Crystal Orientation Dependence Research Articles

Establishing the temperature and orientation dependence of the threshold displacement energy in ThO2 via molecular dynamics simulations

ThO2 is a promising fuel for next-generation nuclear reactors. As a critical quantity measuring its radiation tolerance, the dependence of the threshold displacement energy on temperature and crystal orientation in ThO2 is unclear and established using comprehensive molecular dynamics simulations in this work. For both Th and O primary knock-on atoms (PKAs), the thresholds, denoted as EdTh and EdO, respectively, are calculated using two different interatomic potentials. Similar temperature and orientation dependence are observed, albeit with some quantitative differences. While on average over all orientations, higher energy is required for Th PKAs than O PKAs to displace atoms, the polar-averaged EdTh is significantly lower than that for EdO. Further, EdTh and EdO show different crystal orientation dependence and temperature dependence. Notably, the cubic symmetry in the fluorite structure is followed by EdTh, but does not hold for EdO because of the existence of two sublattices. The much higher average EdO than EdTh and their different temperature dependence are interpreted by the distinct recombination rates of Th and O Frenkel pairs in thermal spikes, resulting from the substantially lower migration barriers of O vacancies and interstitials. The recombination of O vacancies and interstitials, both of which are charged, is further enhanced by the Coulomb interaction at small Frenkel pair separations. The new findings are discussed for their generality in fluorite-structured oxides by comparing the results in ThO2 and UO2.

Strain localisation and grain boundary-mediated deformation in pure titanium at low and high temperatures: In-situ optical microscopy and digital image correlation

Heterogeneous deformation occurs between grains in polycrystalline α-titanium. Understanding the role of temperature in local deformation is essential for its applications in extreme environments such as cryo- or high-temperature, but the underlying mechanism still remains elusive. Here we report a study of grain-scale deformation behaviour in CP-Ti plate at the temperature range from −60 °C to 280 °C. The full-field strain measurements were conducted on the tensile samples loaded parallel (RD) and transverse (TD) to the rolling direction, using in-situ optical microscopy and digital image correlation (OM-DIC). The DIC local strain maps were coupled with scanning electron microscopy and electron backscatter diffraction analysis. The grain boundary sliding and slip transfer occurred depending on the geometric relationship (i.e., deformation compatibility) between adjacent grains at 20 °C, and a micro-crack was observed at the {101‾1} twist boundary. The strain was recovered in the grains in the RD sample favourable for slip, whilst more accumulated in the grains in TD sample unfavourable for slip at 280 °C. The plasticity of grains differs with decreasing temperature to −40 °C, resulting in the presence of soft/hard grain pairs. The strong strain localisation between the soft and hard grains led to the cracking along the GBs (RD) and cross the grains (TD). Interestingly, the cracking was significantly reduced with decreasing temperature to −60 °C due probably to the temperature dependence of the plasticity of grains. The strain-hardening behaviour of α-Ti polycrystals was significantly affected by the temperature and crystal orientation dependence of grain-scale deformation mechanism.

Preparation, Deformation Behavior and Irradiation Damage of Refractory Metal Single Crystals for Nuclear Applications: A Review.

Refractory metal single crystals have been applied in key high-temperature structural components of advanced nuclear reactor power systems, due to their excellent high-temperature properties and outstanding compatibility with nuclear fuels. Although electron beam floating zone melting and plasma arc melting techniques can prepare large-size oriented refractory metals and their alloy single crystals, both have difficulty producing perfect defect-free single crystals because of the high-temperature gradient. The mechanical properties of refractory metal single crystals under different loads all exhibit strong temperature and crystal orientation dependence. Slip and twinning are the two basic deformation mechanisms of refractory metal single crystals, in which low temperatures or high strain rates are more likely to induce twinning. Recrystallization is always induced by the combined action of deformation and annealing, exhibiting a strong crystal orientation dependence. The irradiation hardening and neutron embrittlement appear after exposure to irradiation damage and degrade the material properties, attributed to vacancies, dislocation loops, precipitates, and other irradiation defects, hindering dislocation motion. This paper reviews the research progress of refractory metal single crystals from three aspects, preparation technology, deformation behavior, and irradiation damage, and highlights key directions for future research. Finally, future research directions are prospected to provide a reference for the design and development of refractory metal single crystals for nuclear applications.

Impact of helium and hydrogen plasma exposure on surface damage and erosion of tungsten

The impact of helium plasma exposure on the tungsten surface damage structure development and erosion has been investigated by comparing the impact of hydrogen plasma exposure. Crystal orientation dependence of the undulating surface structure formation and erosion rate is observed on the plasma-exposed tungsten surface independently from the plasma species. The top surface of the plasma exposed tungsten has a tendency to {100} plane independently from the initial surface orientation. Although hydrogen and/or helium cause no erosion in tungsten under incident ion energy exposure conditions below the sputtering threshold, inevitable minute impurities, like oxygen, play an essential role in erosion, and significant erosion can be observed even at 30 eV.

A multiscale FEM-MD coupling method for investigation into atomistic-scale deformation mechanisms of nanocrystalline metals under continuum-scale deformation

This study aims to develop a multiscale bridging method for investigating nanocrystalline metals based on macro-scale deformation. For this purpose, we propose a hierarchical multiscale computational method that can focus on some of the elements in a finite element model for scale bridging to atomistic-scale models. This method assumes that atomistic-scale nanocrystalline models are related to the integration points in a finite element and deform based on the macro-scale deformation. Nanocrystalline aluminum was chosen for the validation of the multiscale method. The finite element method (FEM) and the molecular dynamics (MD) method were used for continuum-scale and atomistic-scale simulations, respectively. We utilized the notion of the CauchyBorn rule (CBR) for communicating deformation information from the continuum scale to the atomistic scale. We studied three different cases with two nanocrystalline models and two loading cases to compare differences resulting from crystal structures and loading. Based on the crystal structure change during relaxation, nonequilibrium grain boundaries (NEGBs) were shown to play a role as deformation mechanisms in the plastic regime and induce the onset and migration of crystal defects, including deformation twins, as reported in the experiment. Furthermore, the crystal orientation dependence of the onset of crystal defects was confirmed by the comparison of the results from the two different nanocrystalline models. The qualitative agreement of the results with experimental observations is also confirmed. The proposed ‘FEM-MD’ method can bridge a large-scale gap, for example, from a nano-scale to a continuum-scale such that an MD model can be coupled to a millimeter or centimeter scale compared to other embedding methods. The present method is ideal for investigating the dislocation behavior of nanocrystalline materials, which contain multi-grained nanostructure at finite temperature, undergoing various loading scenarios at the macro-scale.

Fabrication of Textured Ti<sub>2</sub>AlC-MAX Phase Ceramic by Slip Casting under Strong Magnetic Field and Spark Plasma Sintering, and Its Crystal Orientation Dependence of Room Temperature Deformation Behavior

Fabrication of Textured Ti<sub>2</sub>AlC-MAX Phase Ceramic by Slip Casting under Strong Magnetic Field and Spark Plasma Sintering, and Its Crystal Orientation Dependence of Room Temperature Deformation Behavior

Crystal orientation dependence of photoluminescence of CuCl grown on Si (001) and Si (111)

Crystal orientation dependence of photoluminescence of CuCl grown on Si (001) and Si (111)

Evaluation of Strain-Relaxation of Carbon-Doped Silicon Nanowires and Its Crystal Orientation Dependence Using X-Ray Diffraction Reciprocal Space Mapping

Evaluation of Strain-Relaxation of Carbon-Doped Silicon Nanowires and Its Crystal Orientation Dependence Using X-Ray Diffraction Reciprocal Space Mapping

High-Throughput Investigation of Multiscale Deformation Mechanism in Additively Manufactured Ni Superalloy

In this paper, Inconel 718 (IN718) superalloy was processed by laser powder-bed fusion additive manufacturing (L-PBFAM), followed by heat treatment. High-resolution nanoindentation was used to investigate the complex deformation mechanisms that occurred at various length scales in both conditions. The nanoindentation elastoplastic maps show a strong crystal orientation dependency of modulus and hardness, which is attributed to the high mechanical anisotropy of IN718. The hardness map effectively resolves complex microscale strength variation imparted due to the hierarchical heat distribution associated with the thermal cycles of L-PBFAM. The disproportionately high hardening effect of Nb, Mo-rich chemical segregations and Laves phases in dendritic structures is also observed. The heat treatment resulted in a 67% increase in yield strength (from 731 MPa in the L-PBFAM condition to 1217 MPa in the heat-treated condition) due to the activation of multiple precipitation-strengthening mechanisms. The nanoindentation mapping of a heat-treated sample delineates the orientation-dependent hardness distribution, which apart from high mechanical anisotropy of the alloy, is also contributed to by a high degree of coherency strengthening of the D022 γ″-precipitates oriented parallel to the <001> crystal plane of the γ-matrix. The mean hardness of the sample increased from 13.3 GPa to 14.8 GPa after heat treatment. Evidence of extensive deformation of twin networks and dislocation cells was revealed by transmission electron microscopy of the deformed region under the nanoindentation tip.

Coherent Phonon-Induced Gigahertz Optical Birefringence and Its Manipulation in SrTiO3.

Birefringence, which modulates the polarization of electromagnetic wave, has been commercially developed and widely used in modern photonics. Fostered by high-frequency signal processing and communications, feasible birefringence technologies operating in gigahertz (GHz) range are highly desired. Here, a coherent phonon-induced GHz optical birefringence and its manipulation in SrTiO3 (STO) crystals are demonsrated. With ultrafast laser pumping, the coherent acoustic phonons with low damping are created in the transducer/STO structures. A series of transducer layers are examined and the optimized one with relatively high photon-phonon conversion efficiency, i.e., semiconducting LaRhO3 film, is obtained. The most intriguing finding here is that, by virtue of high sensitivity to strain perturbation of STO, GHz optical birefringence can be induced by the coherent acoustic phonons and the birefringent amplitudes possess crystal orientation dependence. Optical manipulation of both coherent phonons and its induced GHz birefringence by double pump technique are also realized. These findings reveal an alternative mechanism of ultrafast optical birefringence control, and offer prospects for applications in high-frequency acoustic-optics devices.

Experimental and numerical study on strain path and crystal orientation dependences of phase transformation behaviors of QP1180 steel

Phase transformation behaviors of a quenching & partitioning (QP) steel, QP1180, under different strain paths are investigated by both experiment and crystal plasticity simulation. Experiments of various strain paths including uniaxial tension (UT), uniaxial compression (UC), plane strain tension (PST) and equi-biaxial tension (EBT) are conducted, and the mechanical stability of retained austenite (RA) shows obvious strain path and orientation dependences. A crystal plasticity-phase transformation model with multiple-variant kinetics is proposed and used to predict the transformation behaviors. The single crystal analysis with 11 typical orientations is performed under various loading paths. Different variant types and volume fraction evolutions are observed in 11 orientations, which results in the overall path and orientation dependences of phase transformation in polycrystal. Eight major texture components of the RA grains show a similar strain path dependence and determine the transformation behaviors of the QP1180 steel. A numerical simulation comparison between single-variant and multiple-variant kinetics shows that both theories can capture strain path dependence and texture evolution in phase transformation of the current QP1180, but the multiple-variant kinetics can better reproduce the texture of newly formed martensite. The effects of non-proportional loadings on transformation and the related mechanism are also studied with the proposed model.

Electronically Nonadiabatic H Atom Scattering from Low Miller Index Surfaces of Silver.

The reactivity of a surface depends strongly on the surface structure. To study the influence of surface structure on H atom adsorption, we performed inelastic scattering experiments and complementary electronically nonadiabatic molecular dynamics (MD) simulations for H atoms colliding with the three low Miller index surface facets of silver. Experiment reveals very similar energy loss distributions for all three investigated facets. However, for the (100) facet a dependence on the surface orientation is observed that is absent for the other two facets. The nonadiabatic MD simulations manage to describe the experiments well. Despite the observed insignificant influence of the surface geometry on the energy loss distributions, our simulations predict that the capability of the H atoms to penetrate the surface critically depends on the surface structure. The observed crystal orientation dependence of the energy loss distributions in the experiment for Ag(100) cannot be explained with our simulations, and we provide a discussion for a better theoretical description of this system to stimulate future computational investigations.

Effect of wet‐jet milling on the c‐axis orientation of hydroxyapatite ceramics fabricated using a strong magnetic field

AbstractControlling the crystal orientation of hydroxyapatite, an inorganic material that is a major component of human hard tissues is important to fabricate better artificial bones and artificial tooth roots. To obtain highly oriented hydroxyapatite ceramics under a strong magnetic field, a good dispersion of the raw materials in the slurry must be obtained. This study investigates the effect of wet‐jet milling of a slurry on the orientation of hydroxyapatite ceramics fabricated using a strong magnetic field. Although the prolonged ball milling with ZrO2 balls of the raw powder fractures the primary particles of hydroxyapatite, wet‐jet milling is used to successfully pulverize agglomerated hydroxyapatite raw powder without changing the morphology of the primary particles. Evidently, ceramics with a highly oriented c‐axis of hydroxyapatite are obtained by molding the wet‐jet milled slurry in a strong rotating magnetic field. They exhibit anisotropy in fracture toughness, and the fracture toughness calculated from the crack length perpendicular to the rotating axis is higher than that calculated from the crack length parallel to it. Furthermore, these values are higher than those of randomly oriented hydroxyapatite ceramics, which may be owing to the crystal orientation dependence of the fracture toughness of the hydroxyapatite grains and grain boundaries.

High Harmonic Generation Driven by Counter-Rotating Bicircular Laser Fields from Polar Chemical Bonds in h-BN

High harmonic generation (HHG) driven by counter-rotating bicircular (CRB) pulses excitation has been observed from several solid targets, where circularly polarized harmonics are emitted. We study this process using time-dependent density functional theory (TDDFT) to calculate the crystal orientation dependence of the circularly polarized high harmonics from a monolayer h-BN. The resulted can be interpreted by the real space electron dynamics of electrons in polar chemical bonds. The yield of circularly polarized high harmonics (CHHs) can be optimized by controlling the direction of valence electron dynamics. Our findings pave the way for exploring the binding potential from spectrum and all-optically processing information.

Orientation dependence of even-order harmonics generation in biased bilayer graphene

We theoretically study the orientation dependence of even-order harmonics in biased bilayer graphene (BLG) by numerically solving the semiconductor Bloch equations in the velocity gauge. It is shown that the yield of even-order harmonics has a perfect periodicity of ${60}^{\ensuremath{\circ}}$ with the rotation of the crystal orientation. The even-harmonic intensity reaches the maximum when the laser field is polarized along with the nearest-neighbor atom. By investigating the crystal orientation dependence for the parallel and perpendicular components of even harmonics, it is found that the Berry curvature plays an insignificant role in the even-harmonic generation of biased BLG. Further analysis reveals that the asymmetric electron density between in-plane adjacent atoms caused by polarization is responsible for the generation of even harmonics in biased BLG. Additionally, we study the ellipticity dependence of even harmonics in biased BLG when the elliptical principal axis of the laser pulse is along with the in-plane nearest-neighbor atoms. As expected, the yield of even harmonics decreases with the ellipticity. Our findings give access to understanding deeply the mechanism of the orientation dependence of even harmonics in biased BLG.

Orientation dependent etching of polycrystalline diamond by hydrogen plasma

The p-type conducting hydrogen-terminated diamond obtained from hydrogen plasma has several applications in the field of power semiconductor devices. Therefore, clarifying the effects and role of hydrogen plasma on diamond surfaces is a critical aspect in the field of hydrogen-terminated diamond devices. In this study, we focused on diamond surface etching by hydrogen plasma and revealed the dependence of the diamond crystal orientation using scanning probe microscopy and electron backscatter diffraction. Crystal grains with a low etching rate were distributed around the {100} and {111} planes, and those with a high etching rate were distributed on the {110} plane. Furthermore, we found that the etching rate increased as the intermediate orientation grains tilted toward the {110} plane. Because the surface carbon atoms that compose the {110} plane have two of their three back bonds on the surface exposed to hydrogen plasma, we expected that the {110} orientation would have a high etching rate. Even for grains with intermediate orientations, the etching tendency corresponded to the location of the back bonds. Our research provides important systematic data on the crystal orientation in diamond electronics.

Crystal orientation dependence of metal–insulator transition for VO2 microwires fabricated on TiO2(110) substrates with step and terrace structures

Vanadium dioxide (VO2) thin films exhibit a metal–insulator transition (MIT) with sensitivity to the lattice strain. Substrates with step and terrace structures are an attractive platform for growing high-quality thin films. Thus, a prominent lattice strain effect could be derived using VO2 thin films on these substrates. In this study, VO2/TiO2 microwires were fabricated following the thin film growth to investigate the microwire-direction dependence of the MIT property. The in-plane crystal orientation dependence of the MIT property was enhanced for VO2/TiO2 microwires with step and terrace structures, which is promising for strain engineering in device applications.

Crystal Orientation Dependence of the Portevin–Le Chatelier Effect in Instrumented Indentation: A Case Study in Twinning-Induced Plasticity Steels

Instrumented indentation can be effectively used to investigate the Portevin–Le Chatelier (PLC) effect at small scales. It has been shown that the PLC effect in single crystals may depend on the crystal orientation, yet the underlying mechanism is unclear. Here, the orientation dependence of the PLC effect was systematically studied by conducting instrumented indentation tests in the [001]-, [101]- and [111]-oriented grains of a polycrystalline twinning-induced plasticity steel. It is found that the crystal orientation does not affect the PLC effect at relatively high indentation strain rates. In contrast, there is a strong orientation dependence at lower rates, with enhanced difficulty in the formation of serrations in the order of the [001], [111] and [101] orientations. This finding contradicts the previous proposals of the orientation effects, which are associated with the dislocation waiting time. On the basis of both the orientation and rate effects observed here, we proposed that the crystal orientation influences the occurrence of serrations in instrumented indentation by affecting the number of activated slip systems and, therefore, the probability of finding sufficient dislocation sources to accommodate the plastic avalanche.

Crystal orientation dependence of spin Hall angle in epitaxial Pt/FeNi systems

The development of spintronics applications using the spin–orbit torque from the spin Hall effect is expected to advance the low-power consumption of various information devices. However, the dependence of the spin Hall effect on the crystal symmetry of the material has not been studied in detail. In this work, we investigate the dependence of the spin Hall angle on crystal orientation for epitaxial Pt in Pt/FeNi systems by using multiple ferromagnetic resonance techniques. Our estimation of the spin Hall angle indicates that the efficiency of spin current generation via the spin Hall effect can vary depending on the direction in which the spin current flows out, while the direction of the charge current has less impact on this efficiency.

Effect of crystal orientation on fatigue crack initiation life in pure aluminum single crystals

The crystal orientation dependence of fatigue crack initiation life was studied on 99.99 % pure aluminum single crystals. The fatigue crack initiation life is not ruled only by Primary slip system, but also affected by the second, the third and the fourth slip systems. Fatigue damage formation is accelerated in the crystal orientation which has equally large Schmid factor both on Primary and Critical slip planes, but it is restrained as Schmid factor on Conjugate slip plane increases. X-ray micro-beam back reflection Laue observation showed that the subgrain structure was remarkably developed. The configuration of crystallographic fatigue damage depends on the crystal orientation. The secondary slip system plays an important role in it. The total misorientation observed at the fatigue crack initiation takes about 5.5 × 10-2 rad, not depending on the crystal orientation and the shear stress amplitude. This suggests that the fatigue crack is initiated when the dislocation density at the subgrain boundary reaches a certain value.