Anisotropy Of Single Crystals Research Articles

Advanced TiAl Based Alloys: From Polycrystals to Polysynthetic Twinned Single Crystals

AbstractTiAl alloys stand out for low density, high specific strength, and excellent creep resistance, making them promising for high‐temperature aerospace applications. However, traditional TiAl alloys suffer from poor room‐temperature ductility and low service temperature that limit their critical applications in aerospace structures. To address these issues, research has focused on improving the mechanical properties of TiAl alloys through alloying and microstructural design. After decades of effort, the evolution of TiAl alloys has progressed from polycrystalline TiAl to high‐performance polysynthetic twinned (PST) TiAl single crystals. The well‐aligned PST TiAl single crystals enriched with Nb enable an excellent combination of strength and ductility, significantly outperforming polycrystalline TiAl alloys. This review summarizes recent progress on TiAl alloys, particularly focusing on newly developed PST single crystals. First, the development history of TiAl alloys is overviewed; then their crystal structures, phase diagrams, and typical microstructures are systematically discussed, along with the design strategies based on alloying elements. Additionally, recent advances in TiAl columnar crystals, which are between polycrystals and single crystals, are reviewed. Subsequently, the mechanical anisotropy, preparation methods, and superior mechanical properties of the PST single crystals are analyzed in detail. The final remark highlights the future development and application prospects of TiAl alloys.

Electrical conductivity anisotropy of InGaZn3O6 single crystals by pressurized optical floating zone growth and its oxygen annealing effect

Electrical conductivity anisotropy of InGaZn3O6 single crystals by pressurized optical floating zone growth and its oxygen annealing effect

Molecular Orientation-Dependent Photonic Polarization Engineering in Organic Single-Crystal-Filled Microcavities.

Designing the polarization degree of freedom of light is crucial in many fields and has widespread application in, for example, all-optical circuits. In this work, we find that in an organic microcavity filled with anisotropic single crystals the cavity modes can be modulated to be elliptically polarized, i.e., partially circularly polarized and partially linearly polarized. The circular polarization component originates from the Rashba-Dresselhaus spin splitting, while the linear polarization component is due to the dislocation of linearly polarized modes. The dislocation of the linear polarizations is ascribed to the orientation of individual molecules and the molecular packing arrangement; hence, the linear polarizations can be controlled by properly structuring the molecular distributions. Our results pave the way for enriching and engineering the polarization properties of individual optical cavity modes in organic microstructures, which may favor the development of polarized lasers with various polarizations.

Highly-Polarized Solar-Blind Ultraviolet Organic Photodetectors.

Developing high-performance polarization-sensitive ultraviolet photodetectors is crucial for their application in military remote sensing, detection, bio-inspired navigation, and machine vision. However, the significant absorption in the visible light range severely limits the application of polarization-sensitive ultraviolet photodetectors, such as high-quality anti-interference imaging. Here, based on a wide-bandgap organic semiconductor single crystal (trans-1,2-bis(5-phenyldithieno[2,3-b:3',2'-d]thiophen-2-yl)ethene, BPTTE), high-performance polarization-sensitive solar-blind ultraviolet photodetectors with a dichroic ratio close to 4.26 are demonstrated. The strong anisotropy of 2D grown BPTTE single crystals in molecular vibration and optical absorption is characterized by various techniques. Under voltage modulation, stable and efficient detection of polarized light is demonstrated, attributed to the intrinsic anisotropy of transition dipole moment in the bc crystal plane, rather than other factors. Finally, high-contrast polarimetric imaging and anti-interference imaging are successfully demonstrated based on BPTTE single crystal photodetectors, highlighting the potential of organic semiconductors for polarization-sensitive solar-blind ultraviolet photodetectors.

Microscopic mechanism of plastic heterogeneous deformation of columnar-grained polycrystalline ice

Studying ice deformation starting from crystal scale can provide insight into the microscopic mechanisms of ice deformation. In this paper, a crystal plasticity constitutive model of ice considering the dislocation slip mechanism and grain-size dependence was implemented within the finite element framework, and a modeling approach starting from ice single crystals based on the crystal plasticity finite element method (CPFEM) was proposed to study the elastic-viscoplastic deformation of polycrystalline ice samples. Finite element models of ice single crystals and columnar-grained polycrystalline ice characterizing the microstructure were established successively, and the proposed method was validated by comparing the uniaxial compression prediction results of ice single crystals and columnar-grained polycrystalline ice with the existing experimental data. Finally, the microscopic mechanism of plastic heterogeneous deformation of columnar-grained polycrystalline ice under constant strain rate compression conditions was analyzed. The results indicate that the modeling approach from ice single crystals to polycrystalline ice samples based on CPFEM can reasonably describe the mechanical properties of ice deformation. The deformation of columnar-grained polycrystalline ice globally exhibits apparent slip bands around 30° - 60° relative to the loading direction and locally presents intragranular localization bands parallel or perpendicular to the C-axis. This plastic heterogeneous deformation phenomenon is related to the elastic-viscoplastic anisotropy of ice single crystals and the interaction of neighboring grains at grain boundaries (GBs).

Physics-driven neural networks for nonlinear micromechanics

Micromechanical problems seek full-field solutions in response to external and/or internal thermal-mechanical loads, which have been increasingly encountered in material property and process assessments. Here we present a machine learning based strategy of solving micromechanics at finite strains. Deep neural networks (DNNs) are trained by employing the elastic energy together with other physical constraints to formulate the loss function. In particular, the crossover of material points, which can be a common issue for similar DNNs when applied to compression-dominant problems, is shown to be effectively addressed by including a kinematic penalty term that forces the DNN to maintain structural integrity and stability and avoid unphysical behavior. The generality and accuracy of the proposed physics-driven neural networks (PDNNs) are demonstrated through various micromechanical problems, including single crystal anisotropy elasticity, Eshelby's inclusion problem, buckling, and elastic homogenization of polycrystals. The effect of the choice of optimizers and hyperparameters on the PDNN training are discussed and the computational efficiency is also analyzed. It is shown that this novel PDNN framework can completely remove the need of labeled training data and exhibit improved performances as compared to the conventional physics-informed neural networks (PINNs) in terms of avoiding unphysical solutions and attaining higher computational efficiency. PDNNs can thus serve as a promising tool to solve some critical challenges associated with a wide range of nonlinear micromechanical problems.

Magnetic anisotropy, magnetization reversal and switching in Ni4Nb2O9 single crystals

Ni4Nb2O9 is an insulating compensated ferrimagnet with TN = 77 K and T comp= 33 K. We report here the study of the magnetic anisotropy using millimeter-size crystals grown in an image furnace. The magnetization measurements, vs temperature, performed with H aligned along the three main crystallographic axes, show similar Curie–Weiss temperatures (Θp ≈ 190 K) and rather similar effective paramagnetic moments (from 3.5 μ B to 3.6 μ B). This suggests that the strongest magnetic interaction is the antiferromagnetic one, coupling the ferromagnetic distorted honeycomb layers and zigzag ribbons via face sharing NiO6 octahedra. This strong antiferromagnetic coupling is supported by DFT calculations that do not evidence any inter site ferromagnetic interaction, leading to total compensation between magnetic moments of both Ni2+ sites. Measurements vs magnetic field below TN reveal an anisotropic behaviour, with square magnetization loops for H in the ab plane, whereas linear M(H) curves without hysteresis are observed for H‖c. This anisotropy between ab plane and c axis occurs also in the magnetization reversal (MR), which is observed in the ab plane only. Starting from M(H) virgin curves collected just below T comp= 33 K with H‖a or H‖b, the memory-like effect was tested through magnetization switching induced by H or T alternating changes. Below T comp, smaller H is needed to switch M symmetrically for H along b than along a, and, for T switching (2 K interval, constant H), a larger M change is obtained along a than along b. The comparison with ferrimagnetic oxides which exhibit MR, like spinels or rare earth orthoferrites, shows that Ni4Nb2O9 is unique since only one magnetic cation over two sites in octahedral coordination is at play, thus providing a unique platform to study M switching but also a challenge for theoretical interpretation.

Signature of phonon anharmonicity in highly in-plane anisotropic ternary HfGeTe4 single crystals

Signature of phonon anharmonicity in highly in-plane anisotropic ternary HfGeTe4 single crystals

Scanning the time and space evolution of anisotropy in single molecular crystals based on 3-(furan-2-yl)-2-(4-{[(2-hydroxy-5-methylphenyl)methylidene]amino}phenyl)prop-2-enenitrile

The 3-(furan-2-yl)-2-(4-{[(2-hydroxy-5-methylphenyl)methylidene]amino}phenyl)prop-2-enenitrile (LAXYUI)), acting as a model, is a type of outstanding organic fluorophore. Using a transient absorption approach, the excitation relaxation behavior of LAXYUI in the monomolecular system and ordered crystals was identified. The polarization optical properties of LAXYUI crystals, including anisotropy of absorption and photoluminescence, polarization PL decay at different temperatures, and the evolution of optical anisotropy as a function of propagation length, are thoroughly investigated using a series of home-built polarization optical techniques. Our results can help people better understand the time- and space-dependent optical anisotropy that occurs in organic crystals, which will be important in the future to improve or broaden the performance of polarization optical systems based on organic crystals.

Study on the Internal Stress and Thermal Anisotropy in Magnesium Alloys Using a Thermal Elastic Viscoplastic Self-Consistent Model.

A thermal elastic viscoplastic self-consistent model is utilized to examine the thermal stress induced by the thermal anisotropy of single crystals during heat treatments. This model considers temperature-dependent elastic constants and critical resolved shear stress associated with thermal dilation. Simulation results demonstrate that under cooling, the elastic lattice strain increases significantly when constrained compared to unconstrained cooling. The deformation mechanism observed under cooling with constraint resembles tension along the constrained direction at room temperature. Polycrystals offer more deformation mechanisms to accommodate thermal anisotropy compared to single crystals, resulting in lower applied stress at the constrained boundary. Among the various observed textures, the maximum amplitude of residual lattice strain follows the following order: rolled > extruded > random. Lower thermal anisotropy in the entire polycrystal structure leads to reduced internal stress. For a single crystal within aggregates, the {00.2} plane experiences tensile lattice strain, while the {10.0} and {11.0} planes undergo compressive lattice strain due to the greater contraction of single crystals along the <c> direction compared to the <a> direction during cooling.

Estimation of the elastic and piezoelectric tensors of sapphire and lithium niobate from Brillouin light backscattering measurements of a single crystal sample

Brillouin light scattering is a versatile measurement technique of the dispersion of bulk acoustic phonons in amorphous and crystalline solids. It allows contactless and non-destructive characterization of the relevant material tensors of optically transparent materials, provided that the optical refraction indices, and in addition the dielectric tensor for piezoelectric materials, are known beforehand. The complete estimation of the anisotropic material tensors is often performed using many different crystal orientations, and hence different samples of the same crystal. In this paper, we consider the problem of measuring those tensors using a single sample of an anisotropic single crystal with trigonal symmetry. A complete measurement requires sufficient experimental diversity in the phonon wavevectors when sampling the phonon velocity surfaces, hence the consideration of a large number of directions away from crystallographic axes. We estimate all six independent elastic constants of non-piezoelectric sapphire (3¯ m point group) with a single X-cut wafer and the six independent elastic constants together with the four independent piezoelectric constants of piezoelectric lithium niobate (3 m point group) with a single Y-cut wafer. The estimated tensors are in close agreement with those reported in the literature based on resonant ultrasonic techniques.

Strong magnetic anisotropy in PrRu2Ga8 and PrCo2Al8 single crystals

Single crystals of LnRu2Ga8 and LnCo2Al8 (Ln = La and Pr) were grown using a Ga/Al self-flux method. An orthorhombic CaCo2Al8-type structure with space group Pbam (No.55) of them was identified by x-ray diffraction. LaRu2Ga8 and LaCo2Al8 are Pauli paramagnetic down to 2 K, while PrRu2Ga8 and PrCo2Al8 show antiferromagnetic (AFM) order at 2.5 and 5 K, respectively. Strong magnetic anisotropy in PrRu2Ga8 and PrCo2Al8 single crystals was found by an anisotropic magnetic measurement. The field-induced FM state was observed in both PrRu2Ga8 and PrCo2Al8 for H||c. However, in the case of H⊥c, the AFM state is robust. The strong magnetic anisotropy in PrRu2Ga8 FM and PrCo2Al8 is due to their anisotropic magnetic interactions that FM interactions are dominant in the case of H||c while AFM interactions for H⊥c.

Influence of material anisotropy on void coalescence by necking for face-centered cubic single crystals

This paper presents the coalescence of microvoids embedded in an anisotropic copper single crystal using a micromechanics approach. Crystal plasticity framework was used to account for the anisotropy arising from the orientation and slip. A full 3D representative volume element (RVE) with void was considered to circumvent spurious loading. The constant load path parameters were enforced on the RVE using multi-point constraints. Various loading conditions that lead to necking were studied. The key finding from the present study is the interplay of the load path parameters (triaxiality and Lode parameter), material anisotropy, and initial void volume fraction on the void coalescence. It is noticed that at high triaxiality, the ductile failure mechanism is dominated by the necking mechanism, and at medium to low triaxiality, the ductile failure mode is a combination of shearing and necking mechanisms. It was observed that non-homogenous crystallographic slip manifests the material anisotropic effects in void cell RVE across various crystallographic orientations. The crystal orientation [110] exhibited higher shearing than the orientation [100] and [111]. Further, material anisotropy significantly affected void morphology but not the void coalescence strains at high triaxial values.

Face Turning of Single Crystal (111)Ge: Cutting Mechanics and Surface/Subsurface Characteristics

Abstract Single crystal Ge is a semiconductor that has broad applications, especially in manipulation of infrared light. Diamond machining enables the efficient production of surfaces with tolerances required by the optical industry. During machining of anisotropic single crystals, the cutting direction with respect to the in-plane lattice orientation plays a fundamental role in the final quality of the surface and subsurface. In this study, on-axis face turning experiments were performed on an undoped (111)Ge wafer to investigate the effects of crystal anisotropy and feedrate on the surface and subsurface conditions. Atomic force microscopy and scanning white light interferometry were used to characterize the presence of brittle fracture on the machined surfaces and to evaluate the resultant surface roughness. Raman spectroscopy was performed to evaluate the residual stresses and lattice disorder induced by the tool during machining. Nanoindentation with Berkovich and cube corner indenter tips was performed to evaluate elastic modulus, hardness, and fracture toughness of the machined surfaces and to study their variations with feedrate and cutting direction. Post-indentation studies of selected indentations were also performed to characterize the corresponding quasi-plasticity mechanisms. It was found that an increase of feedrate produced a rotation of the resultant force imparted by the tool indicating a shift from indentation-dominant to cutting-dominant behavior. Fracture increased with the feedrate and showed a higher propensity when the cutting direction belonged to the &amp;lt;112¯&amp;gt; family.

Predictive modelling for enhanced scratching of brittle ceramics with magneto-plasticity

The micro-cutting of brittle ceramics starts with elastic-plastic deformation followed by severe brittle fracture, which significantly restricts the machinability and application of these materials. Magnetic field-enhanced plasticity (i.e., magneto-plasticity) has shown a positive potential to augment the ductile-brittle transition and machinability but the material removal mechanism of brittle ceramics with magneto-plasticity and its dependence on strain rate remains unclear. In this study, a deformation energy-based model was developed to bring light to the deformation mechanisms affecting the ductile-brittle transition under magneto-plasticity and the corresponding dependency on strain rate. MD simulations assisted the analytical modeling by deriving the orientation-dependent dislocation movement and stress distribution that contribute to single-crystal anisotropy in the surface formation and ductile-brittle transitions during scratching. The proposed model accurately predicted the improvement in ductile-brittle transition under various magnetic field intensities and orientations as confirmed in the experimental magnetic field-enhanced micro-scratching tests on single-crystal calcium fluoride (CaF2). Scratched surface morphologies, acoustic emission (AE) signals, and critical loading conditions demonstrated the improvements in ductile-brittle transition with magneto-plasticity. Additionally, the progression of lateral slip trace and tangential surface pile-up under various magnetic field conditions confirm the direct influence of magneto-plasticity on dislocation mobility and the change in surface formation mechanism of brittle ceramics. This study not only deepens the understanding of the manifestation of magneto-plasticity during the deformation of brittle ceramics, but also opens new avenues for the integration of magnetic field-assisted technology in manufacturing processes.

Atomic structure and large magnetic anisotropy in air-sensitive layered ferromagnetic VI3.

We report the air-sensitivity, atomic structure, and magnetic anisotropy of VI3 single crystals. We find that VI3 nanocrystals exhibit a large MR/MS ratio of around 0.75 and a uniaxial anisotropic constant of an order of 105 erg cc-1 below the Curie temperature. Furthermore, density functional theory calculations reveal that both the monolayer and bulk VI3 are ferromagnetic insulators, and the magnetic moment of the system arises mainly from the d orbital of the V atom. These findings open a feasible avenue to fabricating TEM specimens of air-sensitive layered materials, providing an in-depth comprehensive understanding of a layered ferromagnetic VI3.

Anisotropy of the friction forces of rest and sliding frictionof single crystals of ice

The properties of the ice of natural reservoirs are due to its crystalline structure. The dependence of the mechanical and strength characteristics of single ice crystals on the angle between the direction of application of force to a single crystal and the direction of its C-axis is known. Basically, this dependence is due to the possibility of the base plates of a single crystal of ice sliding relative to each other. In physics, in addition to the mechanical and strength characteristics of a solid, the properties of the surfaces of a solid state, manifested in the process of friction, are of great importance. The friction of two single crystals of ice relative to each other has been investigated in experiments on rest friction and sliding friction. From an artificially grown plate of a single ice crystal, samples of single crystals of ice with a certain orientation of the C-axis were made for the experiment. The measurement of the rest friction coefficient of a single ice crystal was realized according to the classical scheme of recording the minimum angle at which the single crystal began to roll down an inclined surface. The dependence of the rest friction coefficient of ice single crystals on the mutual orientation of the C-axes of single crystals relative to each other was discovered. The coefficients of rest friction in the case that the C-axes of single ice crystals are parallel to each other significantly exceed those obtained when the C-axes of single ice crystals are perpendicular to each other. Measurements have shown that the anisotropy of sliding friction increases with increasing initial velocity of the single crystal on the surface of the second single crystal. This is explained by the fact that with the growth of the path traveled by the M2 crystal, the time of interaction of single crystals of ice also increases, during which the properties of the anisotropy of single crystals of ice relative to sliding friction are manifested. Studies have shown the presence of anisotropy of the friction forces of rest and friction of sliding of a pair of single crystals of ice.

Understand anisotropy dependence of damage evolution and material removal during nanoscratch of MgF2 single crystals

To understand the anisotropy dependence of the damage evolution and material removal during the machining process of MgF2 single crystals, nanoscratch tests of MgF2 single crystals with different crystal planes and directions were systematically performed, and surface morphologies of the scratched grooves under different conditions were analyzed. The experimental results indicated that anisotropy considerably affected the damage evolution in the machining process of MgF2 single crystals. A stress field model induced by the scratch was developed by considering the anisotropy, which indicated that during the loading process, median cracks induced by the tensile stress initiated and propagated at the front of the indenter. Lateral cracks induced by tensile stress initiated and propagated on the subsurface during the unloading process. In addition, surface radial cracks induced by the tensile stress were easily generated during the unloading process. The stress change led to the deflection of the propagation direction of lateral cracks. Therefore, the lateral cracks propagated to the workpiece surface, resulting in brittle removal in the form of chunk chips. The plastic deformation parameter indicated that the more the slip systems were activated, the more easily the plastic deformation occurred. The cleavage fracture parameter indicated that the cracks propagated along the activated cleavage planes, and the brittle chunk removal was owing to the subsurface cleavage cracks propagating to the crystal surface. Under the same processing parameters, the scratch of the (001) crystal plane along the [100] crystal-orientation was found to be the most conducive to achieving plastic machining of MgF2 single crystals. The theoretical results agreed well with the experimental results, which will not only enhance the understanding of the anisotropy dependence of the damage evolution and removal process during the machining of MgF2 crystals, but also provide a theoretical foundation for achieving the high-efficiency and low-damage processing of anisotropic single crystals.

Rotating magnetocaloric effect in highly anisotropic TbIII and DyIII single molecular magnets

The magnetocaloric effect (MCE) was investigated in highly anisotropic single crystals of two single molecule magnets (SMMs): [LnIII(ZnIIL)2]CF3SO3, where Ln = Tb, Dy and L = tripodal hexadentate Schiff base ligand. The structure of these paramagnetic compounds consists of identically oriented linear trinuclear clusters in a trigonal system with an easy direction c∥Zn–Ln–Zn array and a hard plane ab⊥Zn–Ln–Zn array. The magnitude of MCE measured for c∥H was significantly greater than MCE for ab∥H at a wide temperature range regardless of the studied SMM. Therefore, the rotating magnetocaloric effect (RMCE) was evaluated. The maxima of the magnetic entropy change for RMCE were obtained at 2.0 K and moderate fields: 3.9 J K−1 kg−1 at µ0H = 1.3 T for Ln = Tb and 3.3 J K−1 kg−1 at µ0H = 1.1 T for Ln = Dy. The relative efficiency of RMCE compared to the MCE measured in c∥H was as high as 99% at low magnetic fields.

In-plane optical anisotropy of two-dimensional VOCl single crystal with weak interlayer interaction

Transition-metal oxyhalides MOX (M = Fe, Cr, V; O = oxygen, X = F, Cl, Br, I), an emerging type of two-dimensional (2D) van der Waals materials, have been both theoretically and experimentally demonstrated to possess unique electronic and magnetic properties. However, the intrinsic in-plane anisotropic properties of 2D VOCl still lacks in-depth research, especially optical anisotropy. Herein, a systematic Raman spectroscopic study is performed on VOCl single-crystal with different incident laser polarization at various temperatures. The polarized-dependent Raman scattering spectra reveal that the A g mode of VOCl show a 2-lobed shape in parallel polarization configuration while a 4-lobed shape in vertical configuration. In addition, the temperature-dependent and thickness-dependent Raman scattering spectra confirm a relatively weak van der Waals interaction between each layers among VOCl single crystal. These findings might provide better understanding on the in-plane anisotropic phenomenon in VOCl layers, thus will accelate further application of 2D single crystals for nanoscale angle-dependent optoelectronics.