Aluminum Single Crystals Research Articles

MOLECULAR DYNAMIC MODELING OF STRESS FIELDS IN PLATES WITH A CENTRAL CRACK MADE OF MATERIALS WITH A FACE-CENTERED CUBIC LATTICE

The purpose of this study is to determine the stress-strain state at the atomistic level using molecular dynamic modeling of the structure of the material, which were selected as single crystals of copper and aluminium. A series of computational experiments were carried out on plates with a central crack in the LAMMPS open code. Also, LAMMPS conducted experiments on cubic samples from the same materials to find the tensor components of elastic modules. The visualizations obtained in the OVITO software package are shown, which show the distribution of stress tensor components over different time steps for copper and aluminum single crystals. The work presents graphs showing the dependence of the stress tensor components on the polar angle, obtained by atomistic and classical approaches. The comparison showed that the stressfields at the nanoscale level are in good agreement with their macroscopic magnitudes, so a continuum approach can be applied at the atomistic level.

Evaluation of the surface and subsurface evolution of single-crystal yttrium aluminum garnet during polishing

Yttrium aluminum garnet (Y3Al5O12/YAG) has attracted extensive research and application interest in the field of optoelectronics. However, the evolution mechanism of the surface and subsurface of YAG single crystals during polishing have not been elucidated, thus hindering the in-depth understanding of the processing mechanism of brittle solids. Herein, rough and fine polished YAG crystals were obtained by designing a processing scheme, and the surface and subsurface were expeditiously evaluated simultaneously by angle-resolved ellipsometry. The dependence between the ellipsometric parameter near the Brewster angle and the surface and subsurface quality was determined by modeling the polished surface and subsurface damage optics. The monotonicity of the quasi-Brewster angle shift (qBAS) reveals the diminishing feature of subsurface damage, and the oscillation of the slope unveils the atomic-level topography evolution of the polished planes. The results were validated using data from surface and subsurface damage tests, thereby proving the feasibility and reliability of the method. Furthermore, the refractive indices (210–1690 nm) of YAG were identified based on the spectroscopic ellipsometry data of the fine polished crystal. These findings provide a promising paradigm for predicting and evaluating the surface and subsurface microstructures and optical properties of brittle solids in material-removal-based surface engineering.

Molecular dynamics analysis of friction-triggering process with spherical probe

Molecular dynamics simulation with embedded atom method/Finnis–Sinclair potential function was utilised to study the friction properties of the friction-triggering process at the mesoscopic scale. The sliding friction process of a spherical probe with different diameters on the surface of a single-crystal aluminum matrix was simulated and the influences of sliding friction on the atomic behaviour of the surface under different contact conditions were analysed. The relationship between contact force and friction coefficient with spherical diameter was studied. The research shows that the plastic groove, normal force and friction force increase with the increase in probe diameter in the sliding friction process. The contribution of increasing spherical diameter to normal force is greater than that of friction force. The coefficient of friction varies with the ratio of the actual contact area to the normal force. After the running-in process is stable, the friction coefficient fluctuates around a certain value as theoretically verified by the two-term friction law. The increase in the diameter of the spherical probe leads to the increase in the number of dislocation atoms in the workpiece and the formation of dislocation rings, which result in the difference in the sliding friction process under indenters with different diameters. The results provide valuable reference for friction-triggering measurement based on 3D nanometre probes such as friction force microscopy.

Investigation of the atomistic behavior in nanofinishing single-crystal aluminium nitride with hydroxyl radical ∙OH environment

The effect of depth of pressing and sliding velocity on chemical reaction, wear and atom removal mechanisms was investigated in nanofinishing single-crystal aluminum nitride with hydroxyl radical ∙OH environment. The simulation results show that aluminum nitride can be oxidized by hydroxyl radical ∙OH from the perspective of free energy change. Only chemical action, the decomposed atoms in the solution form bonds with the Al atoms on the aluminum nitride surface, and the bonds are expressed in the form of O-[Al]n, H-[Al]n and OH–[Al]n. However, no Al atoms were discovered to break away from the substrate. The potential energy of substrate and the number of Al-O bonds are positively correlated with the depth of pressing and sliding velocity, and the degree of chemical reaction can be promoted by strong mechanical behavior. The tangential force relies on interfacial Al-C and AlOC bonds, the more interfacial bonds between substrate and abrasive, the more severe wear of the substrate surface. The sliding of diamond abrasive leads to the amorphous evolution of the crystal structure, and the separation of atoms from the initial position produces defects, which destroys the stable covalent bond structure of aluminum nitride. As a result, the total number of dangling bonds (i.e. unsaturated bond) of surface layer atoms increases, and the greater the depth of pressing and velocity, the more obvious this phenomenon will be, and the more atoms will be removed. In this study, atomic removal behavior of aluminum nitride was systematically analyzed at atomic scale, which will provide theoretical guidance for ultra-precision and low-damage processing of aluminum nitride.

Analytical methods for superresolution dislocation identification in dark-field X-ray microscopy

In this work, we develop several inference methods to estimate the position of dislocations from images generated using dark-field X-ray microscopy (DFXM) -- achieving superresolution accuracy and principled uncertainty quantification. Using the framework of Bayesian inference, we incorporate models of the DFXM contrast mechanism and detector measurement noise, along with initial position estimates, into a statistical model coupling DFXM images with the dislocation position of interest. We motivate several position estimation and uncertainty quantification algorithms based on this model. We then demonstrate the accuracy of our primary estimation algorithm on synthetic realistic DFXM images of edge dislocations in single crystal aluminum. We conclude with a discussion of our methods' impact on future dislocation studies and possible future research avenues.

Modeling of acoustoelastic effects based on anharmonic atomic interaction

The acoustoelastic effects have been widely applied in the fields of nondestructive characterization and optimal design of nanomaterial. However, the traditional acoustoelastic constitutive model is established at the macro scale, which is incompatible with nanomaterials, and the micro mechanism of acoustoelastic effect is neglected. In this work, the acoustoelastic constitutive model is reestablished based on the elastic constitutive relation and the motion equation of elastic wave at the atomic scale. The derivation demonstrates that the acoustoelastic effect is caused by the anharmonic interatomic potential. Furthermore, as the interatomic distance increases and decreases, different elastic constitutive relations are exhibited. As a result, an acoustoelastic constitutive model taking into account different elastic constitutive relations is proposed, and the theoretical acoustoelastic effect of single crystal aluminum with anisotropic characteristics is calculated. To verify the proposed constitutive model, the acoustoelastic effect at the atomic scale is simulated based on molecular dynamics (MD). The outcomes demonstrate the consistency between the simulated and theoretical acoustoelastic effects. That is, even though the magnitude of the tensile and compressive stresses is the same, the material exhibits different acoustoelastic effect. This work presents a fairly comprehensive theoretical model for the acoustoelastic effect at the atomic scale.

Dynamic compaction of aluminum with nanopores of varied shape: MD simulations and machine-learning-based approximation of deformation behavior

We compare two machine-learning-based approaches, artificial neural network (ANN) and micromechanical model with automatic Bayesian identification of the model parameters, in application to mimicking the deformation behavior of nanoporous aluminum extracted from molecular dynamics (MD) simulations. Reference data are generated by means of MD simulation of both hydrostatic and uniaxial deformation with compression of representative volume elements of aluminum single crystal with nanopores of spherical, cubic and cylindrical shapes at the temperatures of 300, 500, 700 and 900 K. Several typical sizes of the nanopores are considered: The smallest one corresponds to the initial porosity less than 1%, while the largest one gives the initial porosity in the range of 30–50%. Plastic collapse of nanopores in all cases occurred by the mechanism of emission of partial Shockley dislocation half-loops from the pore surface. The emission of initial loop occurred earlier in the case of cylindrical pore; however, this did not lead to an explosive increase in the number of dislocations in the system at this stage. In general, flat free surfaces of pores are less subjected to the dislocation nucleation than the rounded ones. A new physically-based micromechanical model of the plastic compaction of nanoporous metal is formulated with accounting of different pore shapes and anisotropy of the compaction process. Both tested machine-learning approaches show an adequate approximation of MD data. The developed ANN and parameterized micromechanical model are applied to simulate the propagation of a shock wave in nanoporous aluminum in comparison with direct MD simulations of this process; this comparison shows an adequate description of the shock wave structure by means of both approaches incorporated into continuum mechanics modeling. Thus, the developed machine-learning-based approaches can be applied as constitutive equations of nanoporous aluminum in macroscopic simulations of the dynamic compaction and shock-wave processes in this material.

Evolution of Preset Void and Damage Characteristics in Aluminum during Shock Compression and Release.

It is well known that initial defects play an essential role in the dynamic failure of materials. In practice, dynamic tension is often realized by release of compression waves. In this work, we consider void-included single-crystal aluminum and investigate the damage characteristics under different shock compression and release based on direct atomistic simulations. Elastic deformation, limited growth and closure of voids, and the typical spall and new nucleation of voids were all observed. In the case of elastic deformation, we observed the oscillatory change of void volume under multiple compression and tension. With the increase of impact velocity, the void volume reduced oscillations to the point of disappearance with apparent strain localization and local plastic deformation. The incomplete or complete collapsed void became the priority of damage growth under tension. An increase in sample length promoted the continuous growth of preset void and the occurrence of fracture. Of course, on the release of strong shock, homogeneous nucleation of voids covered the initial void, leading to a wider range of damaged zones. Finally, the effect of the preset void on the spall strength was presented for different shock pressures and strain rates.

Effect of initial temperature on impact-induced spalling behavior in single-crystal aluminum studied by molecular dynamics simulations

Spallation is a typical dynamic fracture mode under shock loading and has attracted the attention of most researchers. However, due to the difficulty in measuring temperature in dynamic experiments, the effect of initial temperature on spalling response has been rarely investigated. Molecular dynamics simulation perfectly corresponds to the short duration and high strain rate of the spalling process. Therefore, in this work, molecular dynamics simulations are used to study the spalling reaction of single-crystal aluminum at different initial temperatures. The research has shown that the evolution of spallation is related to dislocation and hole nucleation. First, the spall strength of the material decreases as initial temperature increases, while the dislocation density gradually increases. However, when the initial temperature increases to 750 K, the dislocation density decreases. Then, the number of holes and the degree of damage change as initial temperature increases. However, at the low impact strength (v < 2.0 km/s), the changes in the number of holes and the degree of damage are highly dependent on the initial temperature. In the case of high impact strength, the opposite is true. Finally, the thermodynamic path of the material during impact compression is studied. It is found that melting may occur during compression, release or tension, and damage stages, depending on the initial temperature and impact strength. The discovery and research of these systems have laid a solid foundation for subsequent studies.

Plastic Softening Induced by High-Frequency Vibrations Accompanying Uniaxial Tension in Aluminum.

We have investigated the influences of high-frequency vibration (HFV) superimposed onto the monotonic uniaxial tension in single-crystal aluminum (Al) specimens by molecular dynamics simulations. It was found that HFV induces softening, i.e., reduction in peak stress. Similar to previous experimental results, the softening increases with the increasing HFV amplitude. Dependences on lattice orientation, tensile strain rate, and a preset notch are considered. Lattice orientation plays an important role in peak stress and plasticity. The evolution of the atomic structure reveals that dislocations have enough time to annihilate under a lower tensile strain rate, resulting in strong ups and downs in the strain–stress curves. Under a higher strain rate, newly appearing dislocations interact with previous ones before the latter annihilate, densifying the dislocation network. As a result, further dislocation motions and annihilations are considerably impeded, leading to a relatively smooth flow stage. Furthermore, by modifying the propagation direction of shear bands, a preset notch can strengthen the peak tensile stress under low-level amplitude HFVs.

Ultrafast visualization of incipient plasticity in dynamically compressed matter

Plasticity is ubiquitous and plays a critical role in material deformation and damage; it inherently involves the atomistic length scale and picosecond time scale. A fundamental understanding of the elastic-plastic deformation transition, in particular, incipient plasticity, has been a grand challenge in high-pressure and high-strain-rate environments, impeded largely by experimental limitations on spatial and temporal resolution. Here, we report femtosecond MeV electron diffraction measurements visualizing the three-dimensional (3D) response of single-crystal aluminum to the ultrafast laser-induced compression. We capture lattice transitioning from a purely elastic to a plastically relaxed state within 5 ps, after reaching an elastic limit of ~25 GPa. Our results allow the direct determination of dislocation nucleation and transport that constitute the underlying defect kinetics of incipient plasticity. Large-scale molecular dynamics simulations show good agreement with the experiment and provide an atomic-level description of the dislocation-mediated plasticity.

Mechanical Aspects of Nonhomogeneous Deformation of Aluminum Single Crystals under Compression along [100] and [110] Directions

The deformation behavior of aluminum single crystals subjected to compression along the [100] and [110] directions is numerically examined in terms of crystal plasticity. A constitutive model taking into account slip geometry in face-centered cubic crystals is developed using experimental data for the single-crystal samples with lateral sides coplanar to certain crystal planes. Two sets of calculations are performed using ABAQUS/Explicit to examine the features of plastic strain evolution in perfectly plastic and strain-hardened crystals. Special attention is given to the discussion of mechanical aspects of crystal fragmentation. Several distinct deformation stages are revealed in the calculations. In the first stage, narrow solitary fronts of plastic deformation are alternately formed near the top or bottom surfaces and then propagate towards opposite ends to save the symmetry of the crystal shape. The strain rate within the fronts is an order of magnitude higher than the average strain rate. The first stage lasts longer in the strain-hardened crystals, eventually giving way to an intermediate stage of multiple slips in different crystal parts. Finally, the crystal shape becomes asymmetrical, but no pronounced macroscopic strain localization has been revealed at any deformation stage. The second stage in perfectly plastic crystals relates to abrupt strain localization within a through-thickness band-shaped region, accompanied by macroscale crystal fragmentation. Stress analysis has shown that pure compression took place only in the first deformation stage. Once the crystal shape has lost its symmetry, the compressive stress in some regions progressively decreases to zero and eventually turns tensile.

Twins and grain boundaries-dominated the reverse Bauschinger effect and tension-compression asymmetry

The failure modes and mechanisms of single and polycrystalline aluminum under different stress states are very different. Therefore, molecular dynamics methods were used to study the tension-compression asymmetry and Bauschinger effect of single and polycrystalline aluminum under different loads. We found both single and polycrystalline aluminum exhibit work hardening under tensile and compressive loads. The difference in this hardening phenomenon is manifested in that the characteristics of single and polycrystalline aluminum are brittle fracture and ductile fracture, respectively. Furthermore, we found that the tension-compression asymmetry of polycrystalline aluminum is more obvious than that of single crystal aluminum, and it manifested as a reverse tension-compression asymmetry. The Bauschinger effect is caused by the defects introduced by preloading, and the asymmetry of the Bauschinger effect is caused by different microscopic defects. Dislocations introduced by the pre-stretching of single crystal aluminum degrade its performance under reverse loading. Polycrystalline aluminum has more twinned regions under compressive load, which hinder the movement of dislocations and grain boundaries during reverse stretching, showing the reverse Bauschinger effect. This study has important theoretical guiding significance for a comprehensive and in-depth understanding of the mechanical properties and microstructural evolution mechanism of single and polycrystalline aluminum.

Equal channel angular pressing of single crystal aluminum: a molecular dynamics simulation

The equal channel angular pressing (ECAP) of single-crystal aluminum was investigated using molecular dynamics to provide insights into the deformation behavior and polycrystal formation. Three samples with different initial crystallographic orientations were processed by an ECAP die with a curvature angle (Ψ) of 20° and a channel angle (Φ) of 90° at 300 K. The samples were oriented, such that [100], [110], and [111] directions were parallel to the extrusion direction (ED). The shear strain was the predominant mode of deformation during ECAP with the presence of normal strain before entering the deformation zone due to the high compression force. The sample with [100] direction oriented along the ED showed significant grain fragmentation, the highest lattice rotation but the lowest dislocation density and shear strain, whereas the sample with [111] orientation showed the opposite results. A correlation between the number of active slip systems, the magnitude of shear strain, and grain rotation angle was observed with most of the lattice rotation occurring around the transverse direction. Furthermore, the deformation and grain fragmentation processes occurring in the deformation zone were also investigated. Finally, the model could produce results similar to the results obtained experimentally in the literature.

Attenuation of the Bauschinger effect and enhancement of tension-compression asymmetry in single crystal aluminum by temperature.

Temperature has a great influence on the mechanical properties of nano-materials. The molecular dynamics method was used to study the effect of temperature on the tension–compression asymmetry and Bauschinger effect of nano single crystal aluminum (NSCA). The strain-hardening behavior of NSCA in the tensile plastic stage is significantly enhanced when the temperature is higher than 400 K. The plastic deformation mechanism of tensile loading shifts from slip blocking of dislocations in grains to dislocation nucleation. The degradation of the mechanical properties of NSCA under compressive loading increases gradually with the increase of temperature. Dislocation emission is limited under compressive loading. Nonetheless, plastic deformation may still be regulated by dislocation slip during severe plastic deformation stages and at elevated temperatures. Temperature enhancement can effectively promote the movement of pre-dislocations and eliminate residual stresses. A new microscopic insight into the temperature attenuated Bauschinger effect is provided. This study provides important theoretical guidance for a comprehensive and in-depth understanding of the high-temperature mechanical properties and microstructure evolution mechanism of NSCA.

High-temperature diffusion of the acceptor impurity Be in AlN

The high-temperature diffusion of an acceptor impurity of beryllium (Be) into bulk single-crystal aluminum nitride (AlN) has been studied. It is shown that the introduction of Be leads to the appearance of green luminescence of AlN, which is stable at room temperature and is observed over the entire thickness of the sample. It was shown by the method of luminescence analysis that the Be diffusion process is most efficiently realized in the temperature range from 1800 to 2100oC and is characterized by extremely high diffusion coefficients D=10-7 cm2/s and 10-6 cm2/s, respectively. It is shown that a prolonged diffusion process (t≤1 hour) at a temperature of 2100oC leads to concentration quenching of the luminescence of near-surface AlN layers with a thickness of ~80 μm, which makes it possible to estimate the concentration of beryllium impurities in the near-surface layer on the order of ~1019 cm-3. Keywords: AlN PVT doping diffusion.

Высокотемпературная диффузия акцепторной примеси Ве в AlN

The high-temperature diffusion of an acceptor impurity of beryllium (Be) into bulk single-crystal aluminum nitride (AlN) has been studied. It is shown that the introduction of Be leads to the appearance of green luminescence of AlN, which is stable at room temperature and is observed over the entire thickness of the sample. It was shown by the method of luminescence analysis that the Be diffusion process is most efficiently realized in the temperature range from 1800°C to 2100°C and is characterized by extremely high diffusion coefficients D = 10-7 cm2/s and 10-6 cm2/s, respectively. It is shown that a prolonged diffusion process (t ≥ 1 hour) at a temperature of 2100°C leads to concentration quenching of the luminescence of near-surface AlN layers with a thickness of ≈ 80 μm, which makes it possible to estimate the concentration of beryllium impurities in the near-surface layer on the order of 10E19 cm-3.

Sudden change of spall strength induced by shock defects based on atomistic simulation of single crystal aluminum

This Letter interprets the detailed mechanism about the discontinuous variation in spall strength associated with shock-induced microstructure. It is known that elastic deformation, dislocation and stacking fault, as well as shock structural transition will appear in turn with the increase of shock intensity. Our molecular dynamics simulations of single-crystal aluminum reveal that the spall damage shows an evident dependence on the shock-induced microstructure. The nanovoids nucleate homogeneously in the region of elastic deformation or phase transition, resulting in higher spall strength. However, the nanovoids nucleate heterogeneously in the region of dislocation and stacking faults, which leads to a sudden decrease in spall strength. Different nucleation modes have a significant effect on the temperature response of spall strength. Under the condition of heterogeneous nucleation, the spall strength shows different temperature dependence and does not change inversely with temperature.

Cyclic Tension Induced Pattern Formation on [001] Single-Crystal Aluminum Foil

Cyclic tension of (100)[001]-oriented single-crystal aluminum foils with the frequency 5 Hz forms a tweed pattern. Its period is several microns and increases by a factor of 1.5 in the temperature range 233–363 K. A model is proposed for structural relaxation of the medium on spatial and time meso- and macroscales under cyclic loading. Conditions under which a steady pattern forms are found based on the analysis of kinetic equations. The number of bands in the steady pattern is found to be related to the strain rate. The process activation energy is determined.

Exploration of the best reference material on anelastic measurement by cyclic loading under high pressure

ABSTRACT Anelastic measurement by cyclic loading under high pressure has been developed by means of in situ X-ray observation at a synchrotron facility. In this method, the reference material is a key factor to precisely determine attenuation and moduli of unknown materials. We compared the performance of three types of reference materials (dense polycrystalline alumina, alumina single-crystal parallel to c-axis, and flexible graphite) under the pressure of 3 GPa and the temperature range between 1173 and 1373 K. The phase lags of strain between reference materials and samples show that the flexible graphite is less attenuated than dense polycrystalline alumina and alumina single crystal in various periods. The strain ratios show that the flexible graphite is much softer and can produce the measurable strain in the limited displacement. The flexible graphite, as reference material, is more excellent to detect the relatively lower energy dispersion of mantle minerals at high pressure.