Prismatic Planes Research Articles

Planar defects in α-Ga2O3 thin films produced by HVPE

The defect structure of α-phase gallium oxide thin films was investigated using transmission electron microscopy (TEM). Epitaxial Ga2O3 films were grown via halide vapor-phase epitaxy on c-plane sapphire substrates. TEM analysis revealed a high density of extended planar defects within the films, primarily located along prismatic planes of {112¯0} type. Displacement vectors were determined using the invisibility criterion for stacking faults. The study encompassed both planar and cross-sectional views of the films. It is hypothesized that these defects form due to the motion of edge partial dislocations with the 13⟨11¯00⟩ Burgers vector. Various mechanisms of their formation have been explored.

Energetics and stability of screw dislocations in magnesium: A first-principles study

Magnesium (Mg), as the lightest structural metal, suffers from poor ductility and formability. Although in literature, the cross-slips in Mg have been identified as the key factors that are decisive for ductility, there are still many controversies and ambiguities in these cross-slip processes, e.g., energy, stability, etc. In this study, we propose a force correction method to eliminate the surface effects, which thus effectively enhances the efficiency/accuracy of the first-principles DFT calculations of screw dislocations. Based on this method, both a and c+a screw dislocations are calculated with convergence carefully examined. Our results show that a dislocation on the Basal plane has a lower energy than on the Prism plane, by 62 meV/b. The Prism core is energetically unstable since no barrier appears in the Prism-Basal cross-slip path. However, the energy landscape around the Prism core is extremely flat, where the tiny configurational forces could be below the convergence criterion, making the Prism core obtainable in the atomistic simulations. Then for the c+a dislocation, the ground state is revealed to dissociate on the Pyramidal II plane. Since multiple metastable c+a core structures are found on the Pyramidal planes, the energy difference during the c+a cross-slip is thus within a range of 8–19 meV/b. All these findings here provide the basis and guidance for understanding and enhancing the ductility in Mg and Mg alloys.

Continuous precipitation behavior of Mg17Al12 phase during multiple steps of twinning-aging procedure in Mg-Al alloys

Twinning was regarded as an effective way to modify the precipitation in Mg alloys. Two procedures were designed to illustrate twinning-aging consequence on Mg17Al12 phase evolution. The effect of multiple steps of twinning-precipitation on the morphology and density of the continuous precipitates inside the twins and the matrix were comprehensively analyzed through phase-orientation related observation. The results show the shape of precipitates in twins was similar to that in the matrix as the CPs mostly lay on the basal planes with an asymmetric-lozenges shape. The CPs were remarkably refined and its density became much higher inside twins. Moreover, the aspect ratio of the CP plates in twins was around 45% of that in the matrix, which means the CPs inside twins became more globular. The precipitation behavior of Mg17Al12 phase inside double extension twins was different from the primary twin or the matrix. The Mg17Al12 phases lay on both basal and prismatic planes can be observed in double twins of two-step precipitation. The growth behavior of pre-precipitates in twins is different from those in the matrix. The growth rate along its thickness became higher than that in the matrix, which is probably related to dislocation-assisted diffusion at the basal plane.

A Molecular Dynamics Analysis of the Thickness and Adhesion Characteristics of the Quasi-Liquid Layer at the Asphalt-Ice Interface.

The quasi-liquid layer (QLL), a microstructure located between ice and an adhering substrate, is critical in generating capillary pressure, which in turn influences ice adhesion behavior. This study employed molecular dynamics (MD) methods to obtain QLL thickness and utilized these measurements to estimate the adhesive strength between ice and asphalt. The research involved constructing an ice-QLL-asphalt MD model, encompassing four asphalt types and five temperature ranges from 250 K to 270 K. The QLL thickness was determined for various asphalts and temperatures using the tetrahedral order parameter gradient. Additionally, capillary pressure was calculated based on the QLL thickness and other geometric parameters obtained from the MD analysis. These findings were then compared with ice adhesion strength data acquired from pull-off tests. The results indicate that QLL thickness varies with different asphalt types and increases with temperature. At a constant temperature, the QLL thickness decreases in the order of the basal plane, primary prism plane, and secondary prism plane. Furthermore, the adhesion strength of the QLL diminishes as the temperature rises, attributed to the disruption of hydrogen bonds at lower temperatures. The greater the polarity of the asphalt's interface molecules, the stronger the adhesion strength and binding free energy. The MD simulations of the asphalt-ice interface offer insights into the atomic-scale adhesive properties of this interface, contributing to the enhancement in QLL property prediction and calibration at larger scales.

Crack nucleation and dislocation activities in titanium alloys with the strong transverse texture: Insights for enhancing dwell fatigue resistance

Cold dwell sensitivity of near α titanium alloys has posed a significant challenge to the engineering safety within the aerospace industry. However, there exists inconsistency regarding the critical facet formation mechanism between basal and prismatic nucleation. A key revelation in this paper is that the controversy surrounding the facet nucleation plane primarily arises from variations in texture: when the loading direction is parallel to the c-axis or at a 45 ° angle, it leads to basal plane cracking, whereas loading direction perpendicular to the c-axis results in prismatic plane cracking. What's more, a large anisotropy is observed in the dwell fatigue performance in descending order: rolling direction (RD) > transverse direction (TD) >45° direction. To be specific, the dwell fatigue life of RD specimens was 2 times and 4.07 times longer than that of TD and 45 ° specimens, which exhibits excellent dwell fatigue resistance. Furthermore, the high prismatic dislocation density near the facet of RD specimen was confirmed through the Transmission Electron Microscope (TEM) and Transmission Kikuchi Diffraction (TKD) map, which is evidenced that prismatic slips lead to higher strain hardening during cyclic loading. The basal plane of facet grain is generally parallel to the facet line, indicating that the elliptical facet developed by the transgranular fracture through basal plane. Twinning activation in hard grain to accommodate deformation in Ti60 alloy is firstly observed under dwell fatigue tests, which is a product of high stress level in the hard grain. This paper unveils the significant impact of texture on dwell fatigue resistance, offering novel insights into enhancing dwell fatigue performance through texture control.

Oxidation Mechanism and Surface Characterization of Pyrolytic Graphite in Simulated Air for Pyrochemical Reprocessing Application

Accidental air ingress into the containment chamber of pyrochemical reprocessing of spent nuclear fuels can cause severe oxidation of graphite components. Pyrolytic graphite (PyG) coating on conventional high-density graphite (HDG) is proposed for protection against oxidation, molten salt, and molten metal corrosion. The aim of the present study is to evaluate the thermal stability and kinetic parameters and propose a suitable oxidation mechanism of PyG under simulated air using thermogravimetry analysis (TGA). In dynamic oxidation, a linear increase in the oxidation onset, peak, and burnout temperature is observed for PyG synthesized at increasing pyrolysis temperature from 1200 to 2400°C except for 1400°C. The isothermal oxidation study showed a similar trend i.e., a decrease in the linear oxidation rate constant value for any given isotherm with increasing pyrolysis. The Arrhenius plot showed two-stage oxidation regimes operating between 600 to 1000°C. In regime-I, between 600 to ∼800°C controlled by surface chemical reaction kinetics, the activation energy is measured between 100-200 kJ.mol−1. The graphite-oxygen reaction initiates and propagates preferentially along the prismatic planes at the sample edges, where the dangling bonds of stacked graphite layer structures are exposed. In regime-II operating above ∼800°C, the surface reaction rate is infinitely faster, and the diffusional mass transport becomes a rate-limiting factor. In this regime, the graphite-oxygen initiates by pore formation on defective reactive sites on the bulk surface and propagates by pore-widening and pore-coalescence mechanism. The observed differences in the oxidation rates for different pyrolysis PyG attributed to its intrinsic material properties are investigated. This work provides new insights into the oxidation mechanism of the PyG during accidental air ingress in a pyrochemical reprocessing plant.

Atomistically informed dislocation dynamics simulations: application to dislocation-loop interactions in zirconium

Neutron irradiation of zirconium alloys leads to the formation of high densities of small dislocation loops. Their interactions with gliding dislocations are responsible for hardening and early necking of the material. Multi-scale numerical simulations of the interactions between dislocations and loops are undertaken to predict the mechanical properties evolution of these materials due to irradiation. Molecular dynamics simulations are first performed to assess the critical ingredients needed for dislocation dynamics simulations. Appropriate models and associated coefficients are then introduced in dislocation dynamics simulations in order to reliably reproduce the dislocation line energy, the cross-slip process from basal to prismatic planes and the mobility of straight and jogged dislocations. Based on this parametrization, interactions between dislocations and loops are finally computed with the two numerical methods. Careful comparisons between the two types of simulations show qualitative and quantitative agreement, opening the path to investigations of irradiation effects at the grain scale through large scale dislocation dynamics simulations.

Observing growth and interfacial dynamics of nanocrystalline ice in thin amorphous ice films

Ice crystals at low temperatures exhibit structural polymorphs including hexagonal ice, cubic ice, or a hetero-crystalline mixture of the two phases. Despite the significant implications of structure-dependent roles of ice, mechanisms behind the growths of each polymorph have been difficult to access quantitatively. Using in-situ cryo-electron microscopy and computational ice-dynamics simulations, we directly observe crystalline ice growth in an amorphous ice film of nanoscale thickness, which exhibits three-dimensional ice nucleation and subsequent two-dimensional ice growth. We reveal that nanoscale ice crystals exhibit polymorph-dependent growth kinetics, while hetero-crystalline ice exhibits anisotropic growth, with accelerated growth occurring at the prismatic planes. Fast-growing facets are associated with low-density interfaces that possess higher surface energy, driving tetrahedral ordering of interfacial H2O molecules and accelerating ice growth. These findings, based on nanoscale observations, improve our understanding on early stages of ice formation and mechanistic roles of the ice interface.

Oxygen vacancy-induced plasmonic and polaronic excitations in rubidium- and potassium-doped hexagonal tungsten bronzes

Potassium- and rubidium-doped hexagonal tungsten bronzes (K- and Rb-HTBs) were examined in comparison with cesium-doped hexagonal tungsten bronze (Cs-HTB) for their crystal structure and dielectric functions via spectrophotometry, spectroscopic ellipsometry, X-ray diffraction, and X-ray photoelectron spectroscopy. A substantial amount of oxygen vacancy (VO) was detected in all HTBs via chemical analysis, which increased depending on the dopant K → Rb → Cs. A minute (201) peak appeared for K- and Rb-HTBs, indicating symmetry transition from P63/mcm to P6322 due to the pseudo Jahn–Teller distortion; this suggests that K- and Rb-HTBs contain lesser conduction electrons than Cs-HTB. Double-peaked loss functions were decomposed using the Drude−Lorentz theory, and the associated plasma frequency, polaron energies, and W5+ fractions increased as the dopants changed, K → Rb → Cs, with increasing VO content. The origin of the decreased VO content in K- and Rb-HTBs was discussed in relation to water-related defects on prismatic planes.

Research on microstructure and mechanical properties of hot extruded Mg-Gd-Y-Sm-Zr alloy at room and high temperatures

In this study, the age hardening behavior of hot extruded Mg-8Gd-4Y-1Sm-0.5Zr (GWS841) alloy, mechanical properties and microstructural evolution of hot extruded GWS841 alloy during elevated temperature tensile processes were mainly investigated. The results show that the hot extrusion aged GWS841 alloy possess excellent mechanical properties at room and high temperature, which are 397 MPa, 386 MPa, 376 MPa, and 312 MPa at 25 °C, 200 °C, 250 °C, and 300 °C, respectively. After aging for 200 °C × 96 h, the hot extruded GWS841 alloy reaches its maximum hardness and room temperature tensile strength, and strengthening precipitation phase, being nano β′ phase, approximately 12 nm × 20 nm in size, precipitates simultaneously in three prismatic planes, (1‾21‾0)α、 (1‾1‾20)α and (21‾1‾0)α planes, which present precipitation strengthening behavior. During tensile testing at 200 °C, the strengthening precipitate remains β′ phase. During elevated tensile processes at 250 °C and 300 °C, strengthening precipitation phase undergoes significant transformation at grain boundary, β′ phase → β phase, and additionally, nano precipitation zone formed by rare earth atoms aggregation along grain boundary, which strengthen the grain boundary. During elevated tensile deformation at 300 °C, β′ phase completely disappeared and transformed into nano rare earth atom cluster region uniformly distributing inner grain.

Investigating the antifreeze effect of FFIBP protein faces on primary prismatic plane of ice through molecular dynamics simulation

Investigating the antifreeze effect of FFIBP protein faces on primary prismatic plane of ice through molecular dynamics simulation

Enhancing conductivity on p-type (Bi,Sb)2Te3 surface by introducing slip trace

Structural modification can effectively tune and optimize the thermoelectric properties of Bi2Te3-based alloys. Herein, we present an innovative and controllable approach to enhance the conductivity of the Bi2Te3-based alloy by introducing slip traces. The effect of slip traces on the conductivity of the Bi2Te3-based alloy was comprehensively investigated using advanced techniques such as Electron Backscatter Diffraction (EBSD) and conductive Atomic Force Microscopy (c-AFM). The results revealed that slip traces were readily formed in (Bi,Sb)2Te3 grains when a load was applied perpendicular to the prismatic plane. The Point Discharge Effect (PDE) was found to promote carriers moving towards the slip traces, leading to the accumulation of positively charged holes. As a result of this process, a higher current intensity was detected at the slip traces. This innovative strategy offers a convenient and effective way to fabricate high-conductivity thermoelectric materials and thus will inspire new technological possibilities for high-performance thermoelectric devices.

Retarding ice recrystallization by tamarind seed polysaccharide: Investigation in ice cream mixes and insights from molecular dynamics simulation

Food hydrocolloids, such as naturally sourced polysaccharide stabilizers, are becoming potential candidates for retarding ice recrystallization in frozen desserts. Tamarind seed polysaccharide (TSP) and its depolymerized fraction (DTSP) possessed ice recrystallization retardation (IRR) activity by a splat cooling assay. This study is a follow-up work aiming to investigate their application effects to retard ice recrystallization in ice cream mixes (ICMs) and delve into the ice-binding behavior at the ice/water interface. Native TSP worked for or against IRR in the ICMs depending on the concentration, while DTSP possessed an increased IRR effect under the addition of higher concentration (3.0%). Compared to TSP, adding DTSP caused less of an increase in the viscosity of the ICMs. Molecular dynamics was used to simulate ice growth in the presence of TSP (oligomer) and showed that the TSP possessed activity for retarding ice growth along the prismatic planes. During the propagation of ice crystals, the TSP was bound onto the ice and caused evident structural defects of ice layers. The findings of this study proved that TSP and DTSP have a promising application to retard ice recrystallization in ice cream and shed new light on understanding the mechanism of natural polysaccharides to retard ice growth.

Unveiling interface structure and polarity of wurtzite ZnO film epitaxially grown on a-plane sapphire substrate

Atomic-scale structure properties of the epitaxial growth of the wurtzite ZnO film prepared on an a-plane sapphire (α-Al2O3) substrate have been investigated by using aberration-corrected transmission electron microscopy. The crystallographic orientation relationship of (0001)[1¯1¯20]ZnO//(112¯0)[0001]α-Al2O3 has been determined between the ZnO film and the α-Al2O3 substrate. Two types of oxygen-terminated a-plane α-Al2O3 substrate surfaces have been characterized, which leads to the formation of different heterointerface structures and ZnO domains with opposite lattice polarity. The coalescence of opposite polarity domains results in the appearance of inversion domain boundaries (IDBs) on prismatic planes, and kinks occur on basal planes during the propagation of IDBs within the film. Additionally, the structure of stacking mismatch boundaries in the film with threefold coordinated Zn and O atoms has been resolved. We believe that these findings can be helpful to advance the understanding of the complex propagation of planar defects (e.g., IDBs and stacking faults) in wurtzite films and the interface structure and polarity of wurtzite films on the a-plane sapphire substrate.

Discrete dislocation dynamics simulations of [formula omitted]-type prismatic loops in zirconium

Neutron irradiation of zirconium alloys in light water nuclear reactors generates nano-scale defects in the form of vacancy and interstitial 〈a〉-type prismatic loops which lie in prismatic planes of the sample. The dynamics of idealised conservative square 〈a〉-type prismatic loops have been investigated for a range of loop lengths in the framework of linear isotropic elasticity. Three-dimensional dislocation dynamics (DD) simulations of a dislocation-loop interaction have been performed to investigate the dislocation-loop interaction mechanism. For this purpose, a mobility law developed for hexagonally close-packed materials has been implemented and described in detail. Analytical and numerical calculations have been performed to obtain expressions for the restoring force and angular stability of prismatic loops. These analyses have been used to inform a 2.5D discrete dislocation plasticity (DDP) model in order to emulate realistic prismatic loop physics and improve irradiation hardening simulations. From the 2.5D DDP prismatic loop analyses, it has been observed that the stable angle of smaller sized loops is less sensitive to external stresses compared to that of larger loops, which may have implications for the mechanisms of irradiation hardening. Furthermore, initial 2.5D single-slip simulations predict that prismatic loops cause significantly elevated flow stress that increases with increasing loop density in accord with experimental observations, and that the restraining effect of the out-of-plane loop segments (the restoring force) plays an important role in the strengthening caused by loops.

Strain-induced heterogeneous bimodal microstructure behavior of Mg-Gd-Y-Zn-Zr alloy containing LPSO phase during hot compression

In this study, the isothermal compression experiments of Mg-Gd-Y-Zn-Zr alloy at 450 °C and 0.01 s−1 under different strains were carried out to investigate the microstructure evolution and the formation mechanism of the heterogeneous microstructure. The microstructure of the sample with a strain of 1.4 exhibited significant heterogeneous microstructure characteristics with bimodal grain and bimodal texture distribution. The particle-stimulated nucleation (PSN) mechanism activated by intergranular block-shaped LPSO phases and the continuous dynamic recrystallization (CDRX) mechanism developed layer by layer towards grain interior promoted grain refinement and the formation of the bimodal grain distribution characteristics. Moreover, the stress-induced dynamic precipitation of Mg5RE particles became more significant with increasing strain, and the interaction of Mg5RE particles with dynamic recrystallization (DRX) grains further refined the DRX grains and exacerbated the degree of bimodal grain distribution. The grains with the <0001> axis between CD and TD and grains with the <0001> axis perpendicular to CD slipped along basal and prismatic planes, respectively, leading to the gradual deflection of the <0001> axis toward CD and the formation of a strong basal texture with <0001> axis parallel to CD during thermal compression. At the same time, the coarse deformed grains with strong basal texture were preserved during compression deformation due to small strain energy storage. Inhomogeneous plastic deformation, randomly oriented refined DRX-ed grains and dynamically precipitated Mg5RE particles together lead to the formation of the heterogeneous microstructure. This study provides theoretical guidance for preparing of high-performance Mg-RE alloys with heterogeneous microstructure.

Mechanistic investigation of highly bendable magnesium alloy sheet fabricated by short-process manufacturing

In this study, a commercial magnesium alloy AZ31 (Mg-3Al-1Zn-0.3Mn) sheet through a short manufacturing process was found to be ductile and highly formable in bending. Despite possessing a strong basal texture, the short-processed sheet without any annealing can be bent at a small radius, only 0.2 times its thickness in the 90° bending test. Additionally, it could withstand direct deformation by repeated folding-flattening. The in-situ microstructural characterization reveals that extension twin bands with strain localization appear in the bending area. During subsequent flattening, these twin bands underwent detwinning, reducing local strain concentrations and enabling further bending deformation. Such outstanding bend formability originated from the significant 〈a〉-type dislocation loops slipping on the prismatic crystal planes within dynamic-recrystallized grains. These grains underwent a uniform refinement to several microns in the short manufacturing process and exhibited low residual strain. The active prismatic dislocation slip within refined grains was due to its much lower relative activation stress to basal slip (CRSSprism/ CRSSbasal of only ∼1.6) owing to the effective grain boundary hardening. Furthermore, the prismatic dislocation activity was further enhanced when bypassing Al-Mn nano-particles during motion, leaving debris and loops that facilitated easy multiplication.

Pressure-driven structural transformation and lattice deformation in TiB reinforced titanium matrix composites: An in-situ synchrotron x-ray diffraction study

Understanding the mechanical responses of structural materials to extreme pressures has particular implication for not only their intrinsic solid properties, e.g., phase transformation, equation of state, lattice deformation, but also the mechanical engineering for extreme environments. In this work, the structural transformation of TiB reinforced Ti (Ti-TiBw) composite particles under hydrostatic, uniaxial and dynamic pressures has been comparatively investigated by diamond anvil cells combined with in-situ synchrotron x-ray diffraction. According to the variation of critical pressure of α → ω martensitic transformation, it is found that, under uniaxial pressure, the presence of nano-TiBw network could largely enhance structural resistance to deviatoric stress for the composites. A fit to the equation of state of Ti-TiBw composites under hydrostatic and uniaxial pressure yields the bulk modulus of ∼120 and ∼ 133 GPa, respectively, which increases 2.56% and 11.76% relative to pure Ti. Furthermore, the lattice deformation behaviors in Ti-TiBw composites and pure Ti during martensitic and its reversible transformation vary with pressure, deviatoric stress and compression rate. Due to the constraints by rigid TiBw network rather than “soft” grain boundary, the lattice deformation along the basal, prismatic, and pyramidal planes in Ti-TiBw system suggests an isotropic behavior, though the deviatoric strain of basal plane displays obvious dependence upon uniaxial pressure.

Unraveling the Atomic Shuffles of Twinning Nucleation in Hexagonal Close-Packed Rhenium Nanocrystals.

Reining in deformation twinning is crucial for the mechanical properties of hexagonal close-packed (HCP) metals and hinges on an explicit understanding of the twinning nucleation mechanism. Unfortunately, it is often suggested rather than conclusively demonstrated that twinning nucleation can be mediated by pure atomic shuffles. Herein, by utilizing in situ high-resolution transmission electron microscopy, we have dissected the atomic shuffling mechanism during the {101̅2} twinning nucleation in rhenium nanocrystals, which revealed the emergence of an intermediate body-centered tetragonal (BCT) structure. Specifically, the double-layered prismatic planes initially shuffle into single-layered {11̅0}BCT planes; subsequently, adjacent {22̅0}BCT planes shuffle in opposite directions to form the basal planes of the twin embryo. This shuffling mechanism is further corroborated by molecular dynamic simulations. The finding provides direct evidence of shuffle-dominated twinning nucleation with atomic details that may lead to better control of this critical twinning mode in HCP metals.

Room-temperature deformation of single crystals of WC investigated by micropillar compression

The room temperature deformation behavior of single crystals of WC has been investigated as a function of crystal orientation and specimen size by micropillar compression tests. Plastic flow is successfully observed at room temperature by the operation of slip on {011¯0}〈21¯1¯3〉 when the specimen size is reduced to micrometer orders. This slip system is the only operative slip system in WC at room temperature, and thus plastic flow is not observed for crystal orientations close to the c-axis orientation. The bulk critical resolve shear stress (CRSS) is estimated to be 1.2 ± 0.3 GPa from the extrapolation of the size-dependent CRSS. The 1/3〈21¯1¯3〉 dislocation carrying slip on {011¯0} dissociates into two identical collinear partial dislocations separated by a stacking fault with the energy of 211∼264 mJ/m2. The preference of slip along 〈21¯1¯3〉 among possible directions (such as 〈21¯1¯0〉 and [0001]) on the {011¯0} prism plane is discussed in terms of dislocation self-energy based on anisotropic elasticity, stacking fault energy, dislocation dissociation and Peierls stress for dislocation motion. The lowest Peierls stress arising from the shorter Burgers vector (1/6〈21¯1¯3〉) of dislocations as a result of collinear dissociation of dislocations with b = 1/3〈21¯1¯3〉 on the {011¯0} slip plane is considered to be the main reason for the preference of the {011¯0}〈21¯1¯3〉 slip system.