Magnesium Single Crystals Research Articles

Effects of normal strain on pyramidal I and II 〈c + a〉 screw dislocation mobility and structure in single-crystal magnesium

Abstract In this study, atomistic simulations were used to analyze the effects of nonglide stress and temperature on the mobility and structure of pyramidal-I (Pyr-I) and pyramidal-II (Pyr-II) 〈c + a〉 screw dislocations in single-crystal Mg. At a very low temperature (10 K), the pyramidal screw dislocations stably exist on Pyr-II planes and tend to glide on Pyr-I planes. The critical resolved shear stresses (CRSSes) of the pyramidal screw dislocations depend on the migration direction. Once a Pyr-II dislocation is transformed into a stuck core, a very high shear stress (243 and 391 MPa) is required to escape from the immobilized structure. Furthermore, their CRSSes increase with increasing compressive strain and decrease with increasing tensile strain normal to the slip planes. At the intermediate temperature range of 200 K ≤ T ≤ 400 K, the CRSSes of Pyr-I screw dislocations are weakly affected, whereas those of Pyr-II screw dislocations drastically decrease. Thus, both Pyr-I and Pyr-II screw dislocations have similar CRSS values at 400 K. At a higher temperature (500 K), Pyr-I screw dislocations frequently emit basal- 〈 a 〉 \langle a\rangle dislocation loops, and the remained dislocations are momentarily immobilized. The basal- 〈 a 〉 \langle a\rangle dislocation loops emitted from the 〈c + a〉 dislocations are quickly retracted, and the core structure is recovered as the shear deformation continues. This phenomenon can reduce the mobility of Pyr-I 〈c + a〉 screw dislocations at a higher temperature. The emission of basal- 〈 a 〉 \langle a\rangle dislocation loops is enhanced under compression.

Evaluating non-intrinsic contribution in flexoelectric measurements

The characterization of the flexoelectric coefficient is a fundamental issue for the studies of the flexoelectric effect, which describes the coupling between strain gradient and polarization. However, the contribution from non-intrinsic flexoelectricity cannot be ignored in the flexoelectric measurements, bringing challenges for the determination of intrinsic flexoelectric coefficients. In this work, we propose a non-intrinsic flexoelectric factor to evaluate the non-intrinsic flexoelectric contributions to the measured coefficient, based on the crystal-orientation-dependent flexoelectric coefficients measured by cantilever-bending method. The cubic magnesium oxide single crystals with different surface statuses are chosen to obtain the effective flexoelectric coefficients through the cantilever-bending method and first-principles. The results verify the effectiveness of the proposed non-intrinsic flexoelectric factor. This work provides an effective way to evaluate the non-intrinsic flexoelectric contributions in flexoelectric measurements.

Rate-independent mechanism of deformation twinning in single crystal magnesium

The dynamics of {101‾2}〈101‾1〉 extension twins in a single crystal of pure magnesium are studied through uniaxial compression experiments coupled with direct optical imaging. The experiments covered a seven orders of magnitude strain rate, between 10−5−102s−1, covering all rates of engineering applications of magnesium and its alloys. Under uniaxial compression along a direction perpendicular to prismatic planes, only two variants of extension twins are activated, facilitating in-situ tracking of individual twin boundaries throughout the deformation process. Across the entire strain rate range, the twinning transformation evolves in a self-similar pattern of twin nucleation and thickening. Deformation fronts initiate at the edges of the sample and propagate toward the center, creating parallel twins with characteristic thicknesses and distances from each other. The velocity of the deformation fronts scales linearly with the imposed external strain rate and a nearly uniform resolved shear stress is measured over the entire strain rate range. We show that this unique process is attributed to the constant ratio between the nucleation rate and sideways velocity of individual twin boundaries, where both processes are proportional to the imposed strain rate. These findings indicate that, within the large strain rate range investigated in this study, the evolution mechanism of extension twinning is practically rate-independent.

Temperature-Decoupled Single-Crystal MgO Fiber-Optic Fabry-Perot Vibration Sensor Based on MEMS Technology for Harsh Environments.

A high-temperature-resistance single-crystal magnesium oxide (MgO) extrinsic Fabry-Perot (FP) interferometer (EFPI) fiber-optic vibration sensor is proposed and experimentally demonstrated at 1000 °C. Due to the excellent thermal properties (melting point > 2800 °C) and optical properties (transmittance ≥ 90%), MgO is chosen as the ideal material to be placed in the high-temperature testing area. The combination of wet chemical etching and direct bonding is used to construct an all-MgO sensor head, which is favorable to reduce the temperature gradient inside the sensor structure and avoid sensor failure. A temperature decoupling method is proposed to eliminate the cross-sensitivity between temperature and vibration, improving the accuracy of vibration detection. The experimental results show that the sensor is stable at 20-1000 °C and 2-20 g, with a sensitivity of 0.0073 rad (20 °C). The maximum nonlinearity error of the vibration sensor measurement after temperature decoupling is 1.17%. The sensor with a high temperature resistance and outstanding dynamic performance has the potential for applications in testing aero-engines and gas turbine engines.

Subpicosecond laser ablation behavior of a magnesium target and crater evolution: Molecular dynamics study and experimental validation

Abstract The micro-ablation processes and morphological evolution of ablative craters on single-crystal magnesium under subpicosecond laser irradiation are investigated using molecular dynamics (MD) simulations and experiments. The simulation results exhibit that the main failure mode of single-crystal Mg film irradiated by a low fluence and long pulse width laser is the ejection of surface atoms, which has laser-induced high stress. However, under high fluence and short pulse width laser irradiation, the main damage mechanism is nucleation fracture caused by stress wave reflection and superposition at the bottom of the film. In addition, Mg[0001] has higher pressure sensitivity and is more prone to ablation than Mg [ 10 1 ¯ 0 ] . The evolution equation of crater depth is established using multi-pulse laser ablation simulation and verified by experiments. The results show that, under multiple pulsed laser irradiation, not only does the crater depth increase linearly with the pulse number, but also the quadratic term and constant term of the fitted crater profile curve increase linearly.

Slip responses and pre-twin structure evolution based on indentation pile-up in magnesium single crystal

Slip behavior and microstructure evolution caused by the indentation pile-up deformation were studied in Mg single crystal with {10–12} pre-twin structure at room temperature. The anomalous indentation is formed on the surface of the large-size single crystal. It is attributed to plastic flow induced by the combined effects of dislocation slip, detwinning and {10–12} twinning during indentation. At the early stage of indentation, basal slip dominates the pile-up deformation. And the reaction structure of basal slips, i.e. (0001) slip plane in the twin to correspond to (0001) slip plane in the matrix, is frequently formed on the twin boundary in order to facilitate plastic relaxation of the local stress on the twin boundary. Under pile-up pattern, the stress state favors detwinning. And detwinning behavior is subjected to effects of stress-field magnitude and pre-twin size. For the little-volume and large-volume pre-twins, detwinning behaviors play the modes of twin shrinkage and “local detwinning” respectively. The latter might involve reverse nucleation and growth of matrix. Although pile-up pattern is unfavorable to {10–12} twinning, stress concentration caused by the interaction between slip dislocations and twin boundary induces the local twin growth on the interface. In addition, a pyramidal slip is found in matrix, and its slip trace is corresponding to (0001) slip trace in twin.

Synthesis, clarifying properties and refining structures of single-crystal magnesium borohydride di- and triammine complexes

Synthesis, clarifying properties and refining structures of single-crystal magnesium borohydride di- and triammine complexes

A theoretical and experimental investigation on the surface formation mechanism in elliptical vibration cutting of single crystal magnesium fluoride

Manufacturing precise microstructures on magnesium fluoride surface remains challenging due to its brittleness and anisotropy. This study investigates the surface formation mechanism in ductile machining of single crystal magnesium fluoride and explores the feasibility of fabricating sophisticated microstructures on magnesium fluoride surfaces by applying elliptical vibration cutting technology. We conducted a systematic study on the ductile-brittle transition behavior, material removal mechanism, and fabrication performance of microstructures through theoretical analysis and cutting experiments. A theoretical model that takes into account the elliptical trajectory's orientation angle has been established to determine the evolution of instantaneous uncut chip thickness and the characteristic nominal depth of cut in one cycle during elliptical vibration cutting. By comprehensively examining the influence of the elliptical trajectory, determined by vibration amplitudes, orientation angle, and nominal cutting speed, on the dynamic behaviour of the ductile-brittle transition, we verified the proposed theoretical model with convincing experimental results. Larger critical depth of cut for ductile-brittle transition and smaller cutting forces achieved by EVC contribute to the improved ductile machinability of MgF2. With optimal vibration parameters, the critical depth of cut increases by 56 times compared to ordinary cutting. Theoretical findings regarding plastic deformation parameters and cleavage fracture parameters align well with experimental observations, providing a more comprehensive understanding of the anisotropy influence on damage evolution during the surface formation in elliptical vibration cutting of magnesium fluoride. Furthermore, leveraging our fundamental understanding of material removal and surface formation mechanisms, we successfully generated bi-sinusoidal arrays on magnesium fluoride surfaces with exceptional accuracy (machining error < 1.5%) and high efficiency (machining time < 121.5 min) by applying elliptical vibration cutting.

Dielectric properties of polycrystalline and single‐crystal magnesium oxide at high temperatures

AbstractThe electrical properties of high‐purity magnesium oxide (MgO) samples sintered between 1500 and 1650°C were investigated up to 800°C. Dielectric constant, loss tangent and electrical conductivity were measured between 25 and 800°C. Optimum electrical properties were obtained for the sample sintered at 1600°C. The impurities in polycrystalline and single crystal MgO were discussed to understand their effect on electrical properties and the role of grain boundaries. Higher dielectric losses of polycrystalline samples as compared to the single crystal MgO were attributed to the presence of grain boundaries. MgO could be a useful dielectric material for capacitor applications up to 600°C.

Modeling of shock wave propagation in porous magnesium based on artificial neural network

Data transfer from the level of atomistic simulations to the level of continuum modeling is one of the urgent problems in the mechanics of materials. In this work, we perform multiple molecular dynamics (MD) simulations of the uniaxial (along different crystallographic directions) and volumetric (isotropic) compression of porous HCP magnesium single crystals in order to study the atomistic processes of the plastic deformation of anisotropic porous material and to create a reference dataset (strain dependencies of stresses, porosity and dislocation density at different temperatures and initial porosities). The MD data are used to train two artificial neural networks (ANNs): the first ANN is a surrogate of the constitutive equation of porous magnesium, while the second ANN describes the dislocation nucleation/emission as the onset of plastic flow. In contrast to the previously considered case of porous FCC aluminum, the present approach takes into account the anisotropy of the matrix material. The ANN-based constitutive equation is successfully used within the continuum mechanics model for numerical study on the shock wave structure in porous magnesium in comparison with direct MD simulation of the shock wave problem.

Mechanical properties and deformation mechanisms of single crystal Mg micropillars subjected to high-strain-rate C-axis compression

The mechanical properties and deformation mechanisms of single crystal magnesium under c-axis quasi-static and high-strain rate compressions are investigated through in situ scanning electron microscope (SEM) experiments and post-mortem transmission electron microscope (TEM) characterization. The findings revealed that ductility and high rates of hardening are preserved for pillars as large as 15 μm. Furthermore, rate effects result in a mild increase in flow stress with plastic deformations controlled primarily by the slip of <a+c> type dislocations. Importantly and in contrast to other literature reports, plastic deformation occurs in the absence of twining. As the strain increases and plastic deformation exceeds about 4%, crystal rotation activates basal slip, <a> type dislocations, resulting in a more rate independent flow stress. TEM observation on micropillars compressed at a strain rate of 250/s, revealed the activation of {112‾2‾}<1‾1‾23> slip systems and high mobility of screw dislocations as major contributors to plastic strains in excess of 10% without fracture. These findings are relevant to the design of lightweight materials used in transportation systems, e.g., selection of material grain size. Moreover, the experimental data here reported provides the materials science community with a unique opportunity to validate discrete dislocation dynamics (DDD) formulations employed in multiscale design of materials.

Plastic deformation of magnesium single crystal: a crystal plasticity coupled twinning phase field simulation

Plastic deformation of magnesium single crystal: a crystal plasticity coupled twinning phase field simulation

Armoring of MgO by a Passivation Layer Impedes Direct Air Capture of CO2.

It has been proposed to use magnesium oxide (MgO) to separate carbon dioxide directly from the atmosphere at the gigaton level. We show experimental results on MgO single crystals reacting with the atmosphere for longer (decades) and shorter (days to months) periods with the goal of gauging reaction rates. Here, we find a substantial slowdown of an initially fast reaction as a result of mineral armoring by reaction products (surface passivation). In short-term experiments, we observe fast hydroxylation, carbonation, and formation of amorphous hydrated magnesium carbonate at early stages, leading to the formation of crystalline hydrated Mg carbonates. The preferential location of Mg carbonates along the atomic steps on the crystal surface of MgO indicates the importance of the reactive site density for carbonation kinetics. The analysis of 27-year-old single-crystal MgO samples demonstrates that the thickness of the reacted layer is limited to ∼1.5 μm on average, which is thinner than expected and indicates surface passivation. Thus, if MgO is to be employed for direct air capture of CO2, surface passivation must be circumvented.

On the Plasticity and Deformation Mechanisms in Magnesium Crystals

This work presents an overview of the mechanical response and microstructure evolution of specifically oriented pure magnesium single crystals under plane strain compression at room temperature. Crystals of ‘hard’ orientations compressed along the c-axis exhibited limited room temperature ductility, although pyramidal ⟨c + a⟩ slip was readily activated, fracturing along crystallographic 112¯4 planes as a result of highly localized shear. Profuse 101¯2 extension twinning was the primary mode of incipient deformation in the case of orientations favorably aligned for c-axis extension. In both cases of compression along ⟨112¯0⟩ and ⟨101¯0⟩ directions, 101¯2 extension twins completely converted the starting orientations into twin orientations; the subsequent deformation behavior of the differently oriented crystals, however, was remarkably different. The formation of 101¯2 extension twins could not be prevented by the channel-die constraints when c-axis extension was confined. The presence of high angle grain boundaries and, in particular, 101¯2 twin boundaries was found to be a prerequisite for the activation of 101¯1 contraction twinning by providing nucleation sites for the latter. Prismatic slip was not found to operate at room temperature in the case of starting orientations most favorably aligned for prismatic slip; instead, cooperative 101¯2 extension and 101¯1 contraction twinning was activated. A two-stage work hardening behavior was observed in ‘soft’ Mg crystals aligned for single or coplanar basal slip. The higher work hardening in the second stage was attributed to changes in the microstructure rather than the interaction of primary dislocations with forest dislocations.

Structural Evolution of Mg-Doped Single-Crystal LiCoO2 Cathodes: Importance of Morphology and Mg-Doping Sites

Layered lithium cobalt oxide (LiCoO2, LCO), which serves as a structural motif for the widely adopted layered cathodes in lithium-ion batteries, has a long history, and its unstable phase transition during high-voltage operation (∼4.5 V) remains an intractable problem. Many research strategies, such as surface coating and immobile ion doping, have been proposed to address this issue, but a clear understanding of the effects has not been demonstrated because of various potential parameters (e.g., particle size, shape, and dopant content). Herein, we report a molten salt synthesis method that produces sphere-like single-crystal magnesium (Mg)-doped LCO. In situ X-ray diffraction and X-ray absorption fine structure analyses confirmed that the lattice strain was effectively alleviated by the effects of both the particle shape and Mg doping compared to the plate-like and sphere-like single-crystal LCO samples. Furthermore, the preference for Mg doping in the Co site (3b) rather than in the Li site (3a) in the LCO framework is systematically revealed, and a clear understanding of Mg doping that suppresses the monoclinic phase transition is discussed in detail.

Effect of tensile rate on structural transformation and dislocation of magnesium single crystal based on molecular dynamics

Magnesium and its alloys have broad application prospects as lightweight materials, but there are few studies of crystal and dislocation lines based on the molecular level of magnesium single crystal. In this study, the molecular dynamics method was used to establish the magnesium single crystal model, the properties of magnesium single crystal at different tensile rates at 300 K were studied. The results of stress - strain, Potential-strain, crystal structure change and dislocation density were analyzed. The results show that with the increase of tensile rate, the corresponding stress peak increases gradually, the corresponding strain value at the peak also increases, and the emergence of dislocation line generally lags behind the peak stress point. Come first and play a leading role was 1/3<-1100> incomplete dislocation and unknown dislocation, followed by 1/3<11-20> dislocation. At the peak point, the crystal structure begins to change from hexagonal-closest packing structure to other structures, after the peak, body-centered cubic and face-centered cubic structures begin to appear. With the increase of tensile rate, the corresponding strain increases gradually when body-centered cubic and face-centered cubic structures appear. The results are of great significance for the study of the deformation mechanism of magnesium single crystal

Temperature dependent critical stress for {[formula omitted]} twinning in magnesium micropillars at cryogenic temperatures

Single-crystal magnesium micropillars (5 µm diameter) have been fabricated using focused ion beam (FIB) milling and loaded in compression along the [21¯1¯0] direction at cryogenic temperature (T = 184 K) at three different strain rates: 10-4, 5 × 10-3 and 10-1 s-1. The stress-strain curves and post-mortem analyses reveal the predominance of {101¯2} twinning. The critical stress required for twinning is detected to be significantly higher compared with that previously reported at higher temperatures for the same strain rates. From the experimental data, the temperature dependence of the strain rate sensitivity and activation volume for twinning is unveiled and observed to be constant and linearly dependent with T, respectively. By considering the activation energy (Q) in terms of stress-dependent dislocation motion using the rate equation proposed by Schoeck, for {101¯2} twinning, Q is determined to be 174 kJ mol-1 for T < 273 K and 72 kJ mol-1 above.

Deformation behavior of single-crystal magnesium during Nano-ECAP simulation

Molecular dynamics was applied to simulate ECAP of single-crystal magnesium at room temperature. Four samples with different orientations were processed, and the grain structure, grain fragmentation, slip systems, strain, and twin formation were analyzed. The initial orientation played a substantial role in the strain and deformation experienced by the samples during both stages of deformation. Compressions initially occurred before extrusion, and simple shear occurred in the deformation zone during extrusion. The samples nucleated a {101¯2} tension twin during compression, and the tension twin grew to immediately cover the entire sample, effectively changing the orientation of the sample. Additionally, stacking faults acted as a precursor for the {101¯2} tension twin. The strain was strongly correlated with the shear factor, that is, a high shear factor resulted in low strain. Moreover, discrepancy occurred between theoretical and actual shear strain due to two factors. First, theoretical shear is considered to be simple shear occurring entirely in the deformation zone; it does not consider the shear strain due to the normal stress in the compression phase. Second, deformation is considered to be homogenous and isotropic, and it does not take into account the initial grain orientation and the anisotropic nature of magnesium.

Formation of optically-active disordered layers in (001) MgO single crystals

The formation of defective layers and their associated optical properties in (001) magnesium oxide (MgO) single crystals were examined in this study. To form defective layers, 100 keV beams with different doses (1015, 5 × 1015, and 1016 ions/cm2) of atomic nitrogen ions (N+) were used. Following the ion implantation, the surface roughness remained unaffected, while a structurally and chemically distinct surface layer was formed in the MgO single crystal. Spectroscopic methods indicated that defects in the layer consist of nitrogen ions and oxygen vacancies. Moreover, the resultant disordering triggered the softening of optical phonons. Upon using the magnetic resonance technique, it was observed that the F+ center created by oxygen vacancies did not contribute to this optical phenomenon.

Micromechanical response of pure magnesium at different strain rate and temperature conditions: twin to slip and slip to twin transitions

The strain rate (ε˙) and temperature (T) dependent mechanical response of single crystal magnesium (Mg) micropillars compressed along the [2¯110] direction (a-axis) is investigated from room temperature to 573 K and from 10-3 to 100 s-1. The loading direction was chosen to disfavour basal slip activation by a low Schmid factor, allowing the investigation of the rate-sensitivities of extension twinning and prismatic slip. For T ≤ 423 K, the plasticity was governed by extension twinning for the entire range of the applied strain rates. At T > 423 K and for ε˙ ≤ 10 s-1, extension twinning did not occur and a continuous plastic flow induced by prismatic dislocation mediated plasticity was observed: the twin to slip transition takes place due to the decrease of the critical resolved shear stress of non-basal slip. For ε˙ > 10 s-1, however, the accommodation of the plastic deformation by activation of prismatic slip is not enough to match the applied deformation rate, favouring again deformation twinning. The first part of this work provides a complete overview of the mutual effects of T and ε˙ on the transition points of deformation modes in Mg at the microscale. In a second stage, the influence of thermal and kinetic contributions on the evolution of the flow stress leading to the slip to twin transition at 573 K has been assessed in more detail. Within the slip-dominated plasticity regime, this work provides a quantitative assessment of the increases in the saturation stress (stage III) with ε˙ at high temperature, showing how the strain rate dependency of the dislocation generation rate in the pillar and escape rate at the free surfaces of the structure controls the stress evolution in Mg microcrystals.