Magnesium Single Crystals Research Articles

Molecular dynamics simulation of the effects of temperature on the microscopic deformation of single crystal magnesium containing a void and a crack

Mechanical properties of a material are intensively influenced by the presence of flaws. While temperature is the significant factor of the deformation. An embedded atomic potential function is used to simulate the micro-defect deformation behavior under different temperatures loading in HCP Mg with various defects. Defects consisting of a fixed distribution of void and an edge crack. the results of the simulation were analyzed by the stress-strain curve and atomic trajectory diagram. The results show that the tensile deformation behavior of single crystal magnesium with defects changes obviously with temperature. Increases in temperature lead to decreased peak stress and ascending crack propagation speeds, and toughening the material to fracture ultimately. But the increase of crack-propagating speed is not obvious when the temperature is above 600K. The time that the crack extends to the void and in combination with it at low temperature is delayed as compared to that at high temperature.

Molecular dynamics study on the deformation of void single crystal magnesium under uniaxial stress

The uniaxial tension and compression process of single crystal magnesium model with voids along [0001] direction was simulated by using embedded atom potential and molecular dynamics method, and the micro plastic deformation mechanism of voids under tension and compression was studied. The results show that the elastic modulus of the single crystal magnesium model under compression is greater than the elastic modulus under tension, indicating that compression deformation is more difficult; In the process of plastic deformation, the dislocation, stacking fault and twin will be produced in the single crystal magnesium model under tension and compression, but the emission mechanism of dislocation is different. Tension will make the dislocation slip to the edge of the model along the direction of 45 ° and produce four symmetrical slip bands, while compression will produce an annular defect band near the cavity; In addition, the stacking fault area and twin type produced are also different. This asymmetry is mainly caused by the different initial deformation mechanisms under the two loading conditions.

Effects of Yttrium Addition on Bending Deformation Behavior of Magnesium Single Crystals

Effects of Yttrium Addition on Bending Deformation Behavior of Magnesium Single Crystals

Hot Rolling of Magnesium Single Crystals

To analyze the effect of the initial orientation in the activity of twinning and texture development, magnesium single crystals were rolled at 400 °C (nominal furnace temperature) in two specific orientations. In both orientations, the rolling direction of the sheet (RD) was parallel to the c-axis. For orientation 1, the 112¯0 direction was parallel to the normal direction (ND), and for orientation 2, it was parallel to the 101¯0 direction. The samples were rolled at 30%, 50% and 80% of thickness reduction. After rolling, all the samples were quenched in water to retain the microstructure. The microstructure and texture evolution were characterized by X-ray diffraction and Electron Backscatter Diffraction (EBSD). The initial single crystals were turned into polycrystals, where most grains had their c-axis almost parallel to the ND, and this reorientation was explained by extension twinning. The active twin variants in orientation 1 aligned the basal plane ~30° from the sheet plane and caused a weaker basal texture compared to orientation 2, where the twin variants aligned the basal plane almost parallel to the sheet plane. Strain localization inside contraction twins was observed, and consequently, non-basal grains nucleated inside these twins and weakened the final basal texture only in orientation 1.

On the intrusion-like co-zone twin-twin structure: An in situ observation

An in situ optical microscopy combined with ex situ electron backscatter diffraction testing was applied to a pristine single-crystal magnesium specimen under monotonic tension along the c-axis. An intrusion-like co-zone twin-twin structure is observed for the first time at the micron scale. In situ observation reveals that the intrusion-like twin-twin structure consists of multiple twin-twin boundaries (TTBs) and incoherent twin boundaries (I-CTBs) following energetically favorable formation sequences. The initial interaction results in the impinging TTBI and the acute-angle TTBA. In the local junction region on the obtuse angle side, the impinging twinning dislocations (TDs) further deposit near TTBI due to the preferred local twinning shear stress, leading to the incoherent curve of the impinging twin boundary adjacent to TTBI. Shortly after, the barrier twin boundary on the obtuse angle side migrates and encompasses the incoherent impinging twin boundary. The combination of sequential TTBI, TTBA, and I-CTBs formed locally on the obtuse angle side shapes the final configuration of the intrusion-like twin-twin structure at the micron scale.

Effects of Yttrium Addition on Bending Deformation Behavior of Magnesium Single Crystals

Effects of Yttrium Addition on Bending Deformation Behavior of Magnesium Single Crystals

MgO Single Crystals MEMS-Based Fiber-Optic Fabry–Perot Pressure Sensor for Harsh Monitoring

A single crystalline magnesium oxide (MgO) MEMS-based fiber-optic Fabry-Perot (FP) pressure sensor was designed, fabricated and tested for 800 °C high-temperature environmental applications. In this article, MgO was selected as the sensitive materials of the optical sensor owing to its high melting point, and excellent mechanical and optical properties at elevated temperatures. The sensor head was constructed by combining wet chemical etching with direct bonding. This MEMS process technique realized all-MgO sensor head structure, which is beneficial to reduce the temperature gradient within the sensor structure, and avoid sensor failures. In addition, a non-scanning cross-correlated interrogation method is used to realize the demodulation of FP pressure cavity length having multiple reflectors. The experimental tests demonstrate that the proposed sensor can stably operate at an ambient environment of 22-800 °C and 0-0.6 MPa with a pressure sensitivity of 5.21 μm/MPa (room temperature), a repeatability error of 1.67% and a zero drift of 0.00353 μm/°C. In addition, the high-temperature testing proves that the adhesive-free well-sealed pressure cavity is conducive to improve the operating temperature and the performance of the sensor. To our best knowledge, this is the first research to realize fabrication of high-temperature pressure sensor by developing MgO micromachining technology. Our work is of fundamental importance in realization of pressure detecting in harsh environments.

Molecular dynamics simulation of pore - containing single crystal magnesium stretching along different crystalline orientations

The uniaxial stretching process of single crystal magnesium containing holes was simulated by molecular dynamics method, and the influence of the stretching crystal orientation on the growth and deformation of the holes was studied. Tensile simulations of different crystal orientations show that the microscopic mechanism of void growth and deformation in single crystal magnesium was different. For the orientation of the [1210] crystal, there will be small holes in the crystal at first, and with the process of loading, the polymerization of large and small holes will make the holes grow and then lead to the failure of the material. For the [0001] crystal orientation, the deformation mechanism was different from the former. In the early stage, it was caused by dislocation movement and stacking fault deformation, while in the later stage, it was mainly caused by twin nucleation and growth.

Molecular dynamics simulation on the cyclic deformation of magnesium single crystals

Molecular dynamics simulations were performed to investigate the cyclic deformation of magnesium single crystalline nano-pillars in the first three cycles. To study the tension–compression asymmetry, two symmetric uniaxial cyclic loading paths (applied strain amplitude = 10%) were employed, i.e., a triangle wave loading path started by tension and a triangle wave loading one started by compression. The influence of crystalline orientation on the cyclic deformation of magnesium nano-pillars was studied by changing the angle θ between the loading direction and the c-axis of the nano-pillars (e.g., 0°, 45°, and 90°). Under the cyclic loading path started by tension, the full deformation process in each cycle is controlled by twinning- detwinning-pyramidal slip-retwinning at θ = 0° with significant tension–compression asymmetry observed, basal slip + twinning-detwinning-basal slip + retwinning at θ = 45° without any tension–compression asymmetry, and double twinning-detwinning-retwinning-detwinning at θ = 90° with significant tension–compression asymmetry. Under the cyclic loading one started by compression, on the other hand, the deformation process is dominated by pyramidal slip-twinning-detwinning at θ = 0°, basal slip-twinning-detwinning-retwinning at θ = 45°, and prismatic slip-double twinning-detwinning in the first two cycles and twinning-detwinning-retwinning in the third cycle at θ = 90°, while tension–compression asymmetry is observed in the cases at θ = 0° and 90° only due to different deformation modes in the tension and compression stages. Especially, for the loading path started by compression at θ = 90°, cyclic hardening is seen in the tension stage, but softening is observed in the compression stage only in the third cycle due to the change of deformation modes. The current work provides new insights for understanding the mechanical property of Mg crystals subjected to a cyclic loading.

Influence of pore size on the plastic deformation of c-axis-compressed magnesium single crystals

The molecular dynamics method is used to establish a single crystal model of magnesium with different void sizes. Uniaxial compression along the c-axis is carried out at 300 K. Combined with the stress–strain curve, potential energy curve and dislocation density curve of the four models, the compression mechanical energy and structural evolution process of a single crystal of magnesium with different hole sizes are analysed. Results show that when the radius of the single spherical void is large, the elastic modulus is small, the yield stress is low, the potential energy value is large, and the absolute value is small, such conditions facilitate deformation. When the hole radius is small, complete closure under c-axis compression requires minimal time and deformation.

Shock-induced deformation twinning and softening in magnesium single crystals

Magnesium is widely regarded as an excellent structural material, primarily because it forms the basis for a range of light-weight high-strength alloys. Recently, high-strain rate deformation of magnesium has received a great deal of attention due to the complicated deformation modes that involve combinations of dislocation slip and deformation twinning. In this study, single crystal magnesium samples were shock-compressed along the c- and a-axis, then released back to ambient conditions. Post-mortem transmission electron microscopy revealed that extension twins developed for both c- and a-axis shock loading. Also, the nanoindentation hardness values for these shocked samples were compared to those for samples compressed under quasi-static conditions; it was found that the hardness decreased with increasing strain rate for both c- and a-axis loading. Molecular dynamics simulations were performed to elucidate the detailed mechanisms of deformation twinning in terms of inertial confinement of sample geometry and different stress relaxation speed between impact and lateral directions. The conversion from work-done to heat was discussed to explain the influence of shock-induced heating on the residual hardness. These results give new insights into the residual mechanical response in shock-compressed materials and may help to develop a more fundamental understanding of shock phenomena in metallic materials.

Phonon-Polariton Excitations in MgZnO/6H-SiC Structures

Specular infrared reflection spectra in the range of “residual rays” of the film and the substrate and in the case of the E⊥c orientation of the electric field have been simulated for the first time for thin MgxZn1−xO films deposited on optically anisotropic 6H-SiC substrates. The simulation was carried out making use of self-consistent parameters obtained earlier for magnesium oxide, zinc oxide, and silicon carbide single crystals. The film thickness and the Mg content x in the film are demonstrated to considerably distort the reflection spectra and to change the reflectivity of the MgxZn1−xO/6H-SiC structure. Using the Kramers–Kronig relation, the spectral intervals, where the reflectivity is sensitive to the film thickness and to the doping levels of the film and the substrate, are determined. The main attention is paid to analyze results obtained for x = 0.2. The existence of surface polaritons in such structures is theoretically demonstrated for the first time, and the attenuated total reflectance surface I(v)/I0(v) is plotted as a three-dimensional representation of the structure transmittance dependence on the radiation frequency and the incidence angle. A possibility to study the resonant interaction of optical phonons with plasmons in the film and the substrate is demonstrated.

On the long-term correlations in the twinning and dislocation slip dynamics

The present work aims at gaining new insights into the dynamics of the dislocation slip and twinning processes, which govern the majority of possible scenarios of plastic deformation in structural metals and alloys. Using model magnesium single crystals and employing the statistical analysis of acoustic emissions (AE) generated in the course of plastic deformation, it was demonstrated that twinning is a process with a memory of the past whereby the twinning events affect the occurrence of successive events. As opposes to this, the basal dislocation slip appears as a substantially random intermittent process comprising of independent elementary slip events without long-term correlations between gliding dislocations or slip bands emerging due to the collective dislocation glide along basal planes. The single crystals were chosen with axial orientations best suitable to facilitate either basal dislocation slip or tensile twinning under compressive loading, which significantly simplified the interpretations of AE findings. Besides, the AE results were fully corroborated in situ by microstructure observations using scanning electron microscopy imaging.

Amorphous structure in single-crystal magnesium under compression along the c axis with ultrahigh strain rate

It is well known that twinning deformation plays an important role in the plastic deformation of hexagonal close-packed (HCP) metals because of the insufficient independent slip systems in HCP metals. However, recent works show twinning deformation does not happen in compression in the $c$ axis of the magnesium (Mg) single crystal, and it is not clear that how it deforms with limited dislocation glide in absence of twinning deformation. In the present work, molecular dynamics simulations with well validated modified embedded-atom method potential for Mg are performed to reveal the microscopic deformation behaviors of Mg single crystal compressed in the $c$ axis. We focus on the microstructures and the evolution during both the deformation and the relaxation of the Mg single crystal. Twinning deformation is not observed in the simulations as expected, and dislocations and the amorphous regions are found to be dominant under different loading conditions. The amorphous regions are dominant in the deformation under ultrahigh strain rate, and it results in more homogenous deformation and lower flow stress than dislocation glide. The dislocation glide is dominant under lower strain rate, and the insufficient dislocations may lead to quite inhomogeneous plastic deformation. Furthermore, it is revealed that the amorphous regions consist of irregularly distributed atoms with higher per-atom potential than those in the crystalline state. The amorphous regions thus annihilate rapidly once the high-rate deformation is stopped. The annihilated amorphous regions turn into crystalline ones with dense dislocation networks. The annihilation rate of the amorphous regions decreases with increasing pressure, while it hardly depends on temperature.

A Generalization of the von Mises Criterion for a Single Crystal with a Hexagonal Crystal Lattice

A natural generalization of the von Mises flow criterion is obtained taking into account anisotropy on single-crystal materials with a hexagonal crystal lattice. Boundary-value problems are posed for plane deformation by compression with back pressure, for uniaxial tension, compression, and for pure shear. A special case of solving boundary value problems for hexagonal magnesium with a purity of 99.9% at room temperature is considered. The values of coefficients included in the fluidity function and their errors are determined. The calculated and experimental data σ0.2% for magnesium single crystals of various orientations are compared.

Surface characterization of patterning on MgO single crystals using wet chemical etching process to advance MEMS devices

Single-crystalline magnesium oxide (MgO) is a material with outstanding high-temperature resistance and huge potential for use in high-temperature devices, light emitting devices and optical display fields. The investigation of fabricating a microstructure on the MgO substrate using wet etching process is conducted. The temperature and concentration dependence of the etching rate on the materials, and the surface roughness of the microstructure, are explored and analyzed. A microcavity with good profile and low roughness, 80.7 µm in depth and 4 mm in diameter, has been generated on a MgO substrate with a 50% H3PO4 etchant solution at ~100 °C. Optical microscopy, atomic force microscopy, scanning electron microscopy, x-ray photoelectron spectroscope and x-ray diffraction analysis are employed to demonstrate the successful application of wet etching for improving the etching rate and surface morphology without the deterioration of the surface roughness. Our work is of fundamental importance in the fabrication of MgO-based devices (such as pressure sensors, vibration sensors and photonic resonators) and the improvement of growth condition of oxide films on MgO substrates.

Shock compression/release of magnesium single crystals along a low-symmetry orientation: Role of basal slip

To gain insights into the relative contributions of different plastic deformation mechanisms, particularly basal slip, for shocked hexagonal close-packed (hcp) metals, magnesium (Mg) single crystals were subjected to shock compression and release along a low-symmetry (LS) orientation to 1.9 and 4.8 GPa elastic impact stresses. LS-axis is a “nonspecific” direction resulting in propagation of quasilongitudinal and quasishear waves. Wave profiles, measured using laser interferometry, show a small elastic wave followed by two plastic waves in compression; release wave profiles exhibited a structured response for the higher stress and a smooth response for the lower stress. The LS-axis wave profiles are significantly different than profiles published previously for c- and a-axes, demonstrating that Mg single crystals exhibit strong anisotropy under shock compression/release. Numerical simulations, using a time-dependent anisotropic modeling framework, show that shock wave loading along the LS-axis involves the simultaneous operation of multiple deformation mechanisms. Shock compression along LS-axis is dominated by basal slip while prismatic slip and pyramidal I {101¯1}⟨112¯3⟩ slip play a smaller role; coupling between longitudinal and shear deformations was observed. The unloading response is dominated by basal slip with some contribution from prismatic slip; pyramidal I slip is not activated. The present results, unlike results obtained for c- and a-axes, show that the deformation mechanism observed under quasistatic loading conditions along LS-axis is not sufficient to determine the shock response along this orientation. Although requiring numerical simulations for wave analysis, shock propagation along a LS-orientation provides new insights into the plastic deformation response of hcp metal single crystals.

Atomic-scale imaging of incipient interval-layered hydrogenation of single crystal magnesium

A unique inter-layered hydrogenation process of single crystal magnesium has been firstly confirmed by in situ aberration corrected environmental transmission electron microscopy together with density function theoretical calculations. Differing from the surface dominated process, the incipient hydrogenation of magnesium is mainly related to an intercalation reaction mechanism, resulting in the incorporation of hydrogen into the bulk. It provides fundamentals for interpreting hydrogen embrittlement and hydrogen storage of magnesium-based materials.

Effects of Cerium Addition on Non-basal Slip in Magnesium Single Crystals

Effects of Cerium Addition on Non-basal Slip in Magnesium Single Crystals

Polycrystalline infrared-transparent MgO fabricated by spark plasma sintering

Abstract In the present research, polycrystalline magnesium oxide (MgO) bodies were fabricated using spark plasma sintering (SPS) at different temperatures and times from MgO nanopowder. Microstructural development, densification, and optical properties were investigated during SPS. The critical pressure of plastic deformation of the MgO compacts during sintering was also analyzed. The results showed that the plastic deformation phenomenon had a profound effect on the grain size and optical properties. In addition, the optical properties and microstructure of MgO bodies were strongly dependent on sintering temperature and time. Full-dense infrared-transparent magnesium oxide with a relative density of 99.99% was prepared at 1200 °C for 5 min under the pressure of 80 MPa. The spark plasma sintered MgO demonstrated the highest infrared transmittance of 72% in the 3–7 μm wavelength range, which was comparable with the values reported for MgO single crystal.