Magnesium Single Crystals Research Articles

Influence of the anisotropy in lattice friction on the macroscopic behavior of magnesium single crystal: A study by discrete dislocation dynamics

It is known that the lattice friction in hexagonal close-packed (hcp) Mg crystals is anisotropic and depends on the family of slip systems. In this study, the relation between the lattice friction and the single mechanism involving the reaction between dislocations from different families of slip systems, as well as the relation between the lattice friction and the macroscopic behavior are analyzed using discrete dislocation dynamics simulations.In this work, it is demonstrated that for small values of the friction stress, the recombination stress is controlled by the line tension of dislocations, while for large values of the friction stress, the recombination stress is controlled by the lattice friction. Further, for low values of the lattice friction, the macroscopic behavior is controlled by the well known forest mechanism, while when increasing the lattice friction (which can be viewed as a decrease in the temperature), the macroscopic behavior is controlled by the lattice friction itself.

Kinetic study of crystallisation of sol–gel derived calcia–alumina binary compounds

Abstract In-situ High Temperature X-ray Diffraction (HTXRD) and Differential Scanning Calorimetric (DSC) studies were performed on a sol–gel derived binary compound of a calcia–alumina (C12A7) system consisting of calcium oxide (CaO) and aluminium oxide (Al 2 O 3 ) in a ratio of 12:7 for in situ investigation into the phase transformations under progressively increasing thermal activation from room-temperature to 1200 °C. The crystallisation of amorphous samples formulated at room-temperature on magnesium oxide (MgO) single crystal (1 0 0) substrates was found to be complete on heat treatment at 1100 °C for 3 h. This observation was further supported by independent Fourier Transform Infrared (FTIR) and Raman Spectroscopies. Values of 348 kJ/mol and 375 kJ/mol were estimated from Kissinger plots for activation energies of crystallisation of CaO and Al 2 O 3 constituents, respectively.

Ambient Compression–Compression Fatigue Behavior of Magnesium Single Crystal

A magnesium single crystal sample with a near \( 1 1 {\bar{{2}}{0}} \) orientation was tested at room temperature under compression–compression cyclic loading, and the microstructure was characterized to disclose the involved deformation mechanisms. No plastic deformation region appeared on the stress–strain curve during the cyclic loading. The stress–strain curve stabilized at the first cycle, the strain range for each cycle fluctuated slightly around a constant value, and the mean strain for each cycle was in a narrow range from 0.0846 to 0.0863 during the whole test. The ratcheting strain rate decreased exponentially from ~0.0003, and reached a relatively small and stable value of about zero. The observed deformation mechanisms were prismatic slip, compression twinning, and tension twinning. The prismatic dislocation slip roughened the cylindrical sample surface by forming extrusions and intrusions, and small cracks were also observed on the surface.

Compton Profile Study and Electron Momentum Density Reconstruction in Hexagonal Mg

Four directional Compton profiles (CPs) of a hexagonal magnesium single crystal have been measured with high momentum resolution (0.12 a.u.) using 115.6 keV synchrotron radiation in SPring-8. Experimental CP anisotropies were compared with augmented plane waves (APW), Korringa–Kohn–Rostocker (KKR), and the latest density functional theory (DFT) band structure calculations. In order to reconstruct the full three-dimensional valence electron momentum density distribution, maximum entropy method (MEM) was applied. 3D electron momentum densities obtained using MEM were compared (after appropriate integration) with 2D electron momentum density reconstructed using Cormack's method. MEM has proved its advantage as a technique for reconstructing density distributions when initial information is insufficient and noisy. The 3D valence electron momentum density of hexagonal Mg was successfully reconstructed using as few as four experimental or five theoretical Compton profiles. As is shown, despite a clear anisotrop...

Neutron characteristics of single-crystal magnesium fluoride

A simple formula is proposed that permits the calculation of both the neutron reflectivity and attenuation of single-crystal magnesium fluoride (MgF2) as a function of wavelength at both room temperature and 77K. A computer program MgF2 written in FORTRAN-77, has been developed to carry out the required calculations for the single-crystal MgF2, when it is used as a neutron monochromator and/or filter.The monochromatic characteristics of single-crystal MgF2 are detailed in terms of the optimum crystal cutting plane, mosaic spread, thickness and reactor moderating temperature for efficient neutron reflectivity within the wavelength band from 0.1nm up to .45nm. Calculation shows that a 7mm thick MgF2 single-crystal cut along (111) plane having 0.50 FWHM on mosaic spread has the optimum parameters when it is used as a monochromator at neutron wavelengths shorter than 0.2nm. However, the integrated neutron intensity of 2nd and 3rd orders from a thermal reactor flux is even higher than that of the 1st order one at neutron wavelengths longer than 0.2nm. While, from a cold reactor flux, the use of the single-crystal MgF2 (110) as a neutron monochromator free from higher order contaminations at λ⩾0.38nm is more appropriate.A feasibility study of using single-crystal MgF2 as a neutron filter is also detailed in terms of the optimum crystal orientation, mosaic spread, thickness and temperature for efficient transmission of thermal-reactor neutrons. Calculation shows that a 100mm thick MgF2 single-crystal cut along (110) plane cooled to liquid nitrogen temperature, with 0.05o FWHM on mosaic spread, is a good thermal neutron filter, with high effect-to-noise ratio.

Effects of the initial dislocation density on size effects in single-crystal magnesium

Single-crystal magnesium micropillars, ranging in diameter from approximately 600nm to 10μm, are fabricated using focused ion beam machining and are loaded in uniaxial compression along either the [0001] or [23¯14] axis. The influence of initial dislocation density on size effects is investigated for compression along the [0001] axis using two distinct initial dislocation densities. Separately, at fixed low dislocation density, the influence of orientation on size effects is examined by comparing compression along the [23¯14] and [0001] directions. Our microcompression results show that decreasing the initial dislocation density results in a stronger size effect in terms of both increased strength and stochasticity with decreasing pillar size. Comparison with a probability-based model shows good agreement between theoretical predictions and experimental observations. Our results demonstrate that the properties of magnesium micropillars depend on the specimen diameter, the initial dislocation density and the orientation of the basal planes with respect to the loading axis.

Compression deformation mechanisms at the nanoscale in magnesium single crystal

The dominant deformation mode at low temperatures for magnesium and its alloys is generally regarded to be twinning because of the hcp crystal structure. More recently, the phenomenon of a “loss” of the twins has been reported in microcompression experiments of the magnesium single crystals. Molecular dynamics simulation of compression deformation shows that the pyramidal 〈a + c〉 slip dominates compression behavior at the nanoscale. No compression twins are observed at different temperatures at different loadings and boundary conditions. This is explained by the analyses, that is, the $$\left\{ {10\bar 12} \right\}$$ and $$\left\{ {10\bar 11} \right\}$$ twins can be activated under c-axis tension, while compression twins will not occur when the c/a ratio of the hcp metal is below √3. Our theoretical and simulation results are consistent with recent microcompression experiments of the magnesium (0001) single crystals.

OS0419 メタダイナミクス法によるマグネシウム単結晶中での転位ループ生成過程の解析

OS0419 メタダイナミクス法によるマグネシウム単結晶中での転位ループ生成過程の解析

1504 原子シミュレーションによるマグネシウム単結晶中での転位ループ生成の活性化自由エネルギー解析

1504 原子シミュレーションによるマグネシウム単結晶中での転位ループ生成の活性化自由エネルギー解析

Activation Free Energy of Nucleation of a Dislocation Pair in Magnesium

Kink deformation is one of the possible principal deformation modes of alloys with a long-period stacking ordered structure under compression parallel to the basal plane. In this deformation, dislocation pairs are massively nucleated, and these dislocations align in a line to form kink bands. In this study, we investigated the nucleation of a dislocation pair in a pure magnesium single crystal by molecular dynamics simulations. We also evaluated the activation free energy of nucleation of a dislocation pair and investigated the dependence of the activation free energy on the applied shear stress and temperature. [doi:10.2320/matertrans.MI201213]

Epitaxial growth of (104)- and (018)-oriented LiCoO2 films on MgO single crystals prepared by chemical vapor deposition

The lithium cobaltate (LiCoO2) films were deposited onto single-crystal magnesium oxide (MgO) substrates by metal organic chemical vapor deposition, and the in-plane orientation relationship, morphology and deposition rates were investigated. Hetero-epitaxial LiCoO2 films oriented to (104) and (018) were obtained on (100) and (110) MgO substrates. The surface morphology of the (104) LiCoO2 film showed diagonally arrayed rectangular facets, whereas the (018) LiCoO2 film exhibited a roof-like structure extended unidirectionally to [1¯10] MgO. Twin structures formed with in-plane relationships of [010] LiCoO2 // [01¯0] LiCoO2 and [421¯] LiCoO2 // [010] LiCoO2 in the (104) LiCoO2 film and with in-plane relationships of [100] LiCoO2 // [1¯00] LiCoO2 in the (018) LiCoO2 film. The deposition rates of the (104)- and (018)-oriented LiCoO2 films were 10.8μmh−1 and 8.8μmh−1, respectively, which were almost 100 times greater than those reported for LiCoO2 epitaxial films fabricated by pulsed laser deposition.

Nanoindentation study for deformation twinning of magnesium single crystal

The small-scale deformation behavior of single-crystalline Mg was investigated using nanoindentation combined with transmission electron microscopy observation. The nanoindentation on both basal and second-order prismatic planes activated the tensile twin system at different positions. The concept of twin-favorable zones was chosen to explain the tensile twin formation by indentation on both basal and prismatic planes. Besides 〈c + a〉 dislocation, this study proves that the tensile twin formation plays a role in accommodating the strain induced by the nanoindentation of single-crystalline Mg.

Magnetization and electric transport properties of single-crystal MgB2 nanowires

High quality single-crystal magnesium diboride (MgB2) nanowires with lengths exceeding 10 μm were successfully synthesized by hybrid physical chemical vapor deposition. The magnetization and electrical transport properties of single-crystal MgB2 nanowires (NWs) were measured. The superconducting transition temperature of the NWs was 37 K, as confirmed by magnetization measurements. The disordered behavior of the nanowires was observed by four-terminal current–voltage characteristic measurements of an individual NW from T = 10 to 300 K. The temperature-dependent resistivity curves for seven NWs collapsed into a universal curve described by the variable range hopping model, showing intrinsic nonmetallic transport properties. This implies that the granular superconducting defect states are critical to the superconductivity of the individual MgB2 NWs.

Evidence for ductile deformation of single-crystal magnesium oxide subjected to thermal shock

Possible evidence is presented of the ductile deformation of single-crystal magnesium oxide by the multiplication and propagation of surface dislocations by thermal stresses generated during the measurement of its wettability by liquid nickel.

Work Hardening of Magnesium Single Crystals Deformed to Stage B at Room Temperature

Magnesium single crystals of purity (99.8 wt%) deformed to stage B on the work-hardening curve at the temperature of 293 K and at a strain rate of 10−3 s−1 were investigated. The modi ed Bridgman method was used to obtain the crystals of the preferred orientation of (0001)⟨1120⟩ as primary slip system. By using the method based on the experimental gradient matrix, the activity of slip systems was obtained in magnesium single crystals deformed to shear strain 1.2γ, where well developed stage B of work hardening was observed. It was shown that primary (0001)⟨1120⟩ slip system was dominant in the whole investigated range of the examined deformation. The observation and analysis of etch pits on the {1210} plane showed the heterogeneous distribution of dislocations formed during deformation into walls of dislocations perpendicular to the (0001) slip plane. The suggested model of work hardening of magnesium single crystals, which is worth taking into consideration, shows the in uence of the long-range stress eld derivating from the groups of dislocations arranged in dislocation walls.

Fatigue behavior and microstructure of 0001 and 1̄014 magnesium single crystals under compression–compression cyclic loading

Compression–compression cyclic tests were performed on the 0001 and 1̄014 single crystal samples for 50 loading cycles at room temperature. The compression stress varied in the range of 19MPa–97MPa. The tested samples were then studied using various microstructure characterization tools including X-ray diffraction (XRD), scanning electron microscopy (SEM), and high resolution transmission electron microscopy (HRTEM). The two samples with different orientations had quite different mechanical behavior and microstructural features. For 0001 single crystal, the strain range and ratcheting strain were constant and very small. On the contrary, the sample heavily strained and deformed for 1̄014 single crystal, and the ratcheting strain was about ten times of that for 0001 single crystal. There was no cyclic hardening/softening for 0001 single crystal, while 1̄014 single crystal experienced cyclic strain hardening. Microstructural observations indicated that twinning was the main deformation mechanism for 0001 single crystal and basal slip, pyramidal slip, secondary pyramidal slip, and tension and compression twinning operated for 1̄014 single crystal. The basal slip resulted in the initial plastic deformation region for 1̄014 single crystal. The Schmid factor analyses demonstrated that the deformation mechanisms were consistent with those obtained from the microstructural observations.

The first observation of dislocation blocking in pure metal without external stress

Self-blocking of c + a edge dislocations at second-order pyramidal slip in magnesium single crystals whose axis is parallel to the c axis has been found. Self-blocking is confirmed by the dislocation extension along a selected direction in the absence of external stress. Transmission electron microscopy (TEM) images of (c + a) dislocations extended in the \(\left\langle {1\bar 100} \right\rangle\) directions are obtained. A model of a two-valley potential relief in Mg is proposed: the Peierls relief determines the second-order slip of (c + a) dislocation in the pyramid plane, while the immersion in deep valleys is related to splitting of the c + a edge segment (or the spread of its core) in the basal plane.

Optical properties of thin epitaxial Ba0.8Sr0.2TiO3 films

The properties of nanodimensional (Ba0.8, Sr0.2)TiO3 films on single-crystal magnesia substrates are studied. The films are applied by rf sputtering and grow in the layer-by-layer mode. The lattice parameters are measured by the X-ray diffraction method. The transmission of the films with different thicknesses is studied in the wavelength range 190–1100 nm. When analyzing experimental optical parameters, additional relaxation parameters depicting a final lifetime of the oscillator are used to characterize the refractive index and absorption factor in the dispersion relation. Such an approach allows a more accurate approximation of experimental data.

0 0 0 1] Compression response at room temperature of single-crystal magnesium

Compression of single-crystal magnesium along the [0 0 0 1] direction at room temperature confirmed a high yield stress, high work-hardening and minimal plastic deformation prior to fracture. Post-deformation examination revealed the dominance of c + a slip; straight and zig-zag dislocations and numerous fine dislocation loops, all with the 〈11–23〉 Burgers vector, were observed. These are thought to be responsible for the high work-hardening behavior. Extension and contraction twins were present, but only occasionally. Extension twins were shown to occur during unloading.

Effects of Crystallographic Orientation on Corrosion Behavior of Magnesium Single Crystals

The corrosion behavior of magnesium single crystals with various crystallographic orientations was examined in this study. To identify the effects of surface orientation on the corrosion behavior in a systematic manner, single-crystal specimens with ten different rotation angles of the plane normal from the [0001] direction to the \( [ 10\overline{1} 0] \) direction at intervals of 10° were prepared and subjected to potentiodynamic polarization and potentiostatic tests as well as electrochemical impedance spectroscopy (EIS) measurements in 3.5 wt.% NaCl solution. Potentiodynamic polarization results showed that the pitting potential (Epit) first decreased from −1.57 VSCE to −1.64 VSCE with an increase in the rotation angle from 0° to 40°, and then increased to −1.60 VSCE with a further increase in the rotation angle to 90°. The results obtained from potentiostatic tests are also in agreement with the trend in potentiodynamic polarization tests as a function of rotation angle. A similar trend was also observed for the depressed semicircle and the total resistances in the EIS measurements due to the facile formation of MgO and Mg(OH)2 passive films on the magnesium surface. In addition, the amount of chloride in the passive film was found first to increase with an increase in rotation angle from 0° to 40°, then decrease with a further increase in rotation angle, indicating that the tendency to form a more protective passive film increased for rotation angle near 0° [the (0001) plane] or 90° [the \( ( 10\overline{1} 0) \) plane].