Al Bicrystals Research Articles

Effects of Grain Boundary Misorientation Angle on the Mechanical Behavior of Al Bicrystals

This research article explores the effect of grain boundary (GB) misorientation on the mechanical behavior of aluminum (Al) bicrystals by means of molecular dynamics (MD) simulations. The effect of GB misorientation on the mechanical properties, fracture resistance, and crack propagation are evaluated under monotonic and cyclic load conditions. The J-integral and the crack tip opening displacement (CTOD) are assessed to establish the effect of the GB misorientation angle on the fracture resistance. The simulations reveal that the misorientation angle plays a significant role in the mechanical response of Al bicrystals. The results also evidence a gradual change in the mechanical behavior from brittle to ductile as the misorientation angle is increased.

Dynamic behavior of grain boundaries with misorientations in the vicinity of Σ3 coherent and incoherent twin boundaries in Al bicrystals

The migration behaviour of different grain boundaries with misorientations close to the Σ3 CSL orientation relationship in high purity Al bicrystals under the capillary driving force and applied mechanical stress was investigated. The experiments were performed by an in-situ technique to observe and measure the boundary migration with a scanning electron microscope. The ability of the nearly Σ3 60°〈111〉 incoherent {110} and {112} boundaries to move under capillary driving force was found to depend critically on the initial boundary inclination. While boundaries with inclinations near {112} can easily assume a curved shape and migrate, boundaries with an initial {110} plane remain stationary or form non-mobile facets. This is attributed to the essential anisotropy of the inclination dependence of the energy γ(ψ) of 60°〈111〉 tilt boundaries with differently high torque dγ/dψ around {112} and {110} inclinations, as revealed by atomistic simulations of the respective boundaries. The Σ3 70.5°〈110〉 tilt boundary with the geometry corresponding to the coherent {111} twin boundary, was found to be immobile under both driving forces applied. The 59.2° 〈111〉 tilt grain boundary with geometry near the Σ3 {110} incoherent twin boundary was found to be quite mobile under applied shear stress. The measured migration activation enthalpy H = 0.45 eV for this boundary is the lowest among the values obtained in previous experiments for any other stress driven grain boundary in Al bicrystals of the same purity. Moreover, this boundary migrated with a zero coupling factor, i.e. without producing any measurable shear parallel to the boundary plane.

CRACK INTERACTION WITH TILT GRAIN BOUNDARIES IN BRITTLE FRACTURE OF ALUMINUM

This paper presents a molecular dynamics study of crack propagation in aluminum bicrystals with tilt grain boundaries under uniaxial tension. The simulations showed that crack propagation is always preceded by an abrupt increase in the atomic volume at the crack tip as a result of nanopore formation. The beginning of crack growth or crack initiation after arrest is always accompanied by a rapid decrease in the atomic volume at the crack tip. The results obtained in this study show that the crack can cause significant grain boundary migration. The amount of grain boundary migration increases with decreasing grain misorientation angle. Tilt grain boundaries with misorientation angles above 20° fully prevent crack propagation into a neighboring grain. At smaller tilt angles, the crack is temporarily arrested by the grain boundary, and the arrest time increases with increasing tilt angle.

Ubiquity of the kinetic compensation effect: A consequence of the existence of a maximum in energy dissipation

The significance of the often observed kinetic compensation effect (KCE) remains controversial. The assumption of its validity, as well as its ubiquity, has led to speculations on its significance with no general consensus in the scientific community. Efforts to understand the KCE have usually attempted connections between microscopic events, structures, and mechanisms involved in various kinetic processes. An alternative proposition, with no relation to specific mechanisms, is presented suggesting that there is no new conceptual physics in the KCE. An analysis of internal friction (IF) in the framework of linear response to weak perturbations highlights that the concomitant energy dissipation cannot exceed a maximum or peak value which depends solely on the anelastic properties of the material. The KCE is shown to be a consequence of this simple fact. This physical principle underpinning the KCE is able to explain its ubiquity because of its general nature beyond any specific kinetic rate mechanism. Examples of IF in different systems such as polycrystalline thermoelectric composites and Al bicrystals, which show evidence of the KCE, conform to this prediction. While the observation on Al bicrystals has previously been presented and interpreted as signaling a phase transition, the analysis presented here demonstrates the simpler alternative. A similar argument is presented to derive the KCE in the thermal desorption of gases, which involves kinetic processes unrelated to the above examples. It is proposed that a KCE can, in general, be deduced for any set of closely related relaxation processes having close peak values of energy dissipation.

Effect of Symmetric Tilt and Twist Grain Boundaries on the Void Nucleation, Growth and Spall in polycrystalline Al : Multiscale modelling

Multiscale modelling using molecular dynamics (MD) and hydrodynamics (HD) in sequence is carried out to obtain the dynamic spall strength of aluminum (Al). Specifically, we simulate the effect of symmetric tilt and twist grain boundaries (GB) on the void nucleation and growth in Al using MD and transfer the information to HD simulations of plate impact. MD is used to create bi-crystals mimicking 11 tilt and 12 twist grain boundaries (GB) along <100>, with orientation angles spanning 0° to 90° using an NPT ensemble. The grain boundary energy (GBE) is validated with published results. MD simulations of the tri-axial tensile deformation of the Al bi-crystals show that the voids nucleate at the grain boundaries and then grow and coalesce. The time history of the bulk pressure and void volume fraction are recorded for calculating the material specific parameters of a void nucleation and growth (NAG) fracture model. 1-D Hydrodynamic flyer plate impact simulations are performed using these NAG parameters to obtain the free surface velocity vs time curve. The spall strength and spall thickness of Al, estimated from the free surface velocity profile, show a good match with published experimental values of the dynamic spall strength (relative error of 2.2%–6%) and spall thickness (relative error of 2.4%–4%) for three impact velocities viz. 518, 1588, and 2275 m/s respectively.

Grain Boundary Sliding along Special Asymmetric Grain Boundaries in the Al Bicrystals: Atomistic Molecular Dynamics Simulation

Grain Boundary Sliding along Special Asymmetric Grain Boundaries in the Al Bicrystals: Atomistic Molecular Dynamics Simulation

Aluminum depletion induced by co-segregation of carbon and boron in a bcc-iron grain boundary

The local variation of grain boundary atomic structure and chemistry caused by segregation of impurities influences the macroscopic properties of polycrystalline materials. Here, the effect of co-segregation of carbon and boron on the depletion of aluminum at a Σ5 (3 1 0 )[0 0 1] tilt grain boundary in a α − Fe-4 at%Al bicrystal is studied by combining atomic resolution scanning transmission electron microscopy, atom probe tomography and density functional theory calculations. The atomic grain boundary structural units mostly resemble kite-type motifs and the structure appears disrupted by atomic scale defects. Atom probe tomography reveals that carbon and boron impurities are co-segregating to the grain boundary reaching levels of >1.5 at%, whereas aluminum is locally depleted by approx. 2 at.%. First-principles calculations indicate that carbon and boron exhibit the strongest segregation tendency and their repulsive interaction with aluminum promotes its depletion from the grain boundary. It is also predicted that substitutional segregation of boron atoms may contribute to local distortions of the kite-type structural units. These results suggest that the co-segregation and interaction of interstitial impurities with substitutional solutes strongly influences grain boundary composition and with this the properties of the interface.

Grain boundary energy landscape from the shape analysis of synthetically stabilized embedded grains

The Gibbs free energy of grain boundaries (GBs) in Al bicrystals has been investigated by Molecular Dynamics (MD) simulations. In our novel approach, one grain is fully embedded in a large matrix grain with fixed misorientation. Hence all inclinations are considered simultaneously since the boundary covers the full orientation subspace. A synthetical driving force is employed to counteract the shrinkage of the embedded grain by the capillary forces. Hence, the number of atoms of the embedded grain is kept constant, but its shape adjusts itself at elevated temperatures in order to minimize the total GB energy. The quasi-equilibrium shapes are used to derive the GB energy γ(n) as functions of the GB plane normal n. For GBs with the misorientations Σ5〈001〉 and Σ7〈111〉, analytical functions were derived and validated in a mesoscopic front-tracking simulation: the latter simulations recovered the grain shapes observed in MD simulations. For the Σ5〈001〉 misorientation it is shown that the anisotropy of γ(n) varies quite strongly with temperature. For a Σ9〈110〉 misorientation, the derived numerical energy function was found to be rather complex, showing pronounced energy minima for mixed tilt/twist GBs parallel to 111 crystal planes.

Effect of slip transmission at grain boundaries in Al bicrystals

The effect of slip transfer on the deformation mechanisms of Al bicrystals was explored using a rate-dependent dislocation-based crystal plasticity model. Three different types of grain boundaries (GBs) were included in the model by modifying the rate of dislocation accumulation near the GB in the Kocks-Mecking law, leading to fully-opaque (dislocation blocking), fully-transparent and partially-transparent GBs. In the latter, slip transmission is only allowed in pairs of slip systems (SS) in neighbour grains that are suitably oriented for slip transfer according to the Luster-Morris parameter. Modifications of the GB character led to important changes in the deformation mechanisms at the GB. In general, bicrystals with fully-opaque (dislocation blocking) boundaries showed an increase in the dislocation density near the GB, which was associated with an increase in the Von Mises stress. In contrast, the dislocation pile-ups and the stress concentration were less pronounced in the case of partially-transparent boundaries as the slip in one grain can progress into the next grain with some degree of continuity. No stress concentrations were found at these boundaries for fully-transparent boundaries, and there was continuity of strain across the boundary, which is not typical of most experimentally observed GBs (Hémery et al., 2018; Bieler et al., 2019). Simulations of ideal bicrystals oriented for favorable slip transfer on the most highly favored slip system in grains with high Schmid factors for slip transfer depends on the number of active SS in operation in the neighborhood indicating that most boundaries will lead to nearly opaque conditions while some boundaries will be transparent. Finally, the model was applied to a particular experimentally observed GB in which slip transfer was clearly operating indicating that the model predicted a nearly transparent GB.

The role of the grain boundary in the fracture toughness of aluminum bicrystal

Complex interactions of defects such as dislocations and grain boundaries determine the deformation mechanism of nanocrystalline materials. Using molecular dynamics simulations, in this paper, we study crack propagation under mode I. Single crystals and bicrystals aluminum (Al) with different initial edge crack lengths were loaded until failure under controlled deformation. The effect of the grain boundary on the fracture toughness was analyzed. Tearing was observed and investigated in Al bicrystals. A novel methodology is proposed to estimate the fracture toughness in terms of J for Al single crystals and bicrystals. J was computed by means of three different methods, viz., J Contour Integral, Energy Release Rate, and Crack Tip Opening Displacement. Fracture toughness (JC) for bicrystal was 445% higher than for single crystal. JC for the single crystal was approximately 3 J/m2 and 13 J/m2 for the bicrystal. Comparison between first tearing in Al bicrystals and failure in single crystals was performed. Finally, it was evidenced that for Al bicrystals ductile fracture occurred during final fracture and the grain boundary had a large effect on the mechanical behavior of Al bicrystals.

Grain-Boundary Shear-Migration Coupling in Al Bicrystals. Atomistic Modeling

The energy of grain boundary shears is calculated for symmetric grain boundaries (GBs) using ab initio methods and molecular-dynamic modeling in order to elucidate mechanisms that control GB shear-migration coupling in typical symmetric GBs, such as Σ3 (111), Σ5 (012), Σ5 (013) and Σ11 (113) tilt GBs, in Al bicrystal. The energy of generalized grain-boundary stacking faults (GB–SF) is determined, and the preferred directions and the energy barrier are established for grain-boundary slippage. It is shown that the relative slippage of neighboring grains at certain directions of particle shears is accompanied by conservative migration of GB in the direction perpendicular to its plain. The modeling data are comparative to known grain-boundary shear-migration coupling mechanisms in Al.

Зернограничное проскальзывание и миграция специальных границ зерен в бикристаллах Al. Атомистическое моделирование

AbstractThe energy of grain boundary shears is calculated for symmetric grain boundaries (GBs) using ab initio methods and molecular-dynamic modeling in order to elucidate mechanisms that control GB shear-migration coupling in typical symmetric GBs, such as Σ3 (111), Σ5 (012), Σ5 (013) and Σ11 (113) tilt GBs, in Al bicrystal. The energy of generalized grain-boundary stacking faults (GB–SF) is determined, and the preferred directions and the energy barrier are established for grain-boundary slippage. It is shown that the relative slippage of neighboring grains at certain directions of particle shears is accompanied by conservative migration of GB in the direction perpendicular to its plain. The modeling data are comparative to known grain-boundary shear-migration coupling mechanisms in Al.

Effect of grain boundary segregation of Co or Ti on cyclic deformation of aluminium bi-crystals

There exist many factors affecting fatigue behavior of metallic materials, among them the effect of grain boundary (GB) segregation of alloying elements remains largely unexplored. To exploit this factor, an atomistic simulation is performed to investigate the cyclic behavior of Al bi-crystals with GB segregation of Co or Ti during their mode I cyclic loading perpendicular to GB plane. Results are given in comparison with the case of pure Al bi-crystal. Detailed analysis of the structure evolution during cyclic loading for the considered bi-crystals is presented. It is found that during the cycling, plastic deformation of the pure Al bi-crystal occurs through perfect dislocation sliding followed by twinning, while the GB segregations inhibit the formation of both perfect dislocations and twins in an Al matrix leading to decrease in ductility of the bi-crystals and intergranular crack propagation rate. The cyclic loading of the samples with GB segregation is accompanied with a generation of plenty of Shockley partials. Both the alloying elements in GBs can improve the fatigue lifetime of the Al bi-crystals. The ability of GB segregations in bi-crystalline and polycrystalline Al to retard the dislocation emission from GBs that has been first theoretically revealed in the study could be successfully used to improve fatigue characteristics of metals, especially nanocrystalline metals with a dense GB network.

Deformation of nanocrystalline binary aluminum alloys with segregation of Mg, Co and Ti at grain boundaries

The influence of the temperature and sort of alloying element on the deformation of the nanocrystalline (NC) binary Al alloys with segregation of 10.2 at % Ti, Co, or Mg over grain boundaries has been studied using the molecular dynamics. The deformation behavior of the materials has been studied in detail by the simulation of the shear deformation of various Al bicrystals with the grain-boundary segregation of impurity atoms, namely, Ti, Co, or Mg. The deformation of bicrystals with different grain orientation has been studied. It has been found that Co introduction into grain boundaries of NC Al has a strengthening effect due to the deceleration of the grain-boundary migration (GBM) and difficulty in the grain-boundary sliding (GBS). The Mg segregation at the boundaries greatly impedes the GBM, but stimulates the development of the GBS. In the NC alloy of Al–Ti, the GBM occurs actively, and the flow-stress values are close to the values characteristic of pure Al.

Quantitative measurements of grain boundary excess volume from HAADF-STEM micrographs

A novel approach for quantitative measurements of grain boundary (GB) excess volume has been developed using correlative analytical transmission electron microscopy (TEM) and successfully demonstrated for several simulated symmetrical [100] tilt GB configurations as well as for the experimental case of an Al bicrystal containing a near Σ13 GB with an additional twist component. The reliability and precision of this new approach is analyzed and the limitations are discussed.

Crystal plasticity FEM study of nanoindentation behaviors of Cu bicrystals and Cu–Al bicrystals

Abstract

Effect of grain boundary segregations of Fe, Co, Cu, Ti, Mg and Pb on small plastic deformation of nanocrystalline Al

The paper studies the effect of grain boundary (GB) segregation of impurity atoms (Fe, Co, Cu, Ti, Mg and Pb) on the stress–strain relations of nanocrystalline Al and its alloys subjected to both shear plastic deformation and hydrostatic pressure at room temperature using molecular dynamics (MD) simulations. It is found that the GB segregations of Fe and Co atoms in the nanocrystalline Al–Fe and Al–Co alloys, respectively, strengthen their GBs, while those of Cu and Pb atoms in GBs of the nanocrystalline Al–Cu and Al–Pb alloys have the opposite effect. Furthermore, the GB strengthening effect of the mentioned impurity atoms in both twist and tilt GBs of Al bicrystals are analyzed via molecular statics (MS) simulations. Similar to the MD simulations, the results show that the GB segregations of Co atoms have a noticeable positive effect on the strength of the GBs while those of Pb atoms have a negative effect. Moreover, both MD and MS studies reveal that the Co, Mg and Pb atoms tend to occupy GBs of polycrystalline Al, while Ti, Fe and Cu atoms do not.

Dislocation density based crystal plasticity finite element simulation of Al bicrystal with grain boundary effects

ABSTRACTCrystal plasticity finite element method is a useful tool to investigate the anisotropic mechanical behaviors as well as the microstructure evolution of metallic materials and it is widely used on single crystals and polycrystalline materials. However, grain boundary involved mechanisms are barely included in the polycrystalline models, and modeling the interaction between the dislocation and the grain boundaries in polycrystalline materials in a physically consisstent way is still a long-standing, unsolved problem. In our analysis, a dislocation density based crystal plasticity finite element model is proposed, and the interaction between the dislocation density and the grain boundaries is included in the model kinematically. The model is then applied to Al bicrystals under 10% compression to investigate the effects of grain boundary character, e.g. grain boundary misorientation and grain boundary normal, on the stress state and the microstructure evolution. The modeling results suggest a reasonable correspondence with the experimental result and the grain boundary character plays a crucial role in the stress concentration and dislocation patterning.

Stress Induced Grain Boundary Motion in Al Bicrystals

Recent results of experimental research into stress induced grain boundary migration in aluminum bicrystals are briefly reviewed. Boundary migration under a shear stress was observed to be coupled to a lateral translation of the grains for any &lt;100&gt; tilt boundary in the entire misorientation range (0-90°). Measurements of the temperature dependence of coupled boundary migration revealed that there is a specific misorientation dependence of migration activation parameters. Grain boundaries can act during their motion under the applied stress as sources of lattice dislocations that leads to the generation and growth of new grains in the boundary region. The rate of stress induced boundary migration decreases with increasing solute content in aluminum. Both the migration activation enthalpy and the pre-exponential mobility factor were found to increase with rising impurity concentration.

Observation of Stress-Driven Migration of Specific Planar Grain Boundaries in Al Bicrystals

Stress induced grain boundary migration was experimentally investigated in aluminum bicrystals. Migration of planar symmetrical &lt;100&gt; and &lt;111&gt; tilt boundaries under a shear stress was observed to be accompanied by a lateral translation of the adjacent grains. This coupling proved to be the typical migration mode for all investigated boundaries, no matter whether low-or high angle, low Σ CSL coincidence or non-coincidence boundary. The migration-shear coupling was also observed for asymmetrical tilt boundaries. Measurements of the temperature dependence of coupled boundary migration revealed that there is a specific misorientation dependence of the migration activation parameters. Contrary to expectations, a high angle Σ7 tilt boundary moved under an applied stress, but produced practically no shear during its migration.