Al Single Crystals Research Articles

Dislocation nucleation in Al single crystal at shear parallel to (111) plane: Molecular dynamics simulations and nucleation theory with artificial neural networks

We develop the theory of homogeneous nucleation of dislocations, which predicts plasticity incipience in FCC single crystal using material properties at elastic stage. In contrast to previous nucleation theories, we show that the work of shear stress is compensated by variation of the generalized stacking fault (GSF) energy during the loop formation. As a result, the energy barrier of nucleation includes only the line defect energy of the critical loop, which radius is defined by balance between all three contributions including the work of shear stress, the surface defect energy of the GSF and the line defect energy. This distinction is consequence of the difference between the mechanisms of the initial loop formation consisting in gradual shear along the proto-loop area and the following growth by slip of bordering dislocation line. Calculation of the energy barrier requires material properties at elastic stage such as stress-strain relationship, the GSF and shear modulus. These data are determined from molecular dynamics (MD) simulations and approximated in the form of artificial neural networks (ANNs). We perform direct MD simulations of the dislocation nucleation for simple shear in different directions parallel to (111) plane in Al single crystal at initial pressures from −3 to +3 GPa. MD simulations show complex and nonlinear evolution of stresses at simple shear deformation even at elastic stage before the dislocation nucleation. Predictions of ANN-informed theory are in accordance with our MD simulations and with the literature data. The calculated nucleation threshold for shear along the Burgers vector of partial Shockley dislocation in [112¯] direction at zero initial pressure varies from 2.03 to 2.16 GPa at the strain rate increase from 1 to 109 s−1. The conditions for homogeneous nucleation are close to the states of matter in thin metal films under powerful sub-picosecond laser irradiation. It is shown that twins are nucleated at angles less or equal to 7.5° between the shear direction and [112¯] direction. Perfect dislocation loops formed by leading and trailing partial dislocations arise at larger angles. In spite of change in the final form of nucleated defect, the energy barrier of nucleation is well-described by the same theoretical expression. It is explained by the fact that the initial defect in both cases is the loop of leading partial dislocation with the stacking fault over the entire loop area. The formation of twin or perfect dislocation occurs after the nucleation and does not influence the nucleation threshold. The ANNs reveal themselves as an efficient tool for approximation of the complex and non-obvious dependencies, while one should be careful with using them for data extrapolation. The paper is supplemented by files with the parameters of the trained ANNs and description of their structure so that one can use them within the nucleation model or separately.

Substructural Transformations in Cu – 12 at.% Al Single Crystals with [001] Deformation Axis

The paper presents a detailed analysis of the relation between the substructural transformations and plastic deformation stages in [001]-oriented Cu – 12 at.% Al single crystals at room temperature. Panoramic transmission electron microscopy images are obtained for the defect microstructures occurred at different deformation stages. The stress–strain curves are associated with the dislocation substructures forming during the deformation process. The Cu – 12 at.% Al single crystals are characterized by the microtwin substructure, which occurs at the third stage of deformation.

Frustration of the dislocation density in NaCl and its implication on “Harper-Dorn” creep

Static annealing of (100)-oriented NaCl single crystals at a homologous temperature of ~0.92 showed a significant reduction in the etch-pit dislocation density during the first 100 h, which remained constant during subsequent annealing up to 225 h. The existence of the frustration of the dislocation density, at a value of ~109 m-2, in NaCl is similar to the observations in the high purity Al and LiF single crystals. The implication of the frustration dislocation density on the observation of a creep stress exponent of >1 in the Harper-Dorn creep regime is discussed.

Detwinning-twinning behavior during compression of face-centered cubic twin-matrix layered microstructure

In this paper, we have examined the mechanical response of a face-centered cubic layered twin – matrix system, which has been generated in Cu–Al alloy single crystal. This kind of microstructure was obtained by specially designed uniaxial tension experiments of Cu-9at.%Al single crystals oriented for “easy” glide. It has been shown, that detwinning system, which operates along the twinning plane, may at specific crystallographic conditions totally dominate the process of plastic deformation. The detwinning system was activated in a confined area that creates a localized deformation regions in the form of kink bands that were found to be hierarchical in formation (i.e. higher order kinking within already existing kink bands). This mode of plastic deformation generates lattice rotation that was identified to occur around normal to the plane of shear of the active detwinning system. Hence, the paper gives direct experimental prove of the importance of detwinning systems on the mechanical properties and deformation texture analysis of face-centered cubic materials.

An electrochemical cell for 2-dimensional surface optical reflectance during anodization and cyclic voltammetry

We have developed an electrochemical cell for in situ 2-Dimensional Surface Optical Reflectance (2D-SOR) studies during anodization and cyclic voltammetry. The 2D-SOR signal was recorded from electrodes made of polycrystalline Al, Au(111), and Pt(100) single crystals. The changes can be followed at a video rate acquisition frequency of 200 Hz and demonstrate a strong contrast between oxidizing and reducing conditions. Good correlation between the 2D-SOR signal and the anodization conditions or the cyclic voltammetry current is also observed. The power of this approach is discussed, with a focus on applications in various fields of electrochemistry. The combination of 2D-SOR with other techniques, as well as its spatial resolution and sensitivity, has also been discussed.

A modular ultra-high vacuum millikelvin scanning tunneling microscope.

We describe the design, construction, and performance of an ultra-high vacuum (UHV) scanning tunneling microscope (STM) capable of imaging at dilution-refrigerator temperatures and equipped with a vector magnet. The primary objective of our design is to achieve a high level of modularity by partitioning the STM system into a set of easily separable, interchangeable components. This naturally segregates the UHV needs of STM instrumentation from the typically non-UHV construction of a dilution refrigerator, facilitating the usage of non-UHV materials while maintaining a fully bakeable UHV chamber that houses the STM. The modular design also permits speedy removal of the microscope head from the rest of the system, allowing for repairs, modifications, and even replacement of the entire microscope head to be made at any time without warming the cryostat or compromising the vacuum. Without using cryogenic filters, we measured an electron temperature of 184 mK on a superconducting Al(100) single crystal.

Особенности субструктурных превращений в монокристаллах Cu - 12 ат. % Al с осью деформации [001

Особенности субструктурных превращений в монокристаллах Cu - 12 ат. % Al с осью деформации [001

Modeling the Stretch Behavior of the Single-Crystal Ni–Al Alloy and Its Molecular Dynamics Simulation

Molecular dynamics simulation was employed in this study to investigate the atomistic mechanisms involved in the Ni–Al alloy homogeneous deformation and the mechanical properties of single-crystal Ni–Al under periodic boundary conditions (until the fracture begins). It is the objective of the present study to develop a simple mathematical model for calculating stress similar to molecular dynamics simulation while the effects of the components of the interatomic potential energy function and the different neighboring atomic shells are also determined on the total stress in the system. The model results show that the sixth nearest neighboring shell has the greatest effect on the stress-strain curve of the Ni–Al single crystal. The associated calculations in the model also show that, in the absence of the pair interaction type of the sixth nearest neighboring shell, the behavior of the material approaches a linear elastic one.

Enhancement of electrical conductivity in aluminum single crystals by boron treatment in solid state

Electrical conductivity/resistivity of elemental fcc metals, such as Al and Cu, has been investigated intensively for decades, both theoretically and experimentally. Since these metals are of great practical importance for electrical wiring, reducing their resistivity even by a few percent may have very strong impact on their application effectiveness. In this paper, we report on electrical resistivity measurements in Al single crystals grown by the Bridgman method. We found that their resistivity at room temperature decreases by 11.5% upon heat treatment in a boron environment at 600 °C, i.e., well below the melting temperature of Al (Tm = 660 °C). The residual resistivity indeed reaches its lower limit dictated by electron–phonon interaction at room temperature. We explain this effect by the boron-induced formation of distorted regions at the surface of the Al crystals. These regions are 30–50 μm in size and comprise finer grains with an average size of 5 μm, separated by low-angle grain boundaries. Resistivity reduction is mainly due to the getter effect, i.e., the removal of the impurity atoms from the crystal bulk by the outward diffusion to the distorted surface regions.

Short communication: Assessment of the dislocation density using X-ray topography in Al single crystals annealed for long times near the melting temperature

Earlier work by the authors determined the dislocation density as a function of time in aluminum single crystals annealed near the melting temperature using etch-pits. It was found that the dislocation density is stable and unchanged even with annealing times up to one year for statically annealed Al single crystals with an initially low dislocation density. As several investigators have suggested that etch-pits are not reliable compared to transmission electron microscopy in assessing the dislocation density, the current study utilized x-ray topography at a synchrotron for dislocation density measurements. This work, then, is a short paper complementing the authors earlier work by using a new, reliable, but rarely utilized non-destructive technique to measure the dislocation density. The results confirm the trend of the earlier study using etch-pits by the authors.

Hydrogen-induced cracking of an aluminum single crystal: An atomistic simulation

The purpose of this paper is to examine the effect of hydrogen on the crack tip mechanism of an Al single crystal. A (121)[11¯1] (plane/front) crack system model with a varying hydrogen concentration was investigated based on a molecular dynamics (MD) framework employing an angular-dependent interatomic potential (ADP). The incipient crack tip event was analyzed using a mixed-mode displacement solution for mode-I and mode-II crack tip fields. The critical stress intensity factor for the onset of the crack tip deformation event was evaluated. A detailed description based on a von Mises atomic shear strain analysis revealed that the presence of hydrogen near a crack tip significantly inhibited the activities of emitted partial dislocations. The hydrogen locking of dislocations near a crack tip was found to facilitate the local amorphization of the crack tip, which considerably reduced the critical stress intensity factor for the onset of crack growth. The effect of the loading mode on the atomic-scale mechanism of the hydrogen embrittlement (HE) was further analyzed, and the explicit dependence of in-plane shear loading was observed.

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.

Excitation energy dependence for electron traps in CaTiO3:Pr, Al single crystals

We investigated the red afterglow process of CaTiO3:Pr, Al through photoluminescence and thermoluminescence measurements by using fabricated single crystals. The crystals with several cubic millimeter in size were obtained through a flux method. We found that the afterglow properties varied with the excitation energy. At room temperature, the red afterglow was observed for the charge transfer from Pr3+ to Ti4+ excitation and the direct excitation of Pr3+, while not observed for the interband excitation of CaTiO3 due to the relaxation process going through 4d15d1 state of Pr3+. For the direct excitation of Pr3+ at 80 K, the excited electrons did not reached to trap states for afterglow due to a potential barrier, which can overcome with thermal assistance at room temperature. For the charge transfer excitation at 80 K, the excited electrons could reach the traps with excess energy.

Prediction of strain distribution and four, six, or eight ears depending on single-crystal orientation using a new single crystal criterion

Significant progress has been achieved on modeling the influence of plastic anisotropy on forming of polycrystalline metal sheets. In contrast, the effect of crystal orientation on forming of single-crystal sheets has been largely unexplored. In this paper, using a recently developed single crystal criterion, it is shown that the single-crystal orientation has a very strong influence on forming. Results of F.E. simulations of cup drawing and hole expansion are reported. The same set of values of the anisotropy coefficients, which correspond to Al single crystal (with 5% Cu) is used in all simulations. It is predicted that for the {100}〈001〉 orientation four ears develop whereas for the $$ \left\{111\right\}\left\langle 1\overline{1}0\right\rangle $$ and $$ \left\{122\right\}\left\langle 1\overline{1}0\right\rangle $$ crystal orientations six, and eight ears form. Moreover, correlations between the location of the ears and the variation of the Lankford coefficients in the plane of the respective single-crystal sheets are established. F.E. analysis of hole expansion also show a strong influence of crystal orientation on the distribution of thickness strains and strain localization.

Atomistic investigation of the T-stress effect on fracture toughness of copper and aluminum single crystals

The T-stress effect on the fracture toughness of Cu and Al single crystals with the (010)[001] crack system was investigated based on atomistic simulations. The interatomic potential function of the embedded-atom method was used to model the face-centered cubic structures. A molecular statics approach using the conjugate gradient algorithm was employed to simulate mode-I fracture testing, and a K-field displacement with various magnitudes of T-stress was applied to load the simulation system. The critical energy release rates were calculated based on the global energy balance to evaluate fracture toughness, and the results were compared with asymptotic solutions obtained from conventional linear elastic fracture mechanics. The obtained values of fracture toughness exhibit a strong T-stress dependence for both the Cu and Al single crystals. Furthermore, the crack growth behavior is explained from an atomistic view of fracturing to analyze the T-stress effect on crack growth resistance.

The nucleation of cube grains during primary recrystallization of aluminum

The formation of cube grains during recrystallization has been analyzed in commercial AA1050 alloy and pure Al single crystal of S{234}<524> orientation. Samples of AA1050 alloy were deformed along two deformation modes to form different as-deformed texture components and then lightly annealed. One group of the samples was plane strain compressed (PSC) in a channel-die. The second group was deformed by equal channel angular extrusion (ECAE). The results obtained on polycrystalline AA1050 alloy were compared with recrystallization behavior of PSC single crystal of S{234}<524> orientation. The textures were measured by SEM/EBSD and X-ray diffraction. It was found that a very weak residual cube{100}<011> component was observed in the samples of AA1050 alloy after both deformation modes. After annealing cube-oriented grains were extensively formed only in PSC samples. These grains were situated preferentially inside or in between the S-oriented areas with local disorientations about <111> axes. Cube-oriented grains were not formed during annealing of the ECAE samples, where main as-deformed texture components close to two complementary orientations of {124}<651>-type were transformed towards two components of ∼{100}<011> and ∼{221}<114>, essentially by <110> rotations. In single crystalline samples cube grains were extensively formed during annealing despite (near)cube-oriented areas haven’t been observed in the as-deformed microstructure.

Formation of D–VZn complex defects and possible p-type conductivity of ZnO nanoparticle via hydrogen adsorption

The hydrogen adsorption on surfaces and on defect sites of ZnO nanoparticles (NPs) has been studied by using Raman and Fourier transform infrared spectroscopic methods. The presence of hydrogen at defect sites bound to zinc vacancy with different coordinations has been confirmed. To further identify the existence of isolated VZn and H–VZn complexes in the ZnO NPs, coincidence Doppler broadening (CDB) spectroscopic studies have been performed with respect to the CDB spectra of a 99.9999% pure Al single crystal. The broad momentum dip ρL showed between 15–17 × 10−3 m0c suggests the trapping of positrons with the core electrons of 3p Zn. However, positron annihilation takes place between ρL 20–25 × 10−3 m0c and this may occur with an electron belonging to OH bonds (VZn–Hi–O). Here the lattice hydrogen H+ ion acts as a compensating centre, and it can bind with the VZn around the dislocation and stacking faults (SFs) core, which may produce the acceptor-type complex defect for p-type conductivity. Finally, the existence of SFs and dislocation defects, including edges and steps, was confirmed by transmission electron microscopy.

A Novel Approach to Modelling Nanoindentation Instabilities

We review the recently developed models for load fluctuations in the displacement controlled mode and displacement jumps in the load controlled mode of indentation. To do this, we devise a method for calculating plastic contribution to load drops and displacement jumps by setting-up a system of coupled nonlinear time evolution equations for the mobile and forest dislocation densities by including relevant dislocation mechanisms. These equations are then coupled to the equation defining constant displacement rate or load rate. The model for the displacement controlled mode using a spherical indenter predicts all the generic features of nanoindentation such as the elastic branch followed by several force drops of decreasing magnitudes and residual indentation depth after unloading. The stress corresponding to the elastic force maximum is close to the yield stress of an ideal solid. The predicted numbers for all the quantities match experiments on single crystals of Au using a spherical indenter. We extend the approach to model the load controlled nanoindentation experiments that employ a Berkovich indenter. We first identify the dislocation mechanisms contributing to different regions of the F − z curve as a first step for obtaining a good fit to a given experimental F − z curve. This is done by studying the influence of the parameters associated with various dislocation mechanisms on the model F − z curves. The study also demonstrates that the model predicts all the generic features of nanoindentation such as the existence of an initial elastic branch followed by several displacement jumps of decreasing magnitudes and residual plasticity after unloading for a range of model parameter values. Furthermore, an optimized set of parameter values can be easily determined that give a good fit to the experimental load–displacement curves for Al single crystals of ( 110 ) and ( 133 ) orientations. Our model also predicts the indentation size effect in a region where the displacement jumps disappear. The good agreement of the results of the models with experiments supports our view that the present approach can be used as an alternate method to simulations. The approach also provides insights into several open questions.

Anelasticity of iron-aluminide Fe3Al type single and polycrystals

Mechanical spectroscopy and in situ neutron diffraction techniques were employed to study and interpret anelasticity in Fe-27 at.%Al single crystal (SC) and polycrystal (PC) alloys from room temperature to 600 °C. Thermally activated Zener relaxation was recorded in the temperature dependent internal friction (IF) curves for both SC and PC alloys upon heating and cooling. The reversible second-order D03 ↔ B2 phase transition studied by in situ neutron diffraction leads to a transient anelastic effect at ∼550 °C both upon heating and cooling. The transient peak temperature does not depend on measurement frequency, whereas the peak height is roughly proportional to the inverse frequency of measurements according to Landau's theory for the second order transitions. Calorimetry and magnetometry techniques recorded the same temperature range for the D03 ↔ B2 phase transition as mechanical spectroscopy and in situ neutron diffraction. The study of amplitude dependent internal friction (ADIF) revealed that the absolute value of maximum damping Qm−1 in the SC sample is about five times higher than that of in the PC sample at room temperature. The calculated value of internal stresses opposing magnetic domain wall motions for the PC sample is ∼1.5 times larger than that of for the SC sample, which corresponds to the detrimental influence of grain boundaries to magnetomechanical damping. By increasing the tightening force of torque wrench, a higher level of localized stress is induced to the sample, and the damping capacity (Qm−1) in single cantilever configuration of forced bending vibrations reduces monotonously.

The Temperature-Dependent Ideal Shear Strength of Solid Single Crystals

Knowledge of the ideal shear strength of solid single crystals is of fundamental importance. However, it is very hard to determine this quantity at finite temperatures. In this work, a theoretical model for the temperature-dependent ideal shear strength of solid single crystals is established in the view of energy. To test the drawn model, the ideal shear properties of Al, Cu, and Ni single crystals are calculated and compared with that existing in the literature. The study shows that the ideal shear strength first remains approximately constant and then decreases almost linearly as temperature changes from absolute zero to melting point. As an example of application, the “brittleness parameter” of solids at elevated temperatures is quantitatively characterized for the first time.