Aluminum Single Crystals Research Articles

Study on the uniaxial tensile mechanical behavior of two-dimensional single-crystal aluminum nitride

Abstract To investigate the tensile behavior and mechanical properties of single-crystal aluminum nitride (AlN) at the microscopic level, molecular dynamics simulations were used to study the effects of crystal orientation, strain rate, environmental temperature, and hole defect size on fracture strength, fracture mechanism, and potential energy during uniaxial tensile. The results show that the tensile strength of AlN in the [100] crystal direction is stronger. The anisotropic behavior characteristics of Al-N bonds fracture mechanism, crack growth rate, and cracking degree are significant when stretched along the [100], [010], and [110] crystal directions. Under high temperature condition, the lattice structure undergoes changes, causing grain boundaries to move and slip. This facilitates the breaking of bonds, leading to a decrease in tensile strength and a reduction in stored potential energy. Hole defects cause more lattice damage, reducing the energy required for Al-N bonds breakage and facilitating the propagation of microcracks. Additionally, it was found that the strain rate affects the stress-strain behavior of the model. An increase in strain rate leads to an increase in breaking stress, and the rapid deformation of AlN results in more energy being stored in the lattice in the form of potential energy. Therefore, the tensile strength and potential energy are improved.

New insights into the deformation mechanism of orientation-dependent nanoindentation behaviours

Nanoindentation has been widely used to access the micro-/nanomechanical properties of materials. However, the contribution of micro-texture evolution to the anisotropic surface topography, the correlation of dislocation density characteristics with the orientation-dependent plastic volume, and the specifics of inherent slip activities, are still not fully elucidated up to now.Herein, nanoindentation simulations were performed on (001)-, (101)- and (111)-oriented single-crystal aluminium. Dominant slip systems are exactly determined for each surface pile-up for the first time. Moreover, lattice rotation facilitates the formation of pile-ups. The anisotropic plastic volume is controlled by the preference of intrinsic dislocation movement. In (001) sample, dislocations prefer to propagate along the free surface instead of into the depth of the sample, causing its most obvious pile-ups.

Ab initio simulation of the dynamic shock response of single crystal and lightweight multicomponent alloy

The dynamic response of shock wave impact on single crystal aluminium and lightweight multicomponent alloy Al-Cu-Li-Mg is simulated by using the combination of Ab initio Molecular Dynamics (AIMD) and Multi-Scale Shock Technique (MSST), with the analysis carried out at the atomic/electronic levels. The simulation is verified by comparing the particle velocity of single crystal obtained in this work with the data in literature. The shock compression process not only involves the migration of atoms, but also is related to electronic transition. Two stages could be found in the shock compression process: oscillatory compression of the crystal cell and oscillatory migration of the atoms. The crystal structure of the multicomponent alloy could be disordered even at low shock speed, due to the difference in the ability to migrate between different kinds of atoms. As the sample is shock-compressed, the contribution proportion of crystal orbitals shows a sharp decrease for D orbital, while it increases significantly for S orbital and P orbital. The electron structure shows a quicker response to the shock wave compression process than the crystal structure. The orbital contribution from P orbital of the crystal is mainly due to the P orbital of Al atoms, while the orbital contribution from D orbital of the crystal is mainly due to the D orbital of Cu atoms. Total Density of States (TDOS) is mainly contributed by the Projected Density of State (PDOS) of Cu atoms in the occupied state of energy levels, while it is close to the PDOS of Al atoms in the non-occupied state of energy levels.

In situ SEM fatigue testing technology for metallic materials: a review.

Fatigue failure is one of the most common fracture modes of structural materials in the industrial field. The study of material fatigue mechanisms and methods for predicting fatigue life has always been of significant interest to researchers due to the abrupt and catastrophic failure mode. In recent decades, the performance and functionality of scanning electron microscopy (SEM) have been continuously improved and expanded. Based on this, the development of in situ fatigue testing in SEM has been rapidly developed. This technology plays a crucial role in providing insights into the deformation behavior of materials under fatigue. Keeping this in view, a comprehensive review of the development and application methods of in situ SEM fatigue testing technology is provided here. The development of in situ SEM fatigue testing devices is provided in brief overview, and the application and research progress of this technology in some representative metal structural materials (nickel-based single-crystal superalloys, steel, aluminum alloys and additive manufacturing materials) are analyzed in detail. Moreover, the perspectives on evaluating fatigue damage, particularly about small cracks and the plastic accumulations fatigue behavior, are presented in this study, utilizing the latest advancements in in situ SEM fatigue testing. Remarks about the present and outlook for future work to be done are then provided.

Deformation behavior of high purity aluminum single crystals under repeated biaxial compressions

Deformation behavior of high purity aluminum single crystals under repeated biaxial compressions

Development of Microstructure in Aluminum Single Crystal During Complex Shearing of Extruded Tube

The development of the microstructure during severe plastic deformation of an aluminum single crystal by complex shearing of the extruded tube (CSET) was studied in this paper. The research has demonstrated that even in a single crystal, an ultrafine-grained microstructure can be obtained during this one-step process. The size of the grains gradually changes and reaches the minimum size on the level of 1 μm at the inner surface of the resulting tube. Simultaneously, preferential orientations in individual parts of the deformed sample change in a complex way. The main mechanism affecting the final microstructure is continuous dynamic recrystallization. The microhardness also exhibits a gradient character with higher values at the inner surface of the tube compared to its center.Graphical

Molecular dynamics simulation of fatigue damage formation in single-crystal/polycrystalline aluminum

The fatigue plasticity mechanisms and local defect characteristics of engineering materials are critical for addressing fatigue damage. This study aims to identify the formation mechanisms of fatigue damage and delineate the microstructural characteristics that resisted or facilitated fatigue-induced plastic accumulation. The fatigue mechanical properties and microstructural evolution processes of single-crystal/polycrystalline aluminum were investigated through strain-controlled fatigue testing, considering three distinct strain amplitudes. The initiation and cumulative occurrence of local plasticity in single-crystal aluminum occur at the intersections of slip planes, which serve as sources of dislocations. The cumulative formation of irreversible surface steps results in local stress concentration, ultimately leading to fatigue damage at the free surface. Polycrystalline aluminum undergoes grain rotation and merging during cyclic loading, leading to a gradual increase in grain size during plastic deformation. The initiation and cumulative occurrence of local plasticity occur at grain boundaries, ultimately leading to fatigue damage at the grain boundaries.

Temperature-Induced Variations in Slip Behavior of Single Crystal Aluminum: Microstructural Analysis.

The simultaneous increase in strength and plasticity of aluminum and its alloys at cryogenic temperatures has been shown in previous research, but the deformation mechanism was still unclear. Therefore, the purpose of this investigation was to reveal the relationship between slip behavior and mechanical response at low temperatures. A quasi-in situ scanning electron microscope was used to observe the evolution of slip bands in the selected aluminum single crystals with two typical orientations at 25 °C, -100 °C, and -180 °C. The results showed that irrespective of orientation, the density of the slip plane was increased with the decline in temperature, which inhibited slip localization and significantly improved plasticity and work hardening. In detail, at RT, the slip bands were widening until the micro-cracks were generated, causing early failure during deformation. When the temperature was decreased to -180 °C, the slip plane density was increased, and the deformation was more homogenous. Moreover, the slip mode was influenced by orientation and temperature. In particular, a single slip system was activated in the sample with the [112] orientation at all the temperatures investigated. Multiple slip systems were found to activate at 25 °C and -100 °C, and only the primary slip system was activated in the sample with [114] orientation at -180 °C. These findings deepen the understanding of slip behavior at cryogenic temperatures, providing new insights into the deformation mechanism of aluminum and its alloys.

A novel impact indentation technique with dynamic calibration method for measurement of dynamic mechanical properties

High strain rates and continuous real-time data acquisition are difficult to balance in existing impact indentation techniques. Meanwhile, the inaccuracy of the testing P-h curves limits the development of quantitative applications for dynamic testing of mechanical properties at micro region. This article develops a momentum exchange impact indentation testing method, which extends the testing range of strain rate to above 105 s−1, and at the same time avoids secondary impact. Impact indentation tests are conducted on single-crystal aluminum specimens with impact velocity from 76.9 to 176 mm/s. Based on calibrated data, the comparison of calculation methods for dynamic hardness is carried out. The dynamic elastic modulus is obtained by means of the Oliver-Pharr method, which increases from 71.9 to 89.4 GPa with increasing impact velocity. A multi-dimensional data analysis method is proposed based on the obtained experimental data to quantitatively study the damping and stiffness characteristics of materials. This testing method is particularly useful for quantitatively obtaining the micro region dynamic mechanical properties of solid materials at high strain rates.

A Full-Field Crystal Plasticity Study on the Bauschinger Effect Caused by Non-Shearable Particles and Voids in Aluminium Single Crystals

In the present work, the goal is to use two-scale simulations to be incorporated into the full-field open software DAMASK version 2.0.3 crystal plasticity framework, in relation to the Bauschinger effect caused by the composite effect of the presence of second-phase particles with surrounding deformation zones. The idea is to achieve this by including a back stress of the critical resolved shear stress in a single-phase simulation, as an alternative to explicitly resolving the second-phase particles in the system. The back stress model is calibrated to the volume-averaged behaviour of detailed crystal plasticity simulations with the presence of hard, non-shearable spherical particles or voids. A simplified particle-scale model with a periodic box containing only one of the spherical particles in the crystal is considered. Applying periodic boundary conditions corresponds to a uniform regular distribution of particles or voids in the crystal. This serves as an idealised approximation of a particle distribution with the given mean size and particle volume fraction. The Bauschinger effect is investigated by simulating tensile–compression tests with 5% and 10% volume fractions of particles and with 1%, 2%, and 5% pre-strain. It is observed that an increasing volume fraction increases the Bauschinger effect, both for the cases with particles and with voids. However, increasing the pre-strain only increases the Bauschinger effect for the case with particles and not for the case with voids. The model with back stress of the critical resolved shear stress, but without the detailed particle simulation, can be fitted to provide reasonably close results for the volume-averaged response of the detailed simulations.

Friction Coefficient of Single-Crystal Aluminum Oxide under Low Sliding Velocity Conditions with Metal and Polymer Materials

Friction Coefficient of Single-Crystal Aluminum Oxide under Low Sliding Velocity Conditions with Metal and Polymer Materials

Nonlinear optical effects in polycrystalline transparent Al2O3 ceramics using femtosecond laser pulses—supercontinuum generation and laser damage

Nonlinear optical properties play a key role in technologies such as broadband laser light sources and ultrafast laser machining. With the emergence of transparent nanocrystalline Al2O3 ceramics as an alternative to single crystal alumina (sapphire), it is critical to understand their nonlinear optical behavior. Here, we report the demonstration of supercontinuum generation in polycrystalline alumina ceramics. Substantial broadening was observed when a focused 515 nm pulsed (260 fs) laser propagated through the ceramic sample. The broadening increased with increasing laser power and displayed stokes/anti-stokes asymmetries. At higher incident power, permanent damage was observed. Our results show that transparent nanocrystalline Al2O3 ceramics have a higher material removal rate than single crystal alumina. These results have interesting implications for laser machining as well as integrated photonics.

Simplification of selective imaging of dislocation loops: diffraction-selected on-zone STEM

ABSTRACT Contrary to the conventional belief that, in transmission electron microscopy (TEM), selective and sharp imaging of dislocation loops can be realized only by accurate tilting of a specimen from the condition that the symmetrical axis of incident electron beam distribution is parallel to a zone axis of the TEM specimen (on-zone condition), we demonstrate that selective dark-field (DF) imaging of dislocation loops at on-zone condition is possible with a scanning TEM (STEM) mode that uses an objective lens aperture to select a diffraction disk of interest. Diffraction-selected on-zone STEM (DsoZ-STEM) has been applied to selective DF imaging of dislocation loops with a short axis length of <2 nm in a single-crystal aluminum irradiated by argon ions and an electron beam at room temperature. It was found that a Kikuchi line enhances the contrast among the dislocation loops and the matrix of DsoZ-STEM images. DsoZ-STEM obeyed g·b invisibility criterion and showed good agreement with a typical visibility change of a dislocation line and a loop in conventional DF images with a specific pair of ± g. In addition, dislocation loops always showed much higher brightness in the inner side compared to the outer side in DsoZ-STEM images, simplifying the distinction of dislocation loops with apparently the same long-axis direction but different b. Thus, DsoZ-STEM can simplify the selective DF imaging for the determination of the number and the character of dislocation loops.

Probing the small-scale impact deformation mechanism in an aluminum single-crystal

Although the rate-dependence of metals has been widely researched, the deformation mechanism under small-scale impact conditions lacked exploration and in-depth understanding. Using quasi-static nanoindentation (strain rate, SR, < 1 s–1) and high strain-rate nano-impact (SR > 103 s–1) with a pyramidal Berkovich tip, this study investigates the influence of SR on the deformation response of an aluminium single crystal (110). The underlying microstructural variance was analyzed using on-axis TKD and TEM. The results show that the impact deformation involves great elastic recovery and different substructural characteristics. In contrast to the uniform sub-grain substructure with medium and high-angle grain boundaries formed during quasi-static indentation, the substructure formed under impact has a more heterogeneous nature including microbands near the surface and sub-grains underneath with dominant low-angle grain boundaries. The significant change in substructure for the impact deformation comes from suppressed thermally activated dislocation motion, leading to the conversion of dislocation glide from wave-like (quasi-static) to planar regime (impacting), and the insufficient rearrangement of geometrically necessary dislocations (GNDs). The heterogeneous microstructure develops due to the competition between high strain-rate-induced planar-slip and strain gradient-induced GND rearrangement, as well as the uneven distribution of SR and strain gradient. Moreover, the underlying incipient mechanism of the microband is proposed, in which successive primary dislocations are nucleated at the surface and glide perpendicular to flanks to pile up. Finally, the influence of SR on indentation size effects is discussed.

ATOMISTIC AND CONTINUAL DETERMINATION OF STRESS FIELDS AT A CRACK TIP IN ANISOTROPIC ENVIRONMENT WITH CUBIC SYSTEM

The article presents a comparative analysis of stresses associated with a crack tip in an anisotropic medium with cubic symmetry of elastic properties under mixed mode loading conditions, using two different approaches: atomistic and continuum approaches. The atomistic approach is based on the use of the molecular dynamics method implemented in the open source Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS). The continuum approach is based on the classical theory of elasticity of anisotropic media, in which the mechanical fields associated with the crack tip are represented using series that generalize the well-known representation of M. Williams to the case of anisotropic media. Using the method of molecular dynamics, a large series of calculations of loading of single-crystal copper and aluminum plates with a face-centered crystal lattice, weakened by a central crack, was carried out using the embedded atom potential (EAM). Molecular dynamics modeling is aimed at determining atomistic stresses and strains near the crack tip. The calculated atomistic stresses were compared with the stress field determined by the continuum theory of elasticity of anisotropic media for crystal lattices with cubic symmetry of elastic properties. A comparative analysis was carried out for the angular dependences of the stress and strain tensor components at different distances from the crack tip for the entire range of mixed strain modes: from perfect normal tension to loadings close to perfect transverse shear. It is established that the fields found on the basis of two fundamentally different approaches (discrete and continuum) are fully consistent with each other. It is demonstrated that the mathematical methods of continuum fracture mechanics can be used to describe stress, strain, and displacement fields at the atomistic level.

Morphological studies of single crystal Alumina fabricated by wet milling method

Morphological studies of single crystal Alumina fabricated by wet milling method

Influence of preliminary compressive deformation on the spall strength of aluminum single crystal

Spall strength is an important property of material subjected to the shock wave loading. It substantially depends on the initial microstructure and its evolution during the preliminary shock compression. By an example of aluminum single crystal, a number of random axisymmetric compressed states are examined by means of molecular dynamics (MD) simulations, different deformation modes are revealed and the spall strength is calculated during subsequent volumetric tensile tests. A domain of elastic states with the spall strength of about 9 GPa, plastically deformed (dislocation plasticity) states with the spall strength of 6–7 GPa and the states with BCC phase transformations and the spall strength in the range 6–9 GPa are revealed as the main scenarios depending on the combination of strains at preliminary compression. MD results are generalized by means of artificial neural networks (ANNs), which allow interpolation between MD-studied points and plotting of continuous dependencies.

Multiscale modeling of nanoindentation and nanoscratching by generalized particle method

Most concurrent multiscale methods that expand atomistic region by continuum domain suffer from inconsistent material constitutive properties, which affect the integrity of model in the interface of atomistic and continuum domains. In this paper, the Generalized Particle (GP) method is employed to simulate nanoindentation and nanoscratching of a single-crystal aluminum sample. The main advantage of the GP method lies in its ability to extend the simulation model while maintaining consistent atomic properties across all scales. Coarsening of the atomic domain has been conducted through two-scale and three-scale GP model. The results showed a strong consistency between the results of full atomic simulations and those achieved through the GP method for both nanoindentation and nanoscratch simulations. Also, wave reflections were not seen at the interfaces. The study revealed that an increase in the number of distinct particle domains led to a reduction in the accuracy of multiscale simulations.

Mesoscale dislocation dynamics modeling of incipient plasticity under nanoindentation

A mesoscale dislocation dynamics model which couples three-dimensional dislocation dynamics (DD), and finite element method (FEM) was used to investigate the mechanical response of Aluminum (Al) single crystal under spherical nanoindentation. Together with an atomistically informed nucleation model, the dislocation dynamics model can capture both the dynamic evolution of dislocations under the complex stress state of indentation and the corresponding constitutive response of material. The resulting load-displacement curves and the evolution of dislocation microstructures were analyzed to provide insights into the underlying deformation mechanism for incipient plasticity under nanoindentation. Our model could show the transition of the governing mechanism for plasticity from nucleation-controlled plasticity to pre-existing source-driven plasticity with increasing indenter size, as shown in recent experiments. In addition, it could capture a clear picture on incipient plasticity including the formation of prismatic loops through successive cross-slips of nucleated dislocations, such as rhombus loop and prismatic helical structures, which agrees well with atomistic modeling results.

Robust crystal structure identification at extreme conditions using a density-independent spectral descriptor and supervised learning

The increased time- and length-scale of classical molecular dynamics simulations have led to raw data flows surpassing storage capacities, necessitating on-the-fly integration of structural analysis algorithms. As a result, algorithms must be computationally efficient, accurate, and stable at finite temperature to reliably extract the relevant features of the data at simulation time. In this work, we leverage spectral descriptors to encode local atomic environments and build crystal structure classification models. In addition to the classical way spectral descriptors are computed, i.e. over a fixed radius neighborhood sphere around a central atom, we propose an extension to make them independent from the material’s density. Models are trained on defect-free crystal structures with moderate thermal noise and elastic deformation, using the linear discriminant analysis (LDA) method for dimensionality reduction and logistic regression (LR) for subsequent classification. The proposed classification model is intentionally designed to be simple, incorporating only a limited number of parameters. This deliberate simplicity enables the model to be trained effectively even when working with small databases. Despite the limited training data, the model still demonstrates inherent transferability, making it applicable to a broader range of scenarios and datasets. The accuracy of our models in extreme conditions (high temperature, high density, large deformation) is compared to traditional algorithms from the literature, namely adaptive common neighbor analysis (a-CNA), polyhedral template matching (PTM) and diamond structure identification (IDS). Finally, we showcase two applications of our method: tracking a solid–solid BCC-to-HCP phase transformation in Zirconium at high pressure up to high temperature, and visualizing stress-induced dislocation loop expansion in single crystal FCC Aluminum containing a Frank–Read source, at high temperature.