Vicinity Of Grain Boundaries Research Articles

Effect of grain boundary on the friction coefficient of pure Fe under the oil lubrication

In this study, the effect of grain boundary on the frictional behavior under the oil-lubricated condition was investigated by varying the grain size (crystallite size) of the pure Fe samples prepared via physical vapor deposition. A high fraction of grain boundary in the sample yielded a low friction coefficient μ in the case of a lubrication oil that formed a chemisorbed film on the surface of the material. Thick absorbed layer formed on the surface was observed only in the sample with high fraction of grain boundary. These results indicate that a disordered structure in the vicinity of grain boundary preferentially forms chemisorbed films and reduces μ by protecting the surface of the material.

Investigation of a high-dose irradiated beryllium microstructure

The present paper studies the evolving Be microstructure after high-dose neutron irradiation at 660 K, 753 K and 873 K up to 34 dpa and generation of 5524 appm He and 596 appm 3H. At all three temperatures gas bubbles were observed whereby their average size is increasing with the temperature. In addition denuded zones (DZ) formed near grain boundaries. Their width increased with irradiation temperature and was found to be narrower for grains with a random orientation compared to those with a close to basal orientation with respect to the grain boundary. Furthermore, perceptions containing elements such as Al, Cr, Fe, Mg, Si as well as U were found inside single grains, often attached to dislocation lines, and in the vicinity of grain boundaries, mainly at 660 K and 753 K.

Stress fields of finite-size dislocation walls and prediction of back stress induced by geometrically necessary dislocations at grain boundaries

At low strain, geometrically necessary dislocations (GND) confined in the close vicinity of grain boundaries can be approximated as a dislocation wall structure called a GND facet. Analytical solutions derived from Field Dislocations Mechanics (FDM) theory allow calculating the stress components associated with the GND facets but are unable to account for the stress field variation induced by finite size effect. Dislocation dynamics simulation is used to investigate the true stress field of GND facets. The geometry, dimension and dislocation density of three generic types of GND facets (twist, tilt and epitaxial facets) are systematically studied. In all cases, the stress field generated by GND facets is proportional to the surface GND density and its spatial distribution can be recovered using FDM solution combined with two scaling parameters identified from DD simulation results. This calculation procedure can be generalized to any crystal structure by relating the components of the surface Nye’s tensor to the solutions of simple cubic slip systems. Finally, static and dynamic tests are made to validate the calculation of back stress within regular grains bounded by GND facets.

The gradient microstructure and deformation heterogeneity in HPDC AM60 alloy

As an important manufacturing process for Mg-alloy components, high pressure die casting (HPDC) has attracted considerable attention. However, the application of magnesium alloy high pressure die castings is limited by variations in the mechanical properties, which can be ascribed to a heterogeneous distribution of microstructure. In this work, the through-thickness distribution of the microstructure features (i.e. grain size, β-phase and porosity) of HPDC AM60 alloy and their effects on the strain heterogeneity and the microhardness variation were carefully investigated. The results show that the HPDC AM60 alloy exhibited a gradient structured microstructure, i.e., two skin layers sandwiching a core layer. The skin was mainly comprised of fine grains (diameter ≤10 μm), whereas the core predominately consisted of externally solidified crystals (ESCs). Furthermore, the skin shows higher number and area fractions of β-Mg17Al12 particles. Heterogeneous through-thickness distribution of microhardness was observed in the as-cast sample, and the skin showed higher the microhardness than the core. β-Phase was the primary factor accounting for heterogeneous distribution of microhardness. In the as-cast sample, strain heterogeneity was measured at strain of 3% using digital image correlation (DIC) method. The strain in the skin regions was homogeneous, whereas significant strain heterogeneity was presented in the core. After T4 treatment, remarkable strain heterogeneity was still observed, indicating that β-phase did not have profound effect on deformation heterogeneity. Strain localized at the interdendritic regions of ESCs and the vicinity of grain boundaries in fine-grained regions, where porosity occurred. This illustrates that porosity was the primary factor inducing strain localization.

Study of the Electrical Properties of Cu₂ZnSnS₄ (CZTS) Thin Film Using Atomic Force Microscopy (AFM) Techniques.

CZTS is a compound semiconductor made from elements which are plainly available and nonpoisonous having favorable optoelectronic properties for thin film solar cell (TFSC) applications. In this study, Cu-poor CZTS thin film was fabricated on soda lime glass (SLG)/Mo-deposited substrate using cosputtering followed by post sulfurization in H₂S atmosphere. Local electrical transport study was carried out by using conductive atomic force microscopy (C-AFM) for small bias voltage (100 mV). Here we observed that most of the dark current (Idark) flow through grain boundaries (GBs) than grain interiors. The positive high current about 3.4 nA and sharp C-AFM signal at the GBs, dips to the zero (0) value at the grain interior. Local surface potential (Vsurface) study was carried out using kelvin probe force microscopy (KPFM), which showed that the positive Vsurface potential about 175 mV in the vicinity of GBs in a Cu-poor CZTS sample. On the basis of these results we inferred a potential landscape (VL) around the GBs. All result shows that due to variation in elemental composition which creates Cu-deficit or CuZn anti site defects at GBs, leads reduced effective band gap (Eeff) than the bulk towards grain inner to GBs.[-2pt].

Modelling and simulation of dynamic recrystallisation based on multi-phase-field and dislocation-based crystal plasticity models

ABSTRACT The mechanical properties of metals used as structural materials are significantly affected by hot (or warm) plastic working. Therefore, it is industrially important to predict the microscopic behaviour of materials in the deformation process during heat treatment. In this process, a number of nuclei are generated in the vicinity of grain boundaries owing to thermal fluctuation or the coalescence of subgrains, and dynamic recrystallisation (DRX) occurs along with the deformation. In this paper, we develop a DRX model by coupling a dislocation-based crystal plasticity model and a multi-phase-field (MPF) model through the dislocation density. Then, the temperature dependence of the hardening tendency in the recrystallisation process is introduced into the DRX model. A multiphysics simulation for pure Ni is conducted, and then the validity of the DRX model is investigated by comparing the numerical results of microstructure formation and the nominal stress–strain curve during DRX with experimental results. The obtained results indicate that in the process of DRX, nucleation and grain growth occur mainly around grain boundaries with high dislocation density. As deformation progresses, new dislocations pile up and subsequent nucleation occurs in the recrystallised grains. The influence of such microstructural evolution appears as oscillation in the stress–strain curve. From the stress–strain curves, the temperature dependence in DRX is observed mainly in terms of the yield stress, the hardening ratio, and the change in the hardening tendency after nucleation occurs.

Hydrogen-modified interaction between lattice dislocations and grain boundaries by atomistic modelling

Dislocation plasticity in the vicinity of grain boundaries (GBs) plays a critical role in H-induced intergranular failure. Their interaction mechanisms under H environment, however, remain largely unexplored. Here, the underlying interaction of a screw dislocation with [11¯0] symmetric tilt GBs was studied by using molecular dynamics simulation, with special concerns on the role of solute H in it. Our results show several interaction mechanisms including dislocation dissociation, transmission, nucleation and reflection, depending on different glide planes and GB structures. The presence of H tends to transform these reactions into ones involving dislocation absorption due to H-hindered GB migration and H-enhanced localised plasticity. Furthermore, it is quantified that solute H leads to an increase in energy barrier for dislocation-grain-boundary interaction. After dislocation absorption, the GB segregated with H atoms is activated to a more disordered atomic structure, which can be correlated to the crack nucleation and hence the ultimate fracture. These findings advance a mechanistic understanding on H-induced plasticity-mediated intergranular failure.

Comprehensive Elucidation of Grain Boundary Behavior in All‐Inorganic Halide Perovskites by Scanning Probe Microscopy

AbstractAll‐inorganic halide perovskite (AIHP) is becoming one of the most promising generation materials of perovskite photovoltaics for commercialization due to its thermodynamic stability and soared efficiency. Depending on material properties, grain boundary (GB) has detrimental or beneficial effect on device photovoltaic performance. However, less attention is paid to GB behavior in AIHPs. Herein, it is concluded that the microscopic GBs are the major sites for photocarrier generation and transport, as well as the ionic pathway dominating the current hysteresis behavior in AIHP solar cells. Kelvin probe force microscopy (KPFM) measurements reveal a lower surface potential at GBs as compared to grain interiors (GIs), suggesting a significant upward band bending around GBs. Conductive atomic force microscopy (c‐AFM) measurements show a higher current flow in the vicinity of GBs, indicating enhanced carrier separation and collection taking place at GBs. Furthermore, the existence of ion motion is evidenced both by the single‐point c‐AFM measurement and voltage‐controlled KPFM measurement, which accounts for the common current hysteresis behavior in AIHP solar cells. These investigations provide an advanced understanding of the role of GBs in AIHP and further inspiration on how to optimize device performance up to the levels of organic–inorganic hybrid perovskite.

Effect of temperature on deformation mechanisms of AZ31 Mg-alloy under tensile loading

Controlled in situ SEM tensile tests have been carried out between 200 and 300 °C at a constant strain rate of 5.10-5 s-1 to investigate the effect of temperature on deformation mechanisms operating in an Mg–3Al–1Zn (AZ31) Mg-alloy. Fiducial microgrids deposited using electron beam lithography are used to evidence grain boundary sliding as well as to determine the spatial strain heterogeneities as a function of temperature. Dislocation creep and grain boundary sliding coexist between 200 and 300 °C but their respective activity varies significantly as shown by the strain rate sensitivity value m which is about 0.2 at 200 °C but about 0.5 at both 250 and 300 °C. In addition, grain boundary sliding becomes predominant at 250 and 300 °C whereas its occurrence is relatively limited at 200 °C. Slip trace analysis shows that at 200 °C prism and pyramidal <c+a> slip already exhibit a great activity. Spatial strain heterogeneities determined by digital image correlation (DIC) based on microgrid displacements develop during the early stage of plastic deformation and persist at larger strains. It is shown that the strain in the vicinity of grain boundaries intensifies when the temperature rises from 200 to 300 °C while the core of grains accommodates less deformation in agreement with the fact that grain boundary sliding is predominant at 250 and 300 °C.

Ceria thin film memristive device by magnetron sputtering method

CeO2-x nanocrystal (NC) thin film memristive devices are fabricated by magnetron sputtering method. The influence of defect formation and carrier transport properties on memristive devices performances are characterized. The length of nanocrystal CeO2-x grain is about 8–12 nm and the diameter is about 6–8 nm with abundant grain boundaries (GBs). The memristive device exhibits an electronic conductivity. Under the forward bias, electrons accumulate in the vicinity of GBs due to space electrons zone effect and hop among oxygen vacancies to form the conductive filament (CF). The set voltage and reset voltage ranges from ~1.8 to 6 V and from ~-0.3 to -1 V, respectively. The cross-point memory shows an appropriate resistance ratio of high resistance state (HRS) to low resistance state (LRS) is about 104. The retention times of the devices can be more than 104 S, exhibiting a stable performance in terms of information storage capability.

H induced decohesion of an Al grain boundary investigated with first principles: General conditions for instant breakage and local delayed fracture

The uniaxial tensile test response of a H decorated Σ5 [1 0 0] twist grain boundary (GB) in face-centred-cubic Al has been examined with first principles. The impurity shows a strong tendency to relocate during loading. To capture these H movements, the standard model framework was extended to probe loading-unloading hysteresis. Due to the strong monotonic decrease in the H formation energy with rising GB elongation, the maximum tensile stress accepted by the H decorated GB in the slow fracture limit generally is reached before breakage becomes thermodynamically favourable. For any intact GB configuration visited upon exceeding this stress, the assumption of global chemical equilibrium is argued to be violated. Moreover, while breakage remains ensured from the slow fracture limit considerations, it may in practice require the influx of H to the GB vicinity. A quantitative analysis of GB destabilisation in this scenario requires multi-scale modelling even in the slow fracture limit.

The interaction between grain boundary and tool geometry in nanocutting of a bi-crystal copper

Anisotropy is one central influencing factor on achievable ultimate machined surface integrity of metallic materials. Specifically, grain boundary has a strong impact on the deformation behaviour of polycrystalline materials and correlated material removal at the microscale. In the present work, we perform molecular dynamics simulations and experiments to elucidate the underlying grain boundary-associated mechanisms and their correlations with machining results of a bi-crystal Cu under nanocutting using a Berkovich tool. Specifically, crystallographic orientations of simulated bi-crystal Cu with a misorientation angle of 44.1° are derived from electron backscatter diffraction characterization of utilized polycrystalline copper specimen. Simulation results reveal that blocking of dislocation motion at grain boundaries, absorption of dislocations by grain boundaries and dislocation nucleation from grain boundaries are operating deformation modes in nanocutting of the bi-crystal Cu. Furthermore, heterogeneous grain boundary-associated mechanisms in neighbouring grains lead to strong anisotropic machining behaviour in the vicinity of the grain boundary. Simulated machined surface morphology and machining force evolution in the vicinity of grain boundary qualitatively agree well with experimental results. It is also found that the geometry of Berkovich tool has a strong impact on grain boundary-associated mechanisms and resultant ploughing-induced surface pile-up phenomenon.

On a correlation between hydrogen effects on atomic interactions and mobility of grain boundaries in the alpha-iron. Stage II. Mobility of grain boundaries in the H-charged α-iron

Using molecular dynamics, it is shown that, be located in the vicinity of grain boundaries, hydrogen in the α-iron increases their mobility. This effect is attributed to the hydrogen-increased concentration of free electrons, which decreases surface tension at grain boundaries. The obtained results allow to explain the early start of recrystallization in the hydrogenated cold-worked iron-based alloys.

Atomic-Scale Simulations of Grain Boundary Mechanical Properties in the Solid Electrolyte Li7La3Zr2O12

The garnet oxide Li7La3Zr2O12 (LLZO) is a promising solid electrolyte due to its high ionic conductivity, favorable elastic properties, and chemical stability against Li metal. Nevertheless, despite its high shear modulus, recent studies have shown that metallic Li ‘dendrites’ can penetrate LLZO through its grain boundary (GB) network, resulting in a short-circuit. This behavior suggests that microstructural features (such as GB) will play an important role in suppressing this failure mode. The present study examines the possibility for nanoscale softening (i.e., reduced elastic moduli) and reduced strength in the vicinity of GBs in LLZO. Molecular dynamics simulations were performed to predict the Young’s and shear modulus near two prototypical grain boundaries. The ideal shear strength and fracture toughness was also calculated for bulk and GB-containing systems to determine the impact of GBs on plastic deformation and the potential for Li penetration along GBs. Our calculations reveal that all three of these mechanical properties (elastic modulus, ideal strength, and fracture toughness) are reduced in the vicinity of GBs relative to the bulk. This behavior could potentially contribute to the observed penetration of Li along GBs in LLZO at high current densities. Similar behavior is anticipated at defects such as surface cracks or pores.

On the Effect of Quenching on Postweld Heat Treatment of Friction-Stir-Welded Aluminum 7075 Alloy

This work focuses on the effect of postweld heat treatment (PWHT) on the mechanical properties and microstructural evolution of aluminum 7075 alloy processed via friction stir welding (FSW). FSW is known to be capable of grain refinement in the nugget zone (NZ). Two different quench media (water and air) were employed for PWHT. Regardless of the quench media, the PWHT led to the occurrence of grain growth in the NZ of the FSWed aluminum 7075 alloy. Abnormal grain growth occurred in the water quenched specimen. It is shown that ductility and strength of FSWed aluminum 7075 alloy are strongly dependent on the quenching rate. Changes in the mechanical properties and microstructure reveal that only at lower cooling rate this alloy is prone to the formation of precipitate-free zones (PFZs) in the vicinity of grain boundaries. Eventually, the PFZs deteriorate mechanical properties of this alloy.

Influence of crystal orientation and Berkovich tip rotation on the mechanical characterization of grain boundaries in molybdenum

In this study, nanoindentation experiments with a continuously measured stiffness are performed in the vicinity of grain boundaries in technically pure molybdenum. A significant change in hardness as a function of indentation depth is put in context with the corresponding deformation patterns in the indented crystals. The difference in hardness increase of 18% compared to 5% of two different grain boundaries with similar misorientation angle could be explained in this way. Five degrees of freedom have to be determined to fully describe a grain boundary. However, the influence of the rotation angle of a pyramidal indenter geometry around the loading axis as yet another loading parameter is assessed by backscattered electron micrographs and orientation deviation maps. Indentation experiments with different rotation angle at the same grain boundary confirm that a hardness increase close to the grain boundary is only apparent if a significant amount of plastic deformation is introduced towards the interface.

Selection of slip systems in confined single crystal gradient plasticity: coupled effects of slip system orientations, latent hardening, and grain boundaries

In crystal plasticity under prescribed deformation, the incremental material response is potentially non-unique owing to slip system redundancy for most of the crystalline structures. Following Petryk, energy minimizing considerations give the way to select one of these solutions and the set of active systems, which depend on their more or less favorable orientation and their mutual interactions (latent hardening). This variational approach is extended here to confined plasticity in a finite volume, simulating a single crystal embedded in an aggregate. A slip gradient enhanced framework and related micro-hard boundary conditions are considered, using two defect energies introduced by Gurtin and coworkers: the first one takes the slip system polar dislocation densities as internal state variables and the second one is a quadratic potential of the dislocation density tensor. In both cases, micro-hard conditions amount to null flow for the two former quantities. For the classical one dimensional case of a strip in simple shear, the two models yield substantially different solutions, the second one coupling the gradients on the different systems. These results emphasize the necessity for a physically motivated modeling of gradient effects in the vicinity of grain boundary interfaces.

Effect of Nb precipitate on defect formation and migration energies in bi-crystalline Zr

In this article, molecular statics based simulations were performed to study the effect of Nb precipitate on the defect formation and migration energies in the bi-crystals of zirconium and niobium alloy (Zr–Nb). In order to resemble the experimentally observed configurations of grain boundaries in Zr, symmetrical and asymmetrical tilt grain boundaries were generated along [0001] as tilt axis. It has already been established that point defect formation energies are lower in the vicinity of grain boundaries, as compared to bulk values, but literature is almost mute on the effect of Nb precipitate on these energies. A comparison between these energies in Zr–Nb alloy containing (0%, 1%, 2.5% and 5%) Nb is reported in this work. Vacancy migration energy was estimated with the help of nudged elastic band algorithm. Simulations performed in this article lead to a conclusion that addition of Nb in the Zr–Nb alloy significantly effects the formation as well as migration energies of the defects. It can be concluded that an appropriate weight percentage of Nb helps in enhancing the defect sinking capabilities of grain boundaries in bi-crystalline Zr.

Role of multigrain structure on friction of graphene layers

Graphene is a one-atom thick two-dimensional material and has a huge potential as a solid lubricant for industrial application. Graphene is often synthesized by chemical vapor deposition (CVD) because of its simplicity and scalability. However, CVD-grown graphene sheets have the multigrain structure, which may have detrimental effects on the superior properties of single-crystal graphene. In particular, the effects of this polycrystalline structure on graphene friction remain far from completely understood. In this study, we investigate the friction between multigrain graphene layers using molecular dynamics (MD) simulations. The MD simulations examine key factors such as grain size and orientation, and morphology of grain boundaries with several different multigrain configurations. The simulation results reveal that multigrain graphene layers exhibit low, but not completely negligible, friction. The observed frictional behaviors are analyzed in terms of the interactions between grains and grain boundaries (GB). In most models, the contribution of the grain-to-grain interaction to the total friction force is negligible, but some grain pairs exhibit relatively large friction if they configure the commensurate interface due to small mismatch in orientation. The interactions involving grain boundaries often exhibit large friction with the largest at the direct GB-to-GB contacts. An analysis shows that the increase in friction near the grain boundary region is directly related to the decrease in interlayer distance, which is caused by the warping of layers in the vicinity of grain boundaries.

Evaluation of electrochemical impedance and biocorrosion characteristics of as-cast and T4 heat treated AZ91 Mg-alloys in Ringer's solution

Abstract The present study aims at understanding the electrochemical impedance and biocorrosion characteristics of AZ91 Mg-alloy in Ringer's solution. As-cast AZ91 Mg-alloy was subjected to T4 heat treatment in a way to homogenize its microstructure by dissolving most of the β-Mg17Al12 phase at the vicinity of grain boundaries. The electrochemical impedance and biocorrosion performances of these two different microstructures (as-cast and T4 heat treated AZ91 Mg-alloys) in Ringer solution were evaluated by electrochemical impendence spectroscopy, potentiodynamic polarization and weight loss method. EIS spectra showed that both microstructures exhibit similar dynamic response as a function of the immersion time; however, the value of impedance and maximum phase angle are about 50% higher in as-cast AZ91 Mg-alloy as compared to that of homogenized AZ91 Mg-alloy. Weight loss measurement indicated that corrosion resistance of as-cast AZ91 was significantly better than that of homogenized AZ91. Microstructural and XRD analysis revealed that as-cast AZ91 contains a passive film of MgCO3 and CaCO3 precipitates with near spherical morphologies, whereas homogenized AZ91 comprised mainly unstable Mg(OH)2 film featured by irregular plate-like morphologies.