Magnesium Single Crystals Research Articles

NANOSCALE VOID GROWTH IN MAGNESIUM: A MOLECULAR DYNAMICS STUDY

Molecular dynamics calculations were carried out in single crystal magnesium specimens to reveal the dependence of strain rate, temperature, and orientation of the crystal on damage evolution as defined by pore growth. Two specific crystallographic orientations [0001] and [Formula: see text] were examined. During a [0001] tensile test, twin boundaries developed at the void surface leading to a constraint on the [Formula: see text] crystallographic orientation. On the other hand, during the [Formula: see text] tensile deformation, emission of shear loops in the prismatic slip planes arose when void growth initiated. Furthermore, analysis of the damage components (nucleation, growth and coalescence) revealed that a large number of small voids nucleated that rapidly grew and fractured the specimens independent of the temperature and the strain rate.

Crystalline and induced anisotropy and structural defects in single-crystal spinel ferrite films

The crystalline and uniaxial anisotropy constants of single-crystal manganese and magnesium manganese ferrite spinel films have been determined experimentally. The results indicate that the structural perfection of the films, influenced by the film composition and growth conditions, has a significant effect on their anisotropy constants.

Atomic Scale Simulation of Deformation in Magnesium Single Crystals

The deformation behaviour of magnesium single crystals under plane strain conditions has been examined using molecular dynamics modelling. The simulations were based on an existing atomic potential for magnesium taken from the literature. A strain of 10% was applied at rates of 3x109s-1 and 3x107s-1. The simulations predicted the formation of mechanical twins that accommodated extension in the c-axis direction of the hexagonal unit cell. However, the predicted twin is not of the same kind found in magnesium, but is that commonly observed in titanium. It is believed that further analysis of the physical properties predicted by this interatomic potential will shed more light on the atomic processes controlling twinning in Magnesium alloys. It also highlights the need for improvements to the interatomic potential such that more accurate deformation behaviour can be attained.

Study of elementary deformation mechanisms in MgO induced by nanoindentation: experiments and simulation

Nanoindentation test is a common method to study mechanical properties at a nanometre scale such as hardness or Young’s modulus [1]. This mechanical test consists in applying a force on the sample through a diamond tip while the resulting penetration depth is monitored during a loading-unloading cycle. Mechanical properties can be extracted from the so called load-displacement curve thanks to analysis model. The nanoindentation test can be also used to induce locally plastic deformation and thus to study incipient plasticity. In particular, as very high stress can be reached in material volumes small enough to be considered as dislocations free, this method is interesting to investigate the dislocation nucleation process. In the present study this method has been applied in single crystals of Magnesium Oxide (MgO) which have been chosen as a model material due to its rather simple plasticity. A spherical indenter with a high radius (10 μm) has been used for two reasons. Firstly, the elastic contact between a sphere and a flat surface is well described by the Hertz law [2]. And secondly, the stress induced by a spherical contact is easily calculable. Residual deformations around nanoindentation imprint have been observed by Atomic Force Microscope (AFM). The high resolution of the AFM and its ability to measure height variations as low as few Angstroms is of particular interest to analyse slip lines due to the emergence and propagation of dislocations around the indents. AFM has also been associated to a nanoetching technique which consists in revealing the emergence point of dislocations, combined with a fine chemomechanical polishing to perform a 3D reconstruction of the dislocation structure [3].

1125 マグネシウム単結晶の単軸変形挙動に及ぼす亜粒界の影響(OS11. 材料の組織・強度に関するマルチスケールアナリシス(6),オーガナイズドセッション講演)

1125 マグネシウム単結晶の単軸変形挙動に及ぼす亜粒界の影響(OS11. 材料の組織・強度に関するマルチスケールアナリシス(6),オーガナイズドセッション講演)

Microcompression of single-crystal magnesium

Single-crystal magnesium micropillars are fabricated using focused ion beam (FIB) milling. The micropillars are loaded in compression along the [0 0 0 1] c-axis. The stress–strain curves from these microcompression experiments reveal significant strain hardening, and post-mortem microscopy reveals traces of pyramidal slip on the deformed specimens. Transmission electron microscopy (TEM) specimens are excised from the micropillars using FIB, and TEM analysis confirms the presence of pyramidal dislocations as well as 〈c〉 dislocations. No twinning is observed.

Analysis of deformation‐induced twinning at finite strains based on energy minimization

AbstractIn the present contribution, an energy‐based description of phase transformation suitable for the modeling of crystal reorientation during twinning is proposed. A homogenized relaxed stored energy (volume averaging) is utilized for the simulation of the pre‐ and post‐twinning hardening form of magnesium single crystal. The proposed energy‐driven model is applied to analyze the activity of deformation systems inside the twinning related phases. It is shown that the activity of the inter‐twin dislocation systems accommodates the plastic strain at larger deformation. (© 2009 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Generation and removal of pits during chemical mechanical polishing process for MgO single crystal substrate

Magnesium oxide (MgO) single crystal is an important substrate for high temperature superconductor, ferroelectric and photoelectric applications. The function and reliability of these devices are directly affected by the quality of polished MgO surface because any defect on the substrate, such as pit or scratch, may be propagated onto device level. In this paper, chemical mechanical polishing (CMP) experiments were conducted on MgO (1 0 0) substrate using slurry mainly comprised of 1-hydroxy ethylidene-11-diphosphonic acid (HEDP) and silica or ceria particles. Through monitoring the variations of the pits topography on substrate surface, generation and removal mechanism of the pits were investigated. The experimental results indicate that the pits were first generated by an indentation or scratch caused by particles in the slurry. If the rate of chemical etching in the defect area is higher than the material removal rate, the pits will grow. If chemical reaction in the defect area is slower than the material removal rate, the pits will become smaller and eventually disappear. Consequently, these findings may provide insight into strategies for minimizing pits during CMP process.

Comparison of the properties of tonpilz transducers fabricated with ⟨001⟩ fiber-textured lead magnesium niobate-lead titanate ceramic and single crystals

Tonpilz transducers are fabricated from 001 fiber-textured 0.72Pb(Mg(1/3)Nb(2/3))O(3)-0.28PbTiO(3) (PMN-28PT) ceramics, obtained by the templated grain growth process, and PMN-28PT ceramic and Bridgman grown single crystals of the same composition. In-water characterization of single element transducers shows higher source levels, higher in-water coupling, and more usable bandwidth for the 81 vol % textured PMN-28PT device than for the ceramic PMN-28PT element. The 81 vol % textured PMN-28PT tonpilz element measured under large signals shows linearity in sound pressure levels up to 0.23 MV/m drive field but undergoes a phase transition due to a lowered transition temperature from the SrTiO(3) template particles. Although the textured ceramic performs well in this application, it could be further improved with compositional tailoring to raise the transition temperature and better processing to improve the texture quality. With these improvements textured piezoelectric ceramics will be viable options for medical ultrasound, actuators, and sonar applications because of their ease of processing, compositional homogeneity, and potentially lower cost than single crystal.

Multiphonon absorption in a MgO single crystal in the terahertz range

The reflection and transmission spectra of a MgO single crystal at frequencies of 10–1000 cm−1 in the temperature range 10–300 K have been measured using submillimeter and infrared Fourier spectros-copy. The evolution of anharmonic absorption with variations in temperature has been investigated in the frequency range below the frequency of the transverse optical phonon. The parameters of the dispersion model that adequately describes the lattice absorption of terahertz radiation in single-crystal magnesium oxide have been determined.

Development of Fatigue Testing Machine for Thin Sheet Specimen and Fatigue Test for Magnesium Single Crystal

A fatigue testing machine for thin sheet specimens was developed to clarify the crystallographic orientation dependence on fatigue fracture behavior in magnesium single crystals. The size of the thin sheet specimen was 3mm in width and 0.3mm in thickness. One end of the thin sheet specimen was fixed at a voice coil of a loudspeaker and the other end was set free. A bending mode resonance occurs in the specimen due to forced vibration at the fixed end. To estimate stress amplitude, the displacement of the free end of the specimen was measured with a laser displacement meter. The stress amplitude was evaluated as the bending stress in the single cantilever. Fatigue tests were carried out with stress ratio R = –1 at room temperature in laboratory air and the frequency of the cyclic stress was 981Hz. In A-specimen whose surface plane and loading direction were (0001) and [1120], a fatigue crack propagated along [1100] at higher stress level while a crack propagated along [1210] at lower stress level. In B-specimen whose surface and loading direction were (1120) and [1100], a crack propagated partially along {1012} twin interface. As a result, fatigue life of the B-specimen was longer than that of the A-specimen. The fatigue limits of the A- and B-specimens were nearly the same, and it was estimated as 45MPa at 106∼108 cycles. The fatigue testing machine developed in this study showed the crystallographic orientation dependence on fatigue fracture behavior in magnesium single crystals.

Neutron transmission of single-crystal magnesium fluoride

The neutron attenuation through single-crystal magnesium fluoride has been measured as a function of wavelength at both room temperature and 77 K. These data confirm a total cross section that is lower than that of MgO for wavelengths greater than 0.2 nm. MgF2cooled to 77 K is a slightly better filter of epithermal neutrons than MgO for obtaining a thermal neutron beam.

Compression of single-crystal magnesium oxide to 118 GPa and a ruby pressure gauge for helium pressure media

The pressure-volume equation of state (EoS) of single-crystal MgO has been studied in diamondanvil cells loaded with helium to 118 GPa and in a non-hydrostatic KCl pressure medium to 87 GPa using monochromatic synchrotron X-ray diffraction. A third-order Birch-Murnaghan fit to the nonhydrostatic P-V data (KCl medium) yields typical results for the initial volume, V0 = 74.698(7) A 3

The COMPRES/GSECARS gas-loading system for diamond anvil cells at the Advanced Photon Source

We have designed and constructed a new system for loading gases at high pressure into diamond anvil cells at pressures up to 200 MPa. The gases are used either as quasi-hydrostatic pressure media surrounding the sample or as the sample itself. The diamond cell is sealed using a clamping mechanism, which permits nearly any type of diamond anvil cell to be used. Online ruby fluorescence and video imaging systems allow in situ monitoring of the pressure and gasket deformation as the cell is sealed, resulting in a very high success rate in loading cells. The system includes interlocks and computer control that allow it to be safely and easily operated by visiting users at the Advanced Photon Source. We present preliminary X-ray diffraction data on volume compression of single-crystal magnesium oxide (MgO) in helium up to 110 GPa.

Modeling and simulation of a single crystal based on a multiscale field theory

This work presents a multiscale field theory and its applications in modeling and simulation of atomistic systems. The theoretical construction of the multiscale field theory is briefly introduced. A single crystal is discretized into finite element mesh as if it is a continuous medium. However, each node is a representative unit cell, which contains a specified number of discrete and distinctive atoms. Material behaviors of a given atomistic system at nano/micro scale, subject to the combination of mechanical loadings, electromagnetic field and temperature field, can be obtained through numerical simulations. Sample problem of a single crystal (magnesium oxide) subject to large strain tension/compression has been solved to demonstrate the advantage and applicability of this multiscale field theory.

Texture Evolution of Magnesium Single Crystals Deformed by High-Pressure Torsion

Single crystals of technical purity Magnesium (99.8 wt.%) of initial orientations [ ] 2 1 10 and [ ] 2 2 11 were subjected to HPT deformation at room temperature up to strains of 10. The microstructural evolution has been analyzed by X-ray microtexture investigations and by in-situ stress-strain measurements. The results can be described in terms of shear arising from HPT deformation and - with higher strains - in terms of recrystallization. In crystals with hard orientation[ ] 2 2 11 , these features occur at smaller strains than in crystals with soft orientation [ ] 2 1 10 , i.e. with higher symmetry. In general, the observed textures and strength variations are much stronger than those reported for fcc HPT deformed metals.

OS0602 マグネシウム単結晶の非対称硬化挙動の結晶塑性解析(OS06-01 マルチスケール解析1,OS06 微視構造を有する材料の変形と破壊(1))

OS0602 マグネシウム単結晶の非対称硬化挙動の結晶塑性解析(OS06-01 マルチスケール解析1,OS06 微視構造を有する材料の変形と破壊(1))

Simulation of dislocation nucleation and motion in single crystal magnesium oxide by a field theory

This paper presents a multiscale field theory and its application in modeling and simulation of the phenomena that take place at atomic scale. Atomistic formulation of a multiscale field theory is introduced. The governing equations for problems with a given temperature are derived. We have modeled and simulated the nucleation and motion of dislocations and the formation of plastic deformation in a nanoscale ionic material MgO. The mechanism of deformation as well as the evolution of the stress has been elucidated. Results are compared with those from molecular dynamics simulations.

Influence of Impurities to Deformation Behavior in Magnesium Single Crystal

Generally, plastic deformation of magnesium alloys is difficult at room temperature. In order to improve formability of magnesium, impurity elements in magnesium were reduced by vapor deposition technique. Inductively coupled plasma atomic emission spectrometry (ICP-AES) was applied to the determination of trace elements in refined magnesium. To investigate influence of impurity element to deformation behavior, high purity magnesium single crystals were prepared. When the magnesium single crystals are stretched in <11-20> direction, {11-22} <-11-23> pyramidal slips were activated just after yielding in the range of 77K to 293K. The yield stress of high purity magnesium was a half of the stress in raw magnesium.

Deformation Behavior of Magnesium Single Crystals in C-Axis Compression

In general, deformation behavior of magnesium in compression is different from tensile. To investigate deformation behavior of magnesium single crystals, c-axis compression was performed. The crystals were yielded by second order pyramidal slip, and the yield stress shows anomalous temperature dependence (increased with increasing temperature) between 203K and 293K. Yield stress of c-axis compression is bigger than that of a-axis tensile. {10-13} twin and {11-24} twin occurred at 77293K and 77473K respectively. Fracture surface at 77293K was {11-24} and at 473K was {11-22}.