Aluminum Bicrystals Research Articles

Effect of the grain boundary on the fatigue crack growth for aluminum bi-crystals

Grain boundaries (GBs) are relevant as structural barriers for deformations in nano-crystalline materials and their resistance against fatigue crack propagation. The overall behavior of the material's structure is the result of a complex confluence of multiple grains with a variety of relative orientations and their interaction with other structural defects. The effect of the grain boundary on cracked grains is an important factor in order to establish the mechanical behavior of nano-materials under static and dynamic loading conditions. For studying the effect of the grain boundary on the mechanical behavior of nano-crystalline solids a single boundary is placed as a defect that separates two miss-oriented crystals with a relative tilt or twist angle. The purpose of this study is to analyze the effect of the GB and the relative tilt and twist angles on the crack propagation under increasing cyclic loading of cracked FCC bi-crystals. Molecular dynamics simulations at room temperature of fatigue crack growth tests were performed to characterize the retardation effect of different grain miss-orientations on fatigue crack propagation. Al by-cristals with an initial edge crack and different miss-orientattion angles are simutalted, in the present research, under increasing cyclic loading condition. As a general trend high twist angles showed a greater effect on the resistance to crack propagation under single and compound grain miss-orientation.

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.

Prediction of dislocation - grain boundary interactions in FCC aluminum bicrystals using a modified continuum criterion and machine learning methods

Mechanical properties of metals such as strength and toughness are strongly correlated to complex interactions between various defects in the crystalline structure. While elementary interactions between these defects have been investigated using recent micro- and nano-characterization techniques, understanding of the detailed interaction mechanisms has hardly been obtained. To understand defect-driven plasticity at various time and length scales, it is necessary to formulate a general guideline to predict both the interaction type (transmission or reflection) and the dislocation's subsequent slip system after the interaction. Many criteria based on the geometric alignment of the defects have been developed to predict this phenomenon, but these have yet to be found to be accurate when applied to general data sets of grain boundaries (GBs). With this motivation, we conduct a systematic study using molecular dynamics (MD) models of bicrystals to analyze defect interaction process between a prismatic dislocation loop and eleven different grain boundaries of the following character: three tilt, three twist, and five mixed. Based on the MD observations, two new prediction methods are developed: the first is a new data-driven parametric score function based on the classical geometric criteria, and the second is by applying Gaussian process machine learning methods to find the probability distribution of a hidden function. The proposed data-driven prediction methods could pave a new way to predict the unit interaction of dislocations with various GBs, which could show much higher accuracy compared to pre-existing geometric criteria.

Direct measurement of shear stress for dislocation transferring across {111} Σ3 grain boundary in aluminum bicrystal via in situ straining TEM

TEM in situ straining was applied to an Al bicrystal with a {111}Σ3 grain boundary. The activated slip systems in both grains were determined based on the dislocation line configurations and the crystallographic orientation of the applied load. The resolved shear stress for activation of the dislocation transfer at the GB, τgb, was directly evaluated from the dislocation motion and the simultaneously obtained stress-strain curve. The evaluated τgb value agrees with those obtained in a previous study on macroscopic tensile tests. From the geometry of the dislocation reaction, it was presumed that the pile-up lattice dislocations are dissociated into the lattice dislocations in the adjacent grain and the GB dislocation. It was determined that the activated slip system in the adjacent grain was dominated by local energetic and geometric conditions rather than the global condition such as the combination of Schmid low and applied stress.

High-pressure effect on grain boundary wetting in aluminium bicrystals

Abstract The effect of pressure and misorientation on grain boundary wetting in aluminium bicrystals has been investigated. The grain boundaries were of [100] symmetrical tilt type. The wetting liquid was an Sn– Zn alloy. It is shown that the wetting angle is a function of misorientation but not of pressure. The reasons of the above results are discussed, assuming a linear dependence between the interface energy and pressure. It is shown that the difference of energy of the liquid/solid and solid/solid interface as well as the misorientation dependence of energy is simply proportional to the free volume of the interfaces.

Crack Length Effect on the Fracture Behavior of Single-Crystals and Bi-Crystals of Aluminum.

Molecular dynamics simulations of cracked nanocrystals of aluminum were performed in order to investigate the crack length and grain boundary effects. Atomistic models of single-crystals and bi-crystals were built considering 11 different crack lengths. Novel approaches based on fracture mechanics concepts were proposed to predict the crack length effect on single-crystals and bi-crystals. The results showed that the effect of the grain boundary on the fracture resistance was beneficial increasing the fracture toughness almost four times for bi-crystals.

On incipient plasticity in the vicinity of grain boundaries in aluminum bicrystals: Experimental and simulation nanoindentation study

The local mechanical behavior near <100> symmetric tilt grain boundaries in aluminum bicrystals were studied by the nanoindentation technique as well as by computer simulations. Experimentally, grain boundaries with misorientation angles 8.7°, 13.8° and 18.8° were examined. Two well pronounced pop-in events were observed on the load – penetration depth curves measured during indentation in the close vicinity of the studied boundaries. The load of the pop-ins, observed close to the boundaries, practically did not differ from that obtained in the grain interior. This was interpreted as evidence that the boundaries with misorientations in the examined angular range do not represent specific sites for sources of lattice dislocations. The load, at which the second pop-ins took place, substantially increased with increasing misorientation angle of the examined boundaries. Quasistatic molecular dynamics simulations were performed to identify the details of the interaction between grain boundary and dislocations generated during indentation. For this purpose, the bicrystals with similar geometry and misorientation angles, as investigated experimentally, were computed. The simulation results showed that the direct transmission of incoming dislocations across the grain boundary was the primary mechanism for the plastic flow transfer past the boundary. The analysis of the results of both experiments and simulations provided evidence that the capability of grain boundaries to act as a barrier for the motion of incoming dislocations depends crucially on grain boundary structure.

Effect of Inclination Angle on Mechanical Behaviour and Deformation Asymmetry in Aluminium Bicrystal

Effect of Inclination Angle on Mechanical Behaviour and Deformation Asymmetry in Aluminium Bicrystal

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.

On the Influence of Grain Boundary Misorientation on the Severe Plastic Deformation of Aluminum Bicrystals: A Three-Dimensional Crystal Plasticity Finite Element Method Study

A three-dimensional crystal plasticity finite element method model is developed to investigate the influence of the grain boundary (GB) misorientation on the equal-channel angular pressing deformation of aluminum bicrystals. Aluminum bicrystals with symmetric 〈112〉 tilt boundaries and misorientations of 9 deg (low angle), 15 deg (transitional), and 30 deg (high angle) have been designed to study the influence of GB misorientations on the deformed areas near GBs. The numerical results indicate that a high-angle grain boundary acts as a barrier in terms of Mises stress distribution, plastic slip, and lattice rotation, while the aluminum bicrystal with low-angle grain boundary still behaves similarly to a single crystal. An intermediate configuration is found for the aluminum bicrystal with transitional grain boundary. It is also found that the geometry of the GB after deformation depends on the initial orientation of the grain at the lower part of the billet.

Grain boundary effect on nanoindentation: A multiscale discrete dislocation dynamics model

Nanoindentation is a convenient method to investigate the mechanical properties of materials on small scales by utilizing low loads and small indentation depths. However, the effect of grain boundaries (GBs) on the nanoindentation response remains unclear and needs to be studied by investigating in detail the interactions between dislocations and GBs during nanoindentation. In the present work, we employ a three-dimensional multiscale modeling framework, which couples three-dimensional discrete dislocation dynamics (DDD) with the Finite Element method (FEM) to investigate GB effects on the nanoindentation behavior of an aluminum bicrystal. The interaction between dislocations and GB is physically modeled in terms of a penetrable GB, where piled-up dislocations can penetrate through the GB and dislocation debris at GBs can emit full dislocations into grains. In the simulation, we confirmed two experimentally observed phenomena, namely, pop-in events and the dependence of indentation hardness on the distance from GB. Two pop-in events were observed, of which the initial pop-in event is correlated with the activation and multiplication of dislocations, while the GB pop-in event results from dislocation transmission through the GB. By changing the distance between the indenter and GB, the simulation shows that the indentation hardness increases with decreasing GB-indenter distance. A quantitative model has been formulated which relates the dependency of indentation hardness on indentation depth and on GB-indenter distance to the back stress created by piled-up geometrically necessary dislocations in the plastic zone and to the additional constraint imposed by the GB on the plastic zone size.

Effect of Electrical Pulses on the Mechanical Behavior of Single Crystals of Nickel-Based CMSX-4 Superalloy and the Mobility of Low-Angle Grain Boundary in Aluminum Bicrystals

The electroplastic effect is studied on mechanically loaded single crystals of nickel-based CMSX-4 superalloy and pure aluminum bicrystals. Both objects of study exhibit elevated plasticity under electrical pulses. Traces of the electroplastic deformation that occurs in CMSX-4 single crystals at current densities J higher than 3 kA mm−2 and aluminum bicrystals at J exceeding 1.6 kA mm−2 are analyzed using a scanning electron microscope.

Local mechanical properties in the vicinity of [formula omitted]Σ3/[111] symmetric tilt grain boundary in aluminum bicrystal

The local mechanical properties near the (11¯0)Σ3/[111] symmetric tilt grain boundary in a high purity aluminum bicrystal were studied by the nanoindentation technique. The first pop-in load and its width significantly decreased near / at the grain boundary compared to those in the grain interior. The critical stress required to nucleate dislocations at the grain boundary was estimated to be notably lower than that in the grain interior. Moreover, it was found that the slope of the load-penetration curve prior to the first pop-in was smaller near the grain boundary than in the grain interior. This result was discussed in terms of possible micro plasticity effects and decreasing the elastic modulus in the close vicinity of the grain boundary.

A finite-deformation dislocation density-based crystal viscoplasticity constitutive model for calculating the stored deformation energy

A previously-developed infinitesimal strain and dislocation density based crystal viscoplasticity constitutive theory is reformulated in the finite deformation regime in a thermodynamically consistent manner. The constitutive equations are then numerically implemented into the Abaqus/Standard finite element package. The constitutive model is primarily verified and calibrated with respect to single crystal aluminum experiments and then the distribution of stored deformation energy in aluminum bicrystals and polycrystals is investigated to estimate the driving forces for grain boundary migration due to the stored energy difference across grain boundary. Our statistical analysis shows that the onset of strain induced grain boundary migration in a polycrystalline microstructure is basically dependent on the spatial distribution of the stored deformation energy rather than the overall stored deformation energy value; and it preferably occurs in a few predictable specific regions where certain grain orientations meet at a grain boundary across which high values of stored energy difference happens.

Geometrically necessary dislocation density measurements at a grain boundary due to wedge indentation into an aluminum bicrystal

An aluminum bicrystal with a symmetric tilt Σ43 (3 3 5)[1 1 0] coincident site lattice grain boundary was deformed plastically via wedge indentation under conditions that led to a plane strain deformation state. Plastic deformation is induced into both crystals and the initially straight grain boundary developed a significant curvature. The resulting lattice rotation field was measured via Electron Backscatter Diffraction (EBSD). The Nye dislocation density tensor and the associated Geometrically Necessary Dislocation (GND) densities introduced by the plastic deformation were calculated. The grain boundary served as an impediment to plastic deformation as quantified through a smaller lattice rotation magnitude and smaller GND density magnitudes in one of the crystals. There is evidence that the lattice rotations in one grain brought a slip system in that grain into alignment with a slip system in the other grain, upon which the impediment to dislocation transmission across the grain boundary was reduced. This allowed the two slip systems to rotate together in tandem at later stages of the deformation. Finite element crystal plasticity simulations using classical constitutive hardening relationship capture the general features observed in the experiments.

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.

Low cycle fatigue in aluminum single and bi-crystals: On the influence of crystal orientation

Aluminum single crystals with three different double-slip orientations ([2̅32], [3̅3̅2], [130]) and two aluminum bi-crystals – one with a high-angle grain boundary and one with a low-angle grain boundary - were cyclically deformed up to 100 cycles under constant displacement control. The distribution of the local strain and the local strain amplitudes was captured by in-situ digital image correlation (DIC). Dislocation structure analysis was performed by electron channeling contrast imaging (ECCI) and the evolution of local misorientations was recorded by high resolution electron backscatter diffraction (EBSD). The DIC results show a homogeneous strain amplitude distribution in the single crystals while the measured strain amplitude in the low-angle grain boundary bi-crystal sample differs significantly. ECCI observations reveal the presence of dislocation cells elongated along the trace of the primary {111} slip plane in all investigated crystals and the formation of deformation bands parallel to the trace of {110} planes. Deformation bands (DB) were observed in all samples but their frequency and misorientation with respect to the matrix was found to sensitively depend on the crystal orientation and the local strain amplitude. Our results on the bi-crystals show that the grain orientation mainly determines the local stresses and therefore also the formation of the associated dislocation structures rather than the grain boundary character.

Effect of Interface on the Deformation of Aluminium Bicrystal: Atomistic Simulation Study

Molecular dynamic (MD) simulation has been conducted to study the effect of interface structure on the mechanical response of eight symmetric tilt grain boundaries in high stacking-fault Al. It is found that the grain boundaries with E structure unit (SU) have higher energy, but the grain boundary energy alone cannot be used as a parameter to determine the mechanical properties of grain boundary. The SUs, especially E units, do have an influence on the mechanical response of grain boundaries. Our results show that the dislocation imitates from E units preferably, but this depends on the dissociation at grain boundary.

Motion of Grain Boundaries: Experiments on Bicrystals

Recent research on grain boundary migration is reviewed. Novel in-situ measuring techniques based on orientation contrast imaging and the experimental results obtained on specially grown bicrystals are presented. Particularly, the investigated faceting and migration behavior of low angle grain boundaries under the curvature force in aluminum bicrystals was addressed. In contrast to the pure tilt boundaries, which remained straight/flat and immobile during annealing at elevated temperatures, mixed tilt-twist boundaries readily assumed a curved shape and steadily moved under the capillary force. Computational analysis revealed that this behavior is due to the inclinational anisotropy of grain boundary energy, which in turn depends on boundary geometry. The migration of planar grain boundaries induced by a magnetic field was measured in bismuth and zinc bicrystals. Various structurally different boundaries were investigated. The results revealed that grain boundary mobility essentially depends on the misorientation angle and the inclination of the boundary plane. Stress driven boundary migration in aluminium bicrystals was observed to be coupled to a tangential translation of the grains. The activation enthalpy of high angle boundary migration was found to vary non-monotonously with misorientation angle, whereas for low angle boundaries the migration activation enthalpy was virtually the same. The motion of the mixed tilt-twist boundaries under stress was observed to be accompanied by both the translation of adjacent grains parallel to the boundary plane and their rotation around the boundary plane normal.

Grain boundary motion and grain rotation in aluminum bicrystals: recent experiments and simulations

The results of experimental and computational efforts over recent years to study the motion of geometrically different grain boundaries and grain rotation under various driving forces are briefly reviewed. Novel in-situ measuring techniques based on orientation contrast imaging and applied simulation techniques are described. The experimental results obtained on specially grown aluminum bicrystals are presented and discussed. Particularly, the faceting and migration behavior of low angle grain boundaries under the curvature force is addressed. In contrast to the pure tilt boundaries, which remained flat/faceted and immobile during annealing at elevated temperatures, mixed tilt-twist boundaries readily assumed a curved shape and steadily moved under the capillary force. Computational analysis revealed that this behavior is due to the inclinational anisotropy of grain boundary energy, which in turn depends on boundary geometry. The shape evolution and shrinkage kinetics of cylindrical grains with different tilt and mixed boundaries were studied by molecular dynamics simulations. The mobility of low angle <100> boundaries with misorientation angles higher than 10°, obtained by both the experiments and simulations, was found not to differ from that of the high angle boundaries, but decreases essentially with further decrease of misorientation. The shape evolution of the embedded grains in simulations was found to relate directly to results of the energy computations. Further simulation results revealed that the shrinkage of grains with pure tilt boundaries is accompanied by grain rotation. In contrast, grains with the tilt-twist boundaries composed of dislocations with the mixed edge-screw character do not rotate during their shrinkage. Stress driven boundary migration in aluminium bicrystals was observed to be coupled to a tangential translation of the grains. The activation enthalpy of high angle boundary migration was found to vary non-monotonically with misorientation angle, whereas for low angle boundaries the migration activation enthalpy was virtually the same. The motion of the mixed tilt-twist boundaries under stress was observed to be accompanied by both the translation of adjacent grains parallel to the boundary plane and their rotation around the boundary plane normal.