Grain Boundary Misorientation Research Articles

Structure Evolution and Distributions of Grain-Boundary Misorientainons in Submicrocrystalline Molybdenum Irradiated with a Pulsed Electron Beam

Using the methods of electron backscatter diffraction, electron microscopy and X-ray diffraction analysis, it is demonstrated that irradiation of the surface of a submicrocrystalline molybdenum specimen with a pulsed electron beam in a non-melt regime results in the formation of a gradient structure in its bulk. The irradiation temperature is shown to affect the density of defects, the value of stress, and the distributions of grain-boundary misorientations in the surface and bulk of the submicrocrystalline molybdenum specimens.

Intragranular void formation in shock-spalled tantalum: Mechanisms and governing factors

Intragranular void formation in polycrystalline tantalum is investigated with plate impact experiments and molecular dynamics simulations, as regards its mechanisms and governing factors: grain boundary (GB) misorientation, grain orientation, grain size and shock pressure. Free-surface velocity history measurements are performed to obtain spall strength. Electron backscatter diffraction characterizations are used to obtain spatial distribution of voids, GB misorientation, and grain orientation associated with twins and intragranular voids. Intragranular voids are smaller in size but larger in number than intergranular voids, and nucleate in the GB vicinities. Smaller grains are favorable for the formation of intergranular and intragranular voids. At higher shock pressure, intragranular voids increase in fraction and tend to distribute at grain centers. Deformation twinning depends on grain orientation and prefers to form at high-angle GBs. However, intragranular voids have negligible dependence on grain orientation and favor medium-angle GBs, and the preference is enhanced slightly with increasing impact velocity and decreasing grain size. The simulations show GB-induced multiple-slip, slip-slip intersections and strain localizations act as the prerequisites to intragranular void formation.

Dependence of the In-Plane Thermal Conductivity of Graphene on Grain Misorientation

The thermal transport across the grain boundary (GB) is inevitably encountered for large-area polycrystalline graphene. However, the influence of GB configuration on thermal transport is not well understood. Here we investigated the effect of grain misorientation angle on the in-plane thermal conductivity (κ) of suspended graphene by using the optothermal Raman technique. Graphene with well-defined grain orientation was synthesized on an electropolished, annealed, and oxygen plasma-treated single-crystalline Cu(111) substrate by low-pressure chemical vapor deposition. The κ was primarily dependent on the grain size of single-, bi-, and polycrystalline graphene, consistent with the Boltzmann transport model. Surprisingly, κ of bicrystalline graphene dramatically decreased with a slight misorientation (<4°) between two neighboring grains. This phonon-boundary scattering was successfully simulated by the GB misorientation model. The GB length or shape also affected κ as a tertiary parameter. The GB misorient...

Bulk Nanolaminated Nickel: Preparation, Microstructure, Mechanical Property, and Thermal Stability

A bulk nanolaminated (NL) structure with distinctive fractions of low- and high-angle grain boundaries (f LAGBs and f HAGBs) is produced in pure nickel, through a two-step process of primary grain refinement by equal-channel angular pressing (ECAP), followed by a secondary geometrical refinement via liquid nitrogen rolling (LNR). The lamellar boundary spacings of 2N and 4N nickel are refined to ~ 40 and ~ 70 nm, respectively, and the yield strength of the NL structure in 2N nickel reaches ~ 1.5 GPa. The impacts of the deformation path, material purity, grain boundary (GB) misorientation, and energy on the microstructure, refinement ability, mechanical strength, and thermal stability are investigated to understand the inherent governing mechanisms. GB migration is the main restoration mechanism limiting the refinement of an NL structure in 4N nickel, while in 2N nickel, shear banding occurs and mediates one-fifth of the total true normal rolling strain at the mesoscale, restricting further refinement. Three typical structures [ultrafine grained (UFG), NL with low f LAGBs, and NL with high f LAGBs] obtained through three different combinations of ECAP and LNR were studied by isochronal annealing for 1 hour at temperatures ranging from 433 K to 973 K (160 °C to 700 °C). Higher thermal stability in the NL structure with high f LAGBs is shown by a 50 K (50 °C) delay in the initiation temperature of recrystallization. Based on calculations and analyses of the stored energies of deformed structures from strain distribution, as characterized by kernel average misorientation (KAM), and from GB misorientations, higher thermal stability is attributed to high f LAGBs in this type of NL structure. This is confirmed by a slower change in the microstructure, as revealed by characterizing its annealing kinetics using KAM maps.

Role of misorientation in fatigue crack growth behavior for NG-TIG welded joint of Ni-based alloy

The fatigue crack growth (FCG) behavior and microstructure of Ni-based alloy welded joint, fabricated by narrow gap tungsten inert gas welding (NG-TIG), were systematically investigated in this paper. The microstructure of weld metal (WM) and base metal (BM) are columnar grains and equiaxed grains respectively, plentiful irregular carbides aggregate at grain boundaries and the number of twin crystals decreases distinctly in HAZ due to the thermal input. The FCG tests results indicated that a higher FCG rate (da/dN) as well as a lower FCG threshold (ΔKth) for WM were obtained compared to BM and HAZ. It was found that crack deflection and branching in the whole welded joint occurred at FCG path in near-threshold regime. The misorientation of grain boundaries is considered as the main factor to account for the deceleration and retardation behavior revealed by electron back-scattered diffraction (EBSD) analysis. EBSD results also indicate that the misorientation of 20–30° is a critical value from a tortuous path to a relative straight path. A contrasting morphology of Paris regime and near-threshold regime showed the fracture mode transition from tensile mode to intense shear mode.

Experimental quantification of mechanically induced boundary migration in nanocrystalline copper films

A new experimental technique is presented that combines in situ straining with transmission electron microscope-based automated crystal orientation mapping to document microstructural evolution with nanoscale resolution at sequential stages of deformation. Orientation maps of freestanding annealed nanocrystalline Cu films have been collected, and the resultant datasets provide direct measures of grain size, shape and orientation as well as local grain boundary character and position at various stages of applied strain. Numerous examples of stress-driven grain boundary and twin boundary migration were recorded and studied. Detailed analysis of the misorientation of mobile and immobile grain boundaries provided clear experimental evidence that a broad range of grain boundaries are susceptible to stress-assisted migration; no general correlation between grain boundary misorientation and mobility was detected. Incoherent Σ3 boundaries were observed to be significantly more mobile than coherent Σ3 twin boundaries. Nevertheless, deformation twins were observed to nucleate and grow from grain boundaries and triple points.

Development and characteristics of accumulative roll bonded Mg-Zn/Ce/Al hybrid composite

Accumulative roll bonding (ARB) process have been used develop Mg-2%Zn/Ce/Al hybrid composite and microstructure, mechanical and corrosion properties were investigated. The electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) revealed that the grains are significantly reduced and reaches up to 1 μm in Mg-2%Zn layer and 1.8 μm in Al layer having high angle misorientation of grain boundaries after subjected to 5-passes of the ARB process. The Al17Mg12, AlMg4Zn11 and Al11Ce3 intermetallic phases were observed through the XRD analysis. Mechanical properties of the hybrid composite improved with increase in the number of ARB passes which is attributed to work hardening, grain refinement and uniform distribution of Ce particles. Presence of Ce in the hybrid composite restricts the phenomenon of dynamic recrystallization and prevents the grain growth during ARB process. The corrosion rate of Mg-Zn/Ce/Al hybrid composite (0.72 mm/y) improved about 3.3 times as compared to that of Mg-2%Zn alloy (2.37 mm/y).

The grain-boundary structural unit model redux

Properties of grain boundaries (GBs) and their underlying structures are key to understanding polycrystalline material phenomena. The most widely used model for GB structure is the structural unit model (SUM), introduced ∼ 50 years ago. The SUM represents GB structure as a combination of structural units (SUs); this combination evolves systematically with GB misorientation. Despite its successes, many observations suggest the SUM does not completely describe the GB structure; its utility for predicting GB properties is limited. There has been a growing realization that, even for fixed misorientation, multiple stable/metastable structures are common (corresponding to different microscopic degrees of freedom). We generalize the SUM by considering the effect of such metastable structures. While the SUM can describe GB structure evolution between a pair of delimiting boundaries, there will be many such evolutionary paths, corresponding to SUs associated with the metastable structure of the delimiting boundaries. The equilibrium GB energy vs. misorientation does not necessarily correspond to one of these paths, but will have contributions from many. Recognizing this, we propose a new approach to predict GB structure and energy, allowing for accurate determination of the GB energy vs. misorientation based on a very small number of atomistic simulations. For example, we predict the GB energy vs. misorientation for [100] and [111] symmetric tilt boundaries in BCC tungsten over the entire misorientation range to a mean error of <2 % based on atomistic simulations at only three or four misorientations. Our approach allows for the trade-off between computational cost and prediction accuracy.

Uniaxial stress-driven grain boundary migration in Hexagonal Close-packed (HCP) metals: Theory and MD simulations

Stress-driven grain boundary (GB) migration is an important plastic deformation mechanism in nanopolycrystalline metals. Recently, Liu et al. (2014) and Zong et al. (2015) have shown a new mechanism for GB migration in a Hexagonal Close-packed (HCP) bicrystal with 90° GB misorientation under uniaxial stress loading conditions, which is different from shear-coupled GB migration. In real polycrystals, GBs have random orientations. To investigate the effects of GB misorientation and loading directions, we propose a mechanics model for uniaxial stress-driven GB migration in which the GB misorientation varies from 0° to 90°. Two types of imposed uniaxial loadings with loading directions parallel and perpendicular to the GB are considered. The model prediction is compared with that by molecular dynamics (MD) simulations. The results reveal that the uniaxial stress-driven GB migration can only occur in some range of GB misorientation under both loading conditions. The critical strain to trigger the GB migration varies with GB misorientation in a non-monotonic way. The proposed model provides new insights into the GB migration in HCP metals.

Heating rate effect on particle stimulated nucleation and grains structure during non-isothermal annealing of multi-directionally forged solution treated AA2024

Particle stimulated nucleation (PSN) is one of the important mechanisms of recrystallization which is investigated in this research during non-isothermal annealing of multi-directionally forged solution treated aluminum alloy 2024. Non-isothermal annealing is performed up to temperatures of 380°C and 450°C at heating rates of 10, 100 and 200K·min−1 using a Gleeble 3800 thermo-mechanical simulator. In order to investigate the microstructure, field emission gun scanning electron microscope (FEG-SEM) equipped with electron backscatter diffractometer (EBSD) and differential scanning calorimetry (DSC) are utilized. The results show that the heating rate has a significant effect on the PSN and grain structure. An increase in the heating rate decreases the relative frequency and misorientations of high-angle grain boundaries (HAGBs), and suppresses PSN during non-isothermal annealing up to 380°C, but it promotes PSN during annealing up to 450°C. This controversy effect is related to the Zener pinning effect of fine particles of S′/S (Al2CuMg) during recrystallization. In non-isothermal annealing up to 450°C, increasing the heating rate leads to smaller average recrystallized grain size and narrower grain size distribution due to the suppression of recovery; the driving pressure of recrystallization is high, and the critical diameter of coarse particles for PSN is decreased. Texture measurement shows that the recrystallization front migrates and consumes the surrounding Copper component {112} 〈111〉 which is developed during deformation. The final microstructure is approximately randomly oriented and this suggests that the governing nucleation mechanism is PSN.

Microstructural Modeling of Intergranular Fracture in Tricrystals With Random Low- and High-Angle Grain Boundaries

Intergranular (IG) fracture behavior near triple junctions (TJs) in f.c.c. tricrystals with a variety of grain boundary (GB) misorientations has been investigated. Based on a dislocation-density GB interaction scheme, critical fracture conditions were coupled to evolving dislocation-density pileups and local stresses by using a dislocation-density-based crystalline plasticity formulation within a nonlinear finite-element framework to elucidate the effects of local GB structure, dislocation–GB interactions, and misorientations on IG crack propagation in f.c.c. crystalline materials. Tricrystals with low-angle GBs had higher fracture toughness than tricrystals with high-angle GBs. In TJs with a combination of random low- and high-angle GBs, the formation of dislocation-density pileups in the high-angle GB led to IG crack propagation along the high-angle GB rather than along the low-angle GB. These predictions, which are consistent with experimental observations, indicate that fracture behavior near TJs is controlled by highly local, evolving, and interrelated events, such as dislocation-density pileups and GB misorientations.

Coupled motion of grain boundaries and the influence of microcracks: A phase-field-crystal study

With phase-field-crystal, the two-dimensional coupled motion of grain boundary (GB) in triangular lattice crystal is studied. The migration mechanism of symmetric and asymmetric GBs in bicrystals is explored. Considering microcracks can hardly be avoided in materials, it is necessary to study the influence of microcracks on coupled GB motion. As shown in our simulation results, the influence of microcracks can be classified into three categories: (I) GB passes microcracks and is not impeded, cracks maintain unchanged; (II) Microcracks hinder GB motion, cracks migrate along GB and shrink; (III) GB motion is blocked by microcracks, cracks extend along GB and enlarge. The three kinds of results have great dependence on the misorientation and inclination angles of GB. Moreover, coupled GB motion can also lead to grain growth. The shear-induced grain growth process is simulated. As cracks can impede coupled GB motion, the shear-induced grain growth can also be hindered by cracks.

A Predictive Framework for Dislocation-Density Pile-Ups in Crystalline Systems With Coincident Site Lattice and Random Grain Boundaries

Evolving dislocation-density pile-ups at grain-boundaries (GBs) spanning a wide range of coincident site lattice (CSL) and random GB misorientations in face-centered cubic (fcc) bicrystals and polycrystalline aggregates has been investigated. A dislocation-density GB interaction scheme coupled to a dislocation-density-based crystalline plasticity formulation was used in a nonlinear finite element (FE) framework to understand how different GB orientations and GB-dislocation-density interactions affect local and overall behavior. An effective Burger's vector of residual dislocations was obtained for fcc bicrystals and compared with molecular dynamics (MDs) predictions of static GB energy, as well as dislocation-density transmission at GB interfaces. Dislocation-density pile-ups and accumulations of residual dislocations at GBs and triple junctions (TJs) were analyzed for a polycrystalline copper aggregate with Σ1, Σ3, Σ7, Σ13, and Σ21 CSLs and random high-angle GBs to understand and predict the effects of GB misorientation on pile-up formation and evolution. The predictions indicate that dislocation-density pile-ups occur at GBs with significantly misoriented slip systems and large residual Burger's vectors, such as Σ7, Σ13, and Σ21 CSLs and random high-angle GBs, and this resulted in heterogeneous inelastic deformations across the GB and local stress accumulations. GBs with low misorientations of slip systems had high transmission, no dislocation-density pile-ups, and lower stresses than the high-angle GBs. This investigation provides a fundamental understanding of how different representative GB orientations affect GB behavior, slip transmission, and dislocation-density pile-ups at a relevant microstructural scale.

Intergranular Oxidation Effects During Dwell-Time Fatigue of High-Strength Superalloys

The present paper summarizes experimental work to identify the mechanisms of dwell-time cracking during service operation of polycrystalline nickel-base superalloys, such as Alloy 718 and AD730. By means of crack growth monitoring during various kinds of cyclic loading in vacuum and in air using the potential drop technique, it was shown that the combination of sustained tensile stress, load reversal, and oxidizing atmosphere leads to an increase in the crack propagation rate by orders of magnitude, as compared to cyclic reference tests without dwell time and/or under vacuum conditions. By careful metallographic and theoretical analysis, the embrittling effect was attributed to stress-induced oxygen diffusion ahead of the intergranular crack tip followed by decohesion in a nanometer scale and had been termed “dynamic embrittlement.” More recently, atom probe tomography of the near-crack tip region revealed that the damage zone consists of Cr-rich transition oxides rather than elemental oxygen. This is in qualitative agreement with TGA measurements on Alloy 718 specimens without mechanical loading, which shows that crack propagation velocities of 50 µm/s do not allow massive Cr2O3 or NiO scale formation. By means of a quantitative analysis of the fracture surface, it became evident that grain-boundary attack depends on the grain-boundary character. This observation was supported by four-point bending experiments on grain-boundary-engineered samples with a high fraction of coincident site lattice boundaries and bicrystalline samples with well-defined grain-boundary misorientation relationships with respect to the loading axis. Taking the experimental results into account, semiquantitative modeling concepts have been developed to correlate crack propagation rates with the oxygen grain-boundary diffusivity, the local microstructure, and the mechanical stress states. These concepts are discussed in terms to adapt grain size and precipitate microstructure of polycrystalline superalloys.

Effect of microcrystallites formed by deformation on the growth and orientation of grains during recrystallization of iron

The changes of ultradispersed structure in deformed iron after annealing has been investigated. An ultradispersed structure of two different types—cellular and submicrocrystalline (SMC)—has been formed in iron by high pressure torsion deformation. The effect of the deformed-structure type on the temperature of recrystallization onset, the size of recrystallized grains, and the recrystallization texture has been understood. The comparison of recrystallization of the initial cellular and SMC structures should be made to determine the role of microcrystallites. The recrystallization temperature of iron with an SMC structure is 200–250°С lower than that with a cellular structure, because of the presence of microcrystallites, ready recrystallization nuclei. The recrystallization (annealing at 750°C, 1h) of the cellular structure results in the formation of a coarse-grained structure with an average recrystallized grain size that is by an order of magnitude coarser than that after the same annealing of the SMC structure (5 and 180 μm, respectively). The significant distinct feature of the SMC structure in contrast to the cellular one is the annealing-assisted formation of a <110> recrystallization texture in it, whereas the annealing of iron with the cellular structure does not cause the formation of a recrystallization macrotexture. An increase in the sharpness of the recrystallization texture correlates with a decrease in the average grain-boundary misorientation angle.

Multi-scale study of microstructure evolution in hot extruded nano-sized TiB2 particle reinforced aluminum composites

The microstructural evolution of in-situ TiB2 nano-particle reinforced AlZnMgCu composites during hot extrusion was investigated from micro to macro scales by a combination of various techniques, including neutron and synchrotron X-ray diffraction, optical microscopy, scanning and transmission electron microscopy and electron backscatter diffraction (EBSD). The development of microstructure has shown a bimodal grain structure with distinctive spatial distributions of TiB2 particles: the elongated coarse grain structure with smaller dispersed particles and the fine grains mixed with clusters of relatively larger particles. The particle stimulated nucleation occurs at large particle clusters, resulting in recystallized (sub)micron sized fine grains. The dispersed smaller particles are observed to promote dislocation generation and to prohibit recovery. They are shown to reduce the misorientation of low angle grain boundaries due to the pinning effects on independent dislocations, which also lead to the suppression of dynamic recovery and increase of driving force for dynamic recrystallization. Quantitative texture analysis combined with neutron diffraction and EBSD has exhibited the development of a strong 〈111〉 and 〈001〉 dual fiber texture, and both texture volume fractions are changing with the particle content. In addition, the synchrotron diffraction experiments have shown that dislocation density increases with the particle content in both texture components. The microstructure evolution is the result from a complex process of particles/matrix interaction during the deformation and dynamic recrystallization. In comparison with its particle-free alloy counterpart, the thermomechanical response of the composites at high temperature is discussed in terms of aluminum deformation and recrystallization mechanisms combined with nanosized particle effects.

Influences of high strain rate, low temperature, and deformation direction on microstructural evolution and mechanical properties of copper

Step-wise deformations are applied to pure copper along uniaxial direction or multiaxial directions in three conditions of speed and temperature: static and room temperature, dynamic and room temperature, or dynamic and low temperature. Through examinations of total 36 deformation stages, the influences of strain rate, temperature, and deformation direction on microstructural evolution and mechanical properties are investigated. According to the results, the deformation mechanism at extreme conditions (high strain rate and low temperature) changes from only dislocation slip to dislocation slip and deformation twinning. Although deformation-direction change has no significant effect on the final mechanical properties and total dislocation densities measured from X-ray diffraction results, the evolutions of grain-boundary misorientation and geometrically necessary dislocations (GNDs) are clearly influenced. The trend of microstructural evolution, i.e. larger grain size and lower GND densities by multi-axial deformation than the uniaxial case, is reversed when twinning occurred. Based on the results, we suggest four evolution models: uniaxial or multi axial cases with dislocation slip, or with dislocation slip and deformation twining. Even if the homogenous and almost fully twinned microstructure producing improved mechanical strength was obtained by applying the extreme deformations, poor ductility of the processed specimen were observed.

The effect of temperature and topological defects on fracture strength of grain boundaries in single-layer polycrystalline boron-nitride nanosheet

In this paper, molecular dynamic simulations are utilized to investigate the effect of temperature and topological defects on the mechanical response and tensile strength of grain boundaries in single-layer hexagonal boron-nitride (h-BN) nanosheets. Six different cases of grain boundaries are examined including four symmetric and two asymmetric models. We study various types of misorientation angles and defects at different temperatures. The tensile tests of all models are also performed at different strain rates at room temperature. The results clearly show that increasing the misorientation angle of the grain boundary leads to a decreasing tensile strength, elastic modules and strain-at-failure of h-BN nanosheets. The mechanical response of high-angle misorientation grain boundaries were observed to be quite unique under uniaxial tension, since a chain of atoms between two fractured parts was observed even after tensile failure. Increasing the temperature also leads to a decreasing fracture strength and strain-at-failure for all six models. The low-angle misorientation of grain boundaries are more sensitive to variations in the temperature than grain boundaries with high-angle misorientation. The stress-strain curve shows a large drop for low-angles grain boundaries when the temperature elevates from 100K to 300K. In addition, small fluctuations could be observed for all models at high temperature, which does not depend on the misorientation angle of the grain boundary.

Effect of low-angle grain boundaries on morphology and variant selection of grain boundary allotriomorphs and Widmanstätten side-plates

Morphology and variant selection (VS) of grain boundary (GB) allotriomorphs and Widmanstätten side-plates of α phase in an α/β titanium alloy, Ti-6Al-4V (wt%), are investigated using a three-dimensional phase field model. The structures of low-angle GBs (misorientation θm ≤ 10°) are modeled as discrete dislocation networks using Frank-Bilby theory. It is shown that α allotriomorphs and side-plates compete with each other during precipitation and the final morphology and selected α variants exhibit a strong correlation with the GB dislocation structures. While the side-plate morphology is more preferred by a symmetrical tilt GB with θm ∼ 10°, it can also be induced by a pure twist GB with θm ≤ 5°. Quantitative analysis indicates that precipitate morphology and VS are determined by the interplay among (i) elastic interaction between a nucleating α precipitate and the GB dislocation networks, (ii) growth anisotropy determined by the relative inclination of the habit plane with respect to the GB dislocations, (iii) density of nucleation sites for the same variant and coalescence during growth, and (iv) spatial confinement from simultaneously nucleated neighboring α variants of dissimilar types.

Low-energy, Mobile Grain Boundaries in Magnesium

The strong basal texture that is commonly developed during the rolling of magnesium alloy and can even increase during annealing motivates atomic-level study of dislocation structures of both <0001> tilt and twist grain boundaries (GBs) in Magnesium. Both symmetrical tilt and twist GBs over the entire range of rotation angles θ between 0° and 60° are found to have an ordered atomic structure and can be described with grain boundary dislocation models. In particular, 30° tilt and twist GBs are corresponding to energy minima. The 30° tilt GB is characterized with an array of Shockley partial dislocations bp:-bp on every basal plane and the 30° twist GB is characterized with a stacking faulted structure. More interesting, molecular dynamics simulations explored that both 30° tilt and twist GBs are highly mobile associated with collective glide of Shockley partial dislocations. This could be responsible for the formation of the strong basal texture and a significant number of 30° misorientation GBs in Mg alloy during grain growth.