based on Different Strain Rates

Based on the strain rates of 2 × 108 s−1 and 2 × 1010 s−1, molecular dynamics simulation was conducted so as to study mechanisms of crack propagation in nanoscale polycrystal nickel. The strain rate has an important effect on the mechanism of crack propagation in nanoscale polycrystal nickel. In the case of a higher strain rate, local non-3D-crystalline atoms are induced and Lomer-Cottrell locks are formed, which plays a critical role in crack initiation and propagation. Orientation difference between adjacent grains leads to the slipping of dislocations along the different directions, which results in the initiation of a void near the triple junction of grain boundaries and further contributes to accelerating the crack propagation.

In situ observation of nanotwins formation through twin terrace growth in pulse electrodeposited Cu films

Nanotwinned copper (nt-Cu) shows a broad application prospects as interconnection materials in integrated circuit industry, since it combines the excellent mechanical and electrical properties. However, the formation and growth behavior of twin lamellae in pulse electrodeposited copper films are not fully understood. In this work, a series of electroplated copper films are prepared by verifying the electroplating parameters and the microstructures are analyzed using scanning electron microscope (SEM) and transmission electron microscope (TEM). The surface morphology indicates strong evidence of stacked twin terraces and layers, suggesting that nanotwins grow up layer by layer. Combined with in situ characterization by SEM and molecular dynamics (MD) simulation, it is proved that the terraces originate from the triple junction of grain boundaries and grow up by extending along the lateral (111) crystal plane. A twin terrace-growing model for nt-Cu is then deduced, which distinguishes from deformation twins or annealed twins. This growth model would be prospective to help obtain high quality of nt-Cu in industry.

Grain boundary sliding, triple junction disclinations and strain hardening in ultrafine-grained and nanocrystalline metals

A theoretical model is suggested which describes grain boundary (GB) sliding and its accommodation through dislocation slip in ultrafine-grained and nanocrystalline metals. The initial stage of the accommodating dislocation slip represents emission of lattice dislocations from triple junctions into grain interiors. The lattice dislocations emitted from a triple junction slip across a grain and are absorbed by an opposite GB where they are dissociated into GB dislocations that climb along the GB. In the situation where these GB sliding and accommodating processes are dominant, stress-strain dependences are calculated in ultrafine-grained copper. With the calculated dependences, we found that pronounced strain hardening occurs which is related to the accommodation processes and associated formation of disclinations at triple junctions of GBs. It is theoretically revealed that the special (new) strain hardening mechanism under discussion can play a significant role in enhancing ductility of ultrafine-grained and nanocrystalline metals at comparatively low temperatures.

Effect of atomic ordering on the role of grain boundaries in the plastic deformation of Ni3Fe alloy

The structure of dislocations and the defect structure of grain boundaries and their parameters in Ni3Fe alloy with short-range order (SRO) and long–range order (LRO) at different stages of plastic deformation are studied by means of transmission diffraction electron microscopy using thin foils and replicas. It is found that atomic ordering reduces the Σ3 twins plasticizing effect, increases the density of grain boundary defects, slows their annihilation during deformation, and intensifies the microstrains at the triple junctions of grain boundaries.

An atomistic study of Y segregation at a {101¯1}–{101¯2} double twin in Mg

Segregation at a triple junction of grain boundaries has not been explained much because the structure of a triple junction is very complicated. The present paper describes Monte Carlo simulations by which Y segregation was investigated at a triple junction of a {101¯1}–{101¯2} double twin in Mg. Y atoms segregated at the extension sites in the {101¯1} and {101¯2} twin boundaries. However, they were not necessarily more segregated at the triple junction of the double twin, although the free volumes at the extension sites of the triple junction were larger on average than those of the other boundaries. Thus, the Y segregation behavior at the triple junction cannot be explained only by the free volume. The anisotropic factor of the atomic Voronoi polyhedron was developed to explain the Y segregation behavior at the triple junction. In addition, the shortest interatomic distance and coordination number affected Y segregation at the triple junction. Also, segregation at the triple junction strongly depended on the Y concentration, which resulted from variations in the local atomic configuration. Thus, the Y segregation behavior at the triple junction was complicated, in contrast to those at twin boundaries, even when the size effect was predominant.

Mechanisms of crack propagation in nanoscale single crystal, bicrystal and tricrystal nickels based on molecular dynamics simulation

Molecular dynamics simulation was performed in order to investigate mechanisms of crack propagation in nanoscale single crystal, bicrystal and tricrystal nickels, respectively. The grain boundary plays a significant role in the initiation and propagation of crack. As for tricrystal, in particular, a void occurs at the triple junction of grain boundaries and contributes to accelerating the crack propagation. The existence of the grain boundary contributes to the increase of Shockley partial dislocations along with stacking faults. The orientation difference among the grains causes the dislocations to move along different directions in the different slip systems, which lays the foundation for the formation of the crossed or closed Lomer-Cottrell locks. The Lomer-Cottrell locks are able to impede the movement of dislocations and cause the pile-up of dislocations. Amorphous atoms are induced in the region near the Lomer-Cottrell lock. The deformation energy is relaxed when the amorphous atoms are separated one another, which consequently leads to the initiation and propagation of crack.

Pseudopartial wetting of W/W grain boundaries by the nickel-rich layers

The excellent properties of heavy tungsten-based alloys are based on the combination of hardness of W grains, as well on the toughness and ductility of the binder with low melting temperature (usually containing nickel, iron, or copper). The topology (and resulting properties) of binder network are controlled by the complete and incomplete wetting of W/W grain boundaries (GBs) and GB triple junctions (TJ). We observed for the first time that pseudoincomplete (or pseudopartial, or frustrated complete) GB wetting by Ni layers is also present in W–Ni alloys. Namely, the channel of a Ni-rich solid solution in GB TJ forms the non-zero dihedral contact angle not only with “dry” W/W GBs (incomplete GB wetting), but also with W/W GBs containing the uniformly thin (3nm) Ni-rich layer (pseudoincomplete GB wetting).

Barrier-Free Nucleation at Grain-Boundary Triple Junctions During Solid-State Phase Transformations.

Molecular dynamics simulations are used to provide strong evidence for barrier-free nucleation events in a heterogeneous solid-solid system. The barrier-free events are characterized by an absence of an incubation time and a growth rate of the emerging phase that is independent of the system size. Furthermore, an analysis of the size and shape of the critical nucleus using the Winterbottom construction indicates that no solution exists for these barrier-free cases. We propose that barrier-free nucleation, which will have a profound effect on phase transformation kinetics, may be a general phenomenon for any polycrystalline material.

Orientation dependence of void growth at triple junction of grain boundaries in nanoscale tricrystal nickel film subjected to uniaxial tensile loading

Molecular dynamics simulation was performed in order to investigate the dependence of void growth on crystallographic orientation at the triple junction of grain boundaries in nanoscale tricrystal nickel film subjected to uniaxial tensile loading. The nucleation, the emission and the transmission of Shockley partial dislocations play a predominant role in the growth of void at the triple junction of grain boundaries. The orientation factors of various slip systems are calculated according to Schmid law. The slip systems activated in a grain of tricrystal nickel film basically conform to Schmid law which is completely suitable for a single crystal. The activated slip systems play an important role in plastic deformation of nanoscale tricrystal nickel film subjected to uniaxial tensile loading. The slip directions exhibit great difference among the activated slip systems such that the void is caused to be subjected to various stress conditions, which further leads to the difference in void growth among the tricrystal nickel films with different orientation distributions. It can be concluded that the grain orientation distribution has a significant influence on void growth at the triple junction of grain boundaries.

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

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.

The Causes of Formation of the Triple Junctions of Grain Boundaries Containing Excess Free Volume in fcc Metals at Crystallization

The formation conditions of strained (non-equilibrium) triple junctions of grain boundaries were studied by the method of molecular dynamics. It is shown that strained triple junctions, containing excess free volume, mainly forms during crystallization process in the result of "locking" of the liquid phase density at a meeting of the three crystallization fronts and, as a consequence, of the concentration of excess free volume in the triple junction after solidification.

Toughness and strength of nanocrystalline graphene.

Pristine monocrystalline graphene is claimed to be the strongest material known with remarkable mechanical and electrical properties. However, graphene made with scalable fabrication techniques is polycrystalline and contains inherent nanoscale line and point defects—grain boundaries and grain-boundary triple junctions—that lead to significant statistical fluctuations in toughness and strength. These fluctuations become particularly pronounced for nanocrystalline graphene where the density of defects is high. Here we use large-scale simulation and continuum modelling to show that the statistical variation in toughness and strength can be understood with ‘weakest-link' statistics. We develop the first statistical theory of toughness in polycrystalline graphene, and elucidate the nanoscale origins of the grain-size dependence of its strength and toughness. Our results should lead to more reliable graphene device design, and provide a framework to interpret experimental results in a broad class of two-dimensional materials.

Microstructure, Mechanical Properties and Electromagnetic Shielding Effectiveness of Mg-Y-Zr-Nd Alloy

Abstract The as-cast Mg-Y-Zr- x Nd alloys with different Nd contents were prepared, and the effects of Nd content on microstructure, mechanical properties and electromagnetic interference (EMI) shielding properties were investigated. The results show that the grain sizes are refined from 70.1 μm to 45.2 μm, discontinuous bone-shaped β phases are formed mostly in the triple junction of grain boundaries when the content of Nd increases to 2.63 wt%. The mechanical properties and EMI shielding capacity are both enhanced significantly by adding Nd element. The alloy with 2.63 wt% Nd has a good combination of mechanical properties and EMI properties. T6 heat treatment is able to further improve the EMI shielding effectiveness. The above-mentioned experiment results are attributed to the microstructural variation caused by adding different contents of Nd.

結晶粒界三重線における変形不適合の回位モデル

The geometrical concept of Volterra's disclination is applied to model the inhomogeneous deformation that occurs at the triple junction of grain boundaries in deformed polycrystal. It is shown that even though the deformation is restricted to maintain the strain compatibility on the three grain boundaries meeting at the triple junction, the misfit in the dihedral angles of the three grains would occur. The angular misfit gives the angle of the equivalent disclination that is accompanied by a logarithmic stress singularity at the origin, which is similar to the stress concentration at the triple junction in deformed tricrystals.