Low-index Crystallographic Planes Research Articles

Effect of Si doping on elastic-plastic fracture toughness (JIC) of T91 ferritic/martensitic steel in liquid lead-bismuth eutectic

This work was devoted to understanding the effect of Si doping (1.27 wt%) on elastic-plastic fracture toughness (JIC) of T91 ferritic/martensitic (F/M) steel in liquid lead-bismuth eutectic (LBE, Pb44.5Bi55.5, wt%) under various parameters. The results showed that Si doping significantly promoted occurrence of liquid metal embrittlement (LME) and led to more severe degradation of fracture toughness at 350 °C, especially in oxygen-poor LBE. The LME susceptibility of T91-Si steel was found to be nearly independent on the loading speeds, while T91 F/M steel exhibited a stronger LME effect when the loading speed was decreased from 0.02 mm/min to 0.002 mm/min. Oxygen concentration had a mild effect. Detailed microstructural analyses showed that the fracture behavior of the T91-Si steel in LBE environment at 350 °C was analogous to cleavage fracture and the crack propagation was mainly transgranular. The crack wall was quite rough and locally faceted to low-index crystallographic planes of {110}. Numerous dislocations were also present in the vicinity of the crack tip. No solid evidence of diffusion of Pb and Bi into the steel bulk was discovered even at the atomic scale, which strongly suggests a crack tip surface adsorption-based LME mechanism.

Low traces of acetone detection with WO3-based chemical sensors

This work presents the ability of WO3-based sensors to detect low traces of acetone, specifically within the range of 0.25–5 ppm, specific to the in-field atmosphere. The WO3 powder was synthesised through the hydrothermal method. Morpho-structural investigations showed a monoclinic structure and a good crystallization of the WO3 powder, containing well-grown and faceted grains along low-index crystallographic planes. The paste obtained by mixing the powder with propanediol was screen-printed as a thick layer onto commercial alumina substrates, obtaining the chemical sensors. A dynamic computer-controlled Gas Mixing System was utilized to ensure controlled airflow with variable relative humidity and acetone concentrations. The sensor response was explained based on physico-chemical equations, taking into consideration pre-adsorbed species of oxygen and water, both of which are relevant constituents of atmospheric conditions. The results highlight the applicative potential of WO3, having a good signal-to-noise ratio in relative humidity conditions up to 90% and a pronounced sensitive selectivity to acetone.

Microstructural effects on short fatigue crack propagation in cast Al-Si alloys

The role of microstructure on the short fatigue crack behaviour of cast Al-Si alloy under high cycle loading conditions is investigated by tracking the progress of cracks through fine, medium and coarse microstructured samples. Generally, in the early stage of crack growth, short cracks mainly grow transgranularly, with cracks growth close to low index crystallographic planes (mainly {100}) in some grains and frequently inhibited at grain boundaries. In the late stage, the crack growth mode gradually shifts from crystallographic stage-I to non-crystallographic stage-II. The short cracks grow faster and fluctuate significantly according to local microstructure. Our results show that finer microstructures offer greater crack nucleation and propagation resistance.

Crystal orientation relationships and pattern evolution in directionally solidified Al-Cu-Mg ternary eutectic

Understanding the mechanisms of microstructure evolution during alloy solidification is crucial in designing multi-phase, multi-component systems. In the current work, scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD) techniques are deployed to carry out a detailed experimental investigation to better understand the correlation between crystallographic orientation relationships (ORs) and the microstructural pattern evolution of directionally solidified Al-Al2Cu-Al2CuMg ternary invariant eutectic. The behavior of the patterns in response to imposed low (6 K/mm) and high (25 K/mm) thermal gradients is also presented. Close examination of the microstructure revealed an unexpected modification of lamellar growth, with lamellar sheets being limited in length to approximately four times the width of one sheet, and the Al2Cu rod phase aligning along these pre-existing boundaries which correspond to low index crystallographic planes. Alignment of the pattern was found to increase with increasing thermal gradient. The results shed more light on somewhat contradictory literature data and provide an increased understanding of pattern evolution in multi-phase eutectic systems.

Properties of interfaces between copper and copper sulphide/oxide films

We compare different means of predicting and rationalizing properties of corrosion films aiming at gaining deeper understanding of the behaviour of copper in aqueous and sulphide-containing environments. Purely geometrical tools ranging from mean bulk information to anisotropic and facet-specific approaches are discussed, and their performances compared against DFT-based stability and property evaluations of a range of interfaces arising from combining low-index crystallographic planes of the compounds Cu, Cu2O, and Cu2S. We demonstrate the necessity to consider facet-specific interactions to understand nanolevel differences between Cu2S and Cu2O film behaviour, and that thin films cannot be directly described by bulk properties.

Crystallographic characterization of the middle shelf in a two-step ductile to brittle transition for a ferrite + pearlite structure steel sheet

The middle shelf (MS) exhibits medium absorbed energy during a two-step ductile to brittle transition and appears clearly in a ferrite + pearlite steel sheet. The analysis involved using the electron backscattered diffraction technique and indicated that the traces of fracture surface and secondary cracks at MS and lower shelf (LS) correspond to low index crystallographic planes in bcc iron. Although the {001} cleavage plane in the ferrite grain is dominant for the fracture propagation path at LS, the traces at MS correspond to {001}, {112}, and {011} planes. The potential of fracture in the {001} plane at MS (i.e., the number of traces normalized by the number of equivalent planes) exceeds that in the {112} plane. However, the difference between the same is significantly lower than that at LS. Furthermore, the{112} and {011} planes are typical slip planes in bcc iron. This suggests that the fracture at MS propagates on the {001} plane with cleavage fracture as well as on the {112} and {011} planes with a slipping-off mechanism.

Superfast domain walls in KTP single crystals

Potassium titanyl phosphate KTiOPO4 (KTP) crystals with periodical ferroelectric domain structures are one of the most promising materials for nonlinear optics, in which the main types of nonlinear optical interactions have been demonstrated. Despite the crucial importance of the in situ visualization of domain structure kinetics for creation of high quality periodical domain gratings, there are only a few works concerning KTP. We present the results of in situ visualization of domain kinetics in KTP with the time resolution down to 12.5 μs and simultaneous recording of the switching current data. The wide range of wall velocities with two orders of magnitude difference was observed for switching in a uniform electric field. The kinetic maps allowed analyzing the spatial distribution of wall motion velocities and classifying the walls by velocity ranges. The distinguished slow, fast, and superfast types of domain walls differed by their orientation. It was shown that the fast and slow domain walls provided the smooth input to the switching current, whereas the short-lived superfast walls resulted in short current peaks. The mobility and the threshold fields for all types of domain walls were estimated. The revealed increase in the wall velocity with deviation from low-index crystallographic planes for slow and fast walls was considered in terms of determined step generation and anisotropic kink motion. The obtained results are important for further development of domain engineering in KTP required for creation of high power, reliable, and effective coherent light sources.

Boron carbide nanoparticles for high-hardness ceramics: Crystal lattice defects after treatment in a planetary ball mill

Deformation features of the boron carbide lattice treated in a planetary ball mill are studied in our work. It is shown that such treatment causes a decrease in the B4C grain size and cracking along the {202¯1}h planes, whereas the same treatment in the presence of silicon results in the appearance of twins along the {101¯1}h planes. Distorted stacking faults along (101¯4)h are observed. Ab initio simulations shown, that among low-index crystallographic planes {202¯1}h and {101¯1}h have the smallest values of specific surface energy which favors the formation of defects and cracks.

Resonant charging and stopping power of slow channelling atoms in a crystalline metal

Fast moving ions travel great distances along channels between low-index crystallographic planes, slowing through collisions with electrons, until finally they hit a host atom initiating a cascade of atomic displacements. Statistical penetration ranges of incident particles are reliably used in ion-implantation technologies, but a full, necessarily quantum-mechanical, description of the stopping of slow, heavy ions is challenging and the results of experimental investigations are not fully understood. Using a self-consistent model of the electronic structure of a metal, and explicit treatment of atomic structure, we find by direct simulation a resonant accumulation of charge on a channelling ion analogous to the Okorokov effect but originating in electronic excitation between delocalized and localized valence states on the channelling ion and its transient host neighbours, stimulated by the time-periodic potential experienced by the channelling ion. The charge resonance reduces the electronic stopping power on the channelling ion. These are surprising and interesting new chemical aspects of channelling, which cannot be predicted within the standard framework of ions travelling through homogeneous electron gases or by considering either ion or target in isolation.

Growth of Horizontally Aligned Single-Walled Carbon Nanotubes on the Singular R-Plane (10–11) of Quartz

We used both R-cut and R-face crystal quartz substrates for the efficient growth of horizontally aligned single-walled carbon nanotubes (SWNTs). The R-plane (10–11) is one of the low-index crystallographic planes of crystal quartz. The surface cut from a synthetic quartz block parallel to the R-plane was used as R-cut substrates, and the exposed R-plane was used as R-face substrates. We elucidated that the atomic structure of the R-plane causes the alignment of the SWNTs. While a step-and-terrace structure clearly appeared on R-face substrates, SWNTs were aligned on the terraced area of the R-plane, regardless of the direction of the step edges. Comparison between R-face and ST-cut substrates suggests that the ST-cut surface can be considered as a collection of tiny r-plane (01–11) domains, which are very similar to the R-plane (10–11).

Distribution of brine in grain boundaries during static recrystallization in wet, synthetic halite: insight from broad ion beam sectioning and SEM observation at cryogenic temperature

We report observations from room temperature static recrystallization experiments (annealing times from minutes to year) of cold-pressed, synthetic, coarse-grained, wet sodium chloride, prepared by broad ion beam polishing and SEM observations at cryogenic temperature to observe directly the brine in grain boundaries. At all stages of annealing, the majority of the brine in the samples is connected in 2D sections along grain boundaries. Another part of the brine is in isolated brine inclusion arrays along grain boundaries and in brine inclusions left behind by migrating brine-filled grain boundaries. Most of these boundaries are mobile because the aggregate is coarsening. We interpret that the boundaries without observable brine films (\15 nm) and brine inclusion arrays are healed and immobile. Evolution of grain boundary structure involves three major processes. First, dissolution on one side of the grain boundary and precipitation on the other side, result- ing in grain boundary migration. Second, the development of facets formed by low-index crystallographic planes of the grains bounding the grain boundary brine. When both sides of a grain boundary are able to develop low-index facets in a thick brine film, the resulting impingement boundary is interpreted to be immobile and may prevent the new grain from migrating into a deformed neighbor. When one side of a faceted boundary consists of low-index crystallographic planes and the other side passively follows this faceted shape along irrational surfaces, the boundary is mobile. Third, the healing of grain boundary brine films, producing solid-solid grain boundaries without resolvable brine films.

The crystalline-to-amorphous transition in shock-loaded mullite Al 2VI(Al 2+2 xSi 2−2 x) IVO 10− x in the light of shear modulus anisotropy

We present experimental evidence for shock-wave induced amorphization in polycrystalline and single crystal mullite, Al 2 VI(Al 2+2 x Si 2−2 x ) IVO 10− x , at peak pressures above 35 GPa. The transition proceeds along with a network of very thin glass lamellae (planar deformation features (PDFs)) of mullite-normative composition extending parallel to low-index crystallographic planes including {1 2 0}, {2 3 0} and {1 1 0}. Cumulative microstructural evidence from the PDFs derived via analytical transmission electron microscopy suggests a shear-induced formation mechanism. Experimental PDFs match the relative minima of the calculated representation surfaces of the shear modulus suggesting that suitable PDF orientations can be derived from the elastic anisotropy of mullite. PDFs in mullite are in good agreement with those reported for naturally shocked sillimanite. Unlike the formation of shear-induced PDF-type glass lamellae in shocked mullite, the thermal decomposition of mullite following high post-shock temperatures results in a fine-grained phase assemblage consisting of corundum plus amorphous silica, and represents the most abundant transformation mechanism in the shock regime investigated (20–40 GPa). No stishovite was observed. At shock levels beyond 35 GPa thermal decomposition of mullite may occur along with PDFs within the same specimen.

Kinematic grouping of reflections of parametric X-ray radiation

A model of parametric X-ray radiation (PXR) of moderately relativistic particles is developed within the kinematic theory taking into account radiation absorption. The model allows calculation of the PXR reflection intensity in a given direction taking into account diffraction of virtual photons on a set of “incidental” low-index crystallographic planes. It is shown that when a charged particle moves along certain crystallographic directions, the effect of kinematic grouping of reflections, at which lines of parametric radiation from various planes practically coincide, is observed.

Abnormal grain growth of BaTiO 3 by 2D nucleation and lateral growth

The microstructure of abnormal grains in BaTiO 3 based ceramics suggests that 2D nucleation and lateral growth take place in the material. High-resolution transmission electron microscopy investigation reveals that nucleation takes place at triple junctions of abnormal grain and adjacent matrix grains, subsequently form steps and the steps grow laterally along {1 1 1} planes of the abnormal grains. The abnormal grain boundaries are faceted in the growth direction, however, the ledges, which possess boundaries deviated from low-index crystallographic planes of the abnormal grains, could provide high mobility necessary for abnormal grain growth.

The correlation between preferred orientation and performance of ITO thin films

The tin-doped indium oxide (ITO) thin films were prepared by reactive thermal evaporation on the glass substrates. The effects of substrate temperatures (T-s) on the grain preferred orientation, the electrical and optical properties of ITO films were studied. X-ray diffraction (XRD) patterns indicated that the preferred orientation of film changes from (222) to (400) as T, > 200 degrees C. It can be explained by that the low-index crystallographic planes are easier to be formed when the adatoms have high surface mobility. The Hall measurements indicated that both the concentration and mobility of carrier increase with increasing T,,,. The grain orientation of film does not influence the transmissivity and the carrier concentration, but enhances the carrier mobility. The transmissivity of ITO films is over 90% in the visible wavelength region (except that of the film deposited at 125 degrees C). A minimum resistivity of 5 X 10-4 Omega cm is achieved for the (400) preferred orientation film. Thus, the highest figure of merit of 3.5 x 10(-2) square/Omega is obtained for the film with (400) preferred orientation. The correlation between the preferred orientation and electrical and optical properties are discussed.

Observations of the Gibeon meteorite and the inverse Greninger–Troiano orientation relationship

All the possible parallelism conditions associated with low-index crystallographic planes and directions in face- and body-centred cubic crystals are enumerated in terms of their minimum angle–axis misorientations. These include the Kurdjumov–Sachs (K-S), Nishiyama–Wassermann (N-W) and Pitsch orientation relationships, which are expressed in terms of close-packed (and near-close-packed) planes, close-packed directions, or both in the two crystal systems. By classifying these into either coplanar or codirectional relationships, it is possible to specify intermediate relations with either high-index directions or planes. In such a way, the Greninger–Troiano (G-T) relationship is approximated as a coplanar relation lying between the K-S and N-W relations. Based on geometric symmetry, a further correspondence relationship, which falls into the codirectional category and is located between the K-S and Pitsch relations, is represented both in pole-figure form and in Rodrigues–Frank space. It is referred to here as the inverse G-T, or G-T′. Experimental evidence from the plessite regions of the Gibeon meteorite is presented to support the validity of this relationship. It is shown that, in plessite, all the well known models, as well as their intermediates, are applicable to the γ-to-α transformation. By contrast, in the Widmanstätten portions, the misorientations are largely limited to the K-S, G-T and N-W relations.

Beyond Archimedean solids: Star polyhedral gold nanocrystals

We report star polyhedral gold nanocrystals synthesized by colloidal reduction with ascorbic acid in water at ambient conditions. We identify two distinct classes of star nanocrystals: multiple-twinned crystals with fivefold symmetry, and monocrystals. These respective classes correspond to icosahedra and cuboctahedra, two Archimedian solids, with preferential growth of their {1 1 1} surfaces. Due to this preferential growth, the {1 1 1} faces of the original Archimedean solids grow to become tetrahedral pyramids, the base of each pyramid being the original polyhedral face. By assuming a star morphology, gold nanocrystals increase their proportion of exposed {1 1 1} surfaces, which possess the lowest surface energy among low-index crystallographic planes for FCC crystals. Thus, we propose that the driving force for star nanocrystal formation could be the reduction in surface energy that the crystals experience. Interestingly, icosahedrally derived star nanocrystals possess a geometric morphology closely resembling the great stellated dodecahedron, a Kepler-Poinsot solid.

Morphology of InGaAs/GaAs quantum wires prepared byhighly controlled deep-etching techniques

Different types of InGaAs/GaAs deep-etched quantumwire (QWI) structure were successfully fabricated byhigh-energy electron beam lithography on GaAs(100) surfaces.A selective wet-chemical-etching technique, preceded by chemicallyassisted ion-beam etching, reduced the controlled lateraldimensions of the wires to ~10 nm due to strongunder-etching. Various types of wire in the [011̄] and[011] crystallographic directions were prepared by the combinedetching method. The side-walls of the wires were defined by theselectively etched low-index crystallographic planes. Amolecular-beam-epitaxy-grown graded InGaAs/GaAs quantum well wasrealized at the narrow `neck' region of the wires, thusproviding the strongest possible lateral confinement of the QWIstructure. Consequently, similarly to the selective growth ofself-narrowing ridge structures, selective wet-chemical etchinginduced a controlled self-narrowing of the wire structures.Scanning electron microscopy images of the QWI nanostructuresshowed smooth side-walls defined by the crystallographic planes.Low-excitation photoluminescence spectroscopy of the structuresrevealed extremely high quantum efficiency and a size-dependentblue shift as a result of the strong lateral confinement.

Self-organized InAs islands on InP(3 1 1)B substrates emitting around 1.55 μm

The influence of the InP(3 1 1)B high index substrate on InAs island characteristics, is investigated as regards to the (1 0 0) substrate. The AFM images of a nominally 2.1 ML thick InAs layer show that the island density is 10 times higher on the (3 1 1)B substrate than on (1 0 0). The average island diameter and height on (3 1 1)B are half the average values on (1 0 0). Careful observations of the island profiles on AFM images allow to determine facets close to low index crystallographic planes. The islands grown on the (3 1 1)B surface lead to a steady photoluminescence emission at 0.7 eV for a wide range of InAs growth conditions. In order to tune this PL emission wavelength around 1.55 μm (0.8 eV), the islands have been exposed under a phosphorous overpressure for various times up to 20 s before being capped with the InP layer. The As/P exchanges induce a blue shift of the PL emission up to 250 meV.

Pseudo-dendritic growth in lead molybdate single crystal by Czochralski technique

A pseudo-dendritic crystal growth of lead molybdate (PbMoO 4) has been studied with the Czochralski technique. The observed dendritic growth is iso-directional, projecting from the circumference of the growth front, where the content of impurities, melt stoichiometry and temperature are expected to be varied. The dendrite arm length and inter-dendrite angle are closely correlated. It is found that the grown dendrite arms are bounded only by ten low-index crystallographic planes (001), (00 1 ), (101), ( 1 01), (10 1 ), ( 1 0 1 ), (011), (0 1 1), (01 1 ), and (0 1 1 ). Based on this work, a facet transition pattern existing among the dendritic arms has been revealed. Further analysis for crystal grains in as-grown PbMoO 4 single crystals indicates that a similar dendritic growth can also occur under conventional growth conditions. It is further suggested that the constitutional supercooling is a major factor causing the dendritic growth in PbMoO 4 single crystals.