Crystallographic Facets Research Articles

Crystallographic fatigue crack initiation in nickel-based superalloy René 88DT at elevated temperature

The fatigue behavior of a polycrystalline nickel-based superalloy René 88DT was examined in the lifetime regime of 10 5–10 9 cycles at 593 °C in air using an ultrasonic fatigue apparatus operating at frequencies close to 20 kHz. Three experimental techniques were combined to obtain new insights into the crack initiation process: serial sectioning, electron backscatter diffraction and quantitative fractographic analysis. Most fatigue failures initiated from internal microstructural sites comprised of large grains. Large crystallographic facets formed at crack initiation sites due to cyclic strain localization on {1 1 1} slip planes in the region close to Σ3 twin boundaries in large grains having high Schmid factors. The micromechanical mechanism of crystallographic fatigue crack initiation was analyzed in terms of both resolved shear stress and elastic incompatibility stresses in regions close to Σ3 twin boundaries. The influence of critical microstructure features on fatigue crack initiation and fatigue life variability is discussed.

Pitting Behavior of Passivated Cadmium Monocrystals

A combination of aqueous thermodynamics, anodic polarization tests, and scanning electron microscopy was used to study the pitting behavior of Cd monocrystals in a phosphate-buffered (pH 11.2) solution of 0.1 M NaCl. Pitting occurred in the passive region where the most stable passive film was . The film thickness was estimated to be . Pitting potentials were not reproducible, being variable within surfaces of the same orientation and between different orientations. Variability was attributed to the effects of structural defects in the passive film and the potential scan rate influencing pit site nucleation and pit growth behavior. Sustained pitting was not possible below . Early stages of pitting produced crystallographic facets consistent with (0001), , and planes, with the morphology becoming increasingly complex with pit growth. pH decreased to below 8.5 in the growing pit and increased the influence of chloride species on localized dissolution behavior.

Pitting of Passivated Cadmium Monocrystals

A combination of aqueous thermodynamics, anodic polarization tests and scanning electron microcopy was used to study the pitting behavior of Cd monocrystals in a phosphate buffered (pH 11.2) solution of 0.1 M NaCl. Pitting occurred in the passive region where the most stable passive film was Cd (OH)2. The initial film thickness was estimated to be ~4 x 10-9 m. Pitting potentials were not reproducible, being variable within surfaces of the same orientation and between different orientations. Variability was attributed to structural defects in the passive film playing a major role in pit site nucleation. Early stages of pitting produced crystallographic facets consistent with (0001), and planes, the morphology becoming increasingly complex with pit growth. The pH decreased in the growing pit and increased the influence of chloride species on localized dissolution behavior.

Nominal vs Local Shot-Peening Effects on Fatigue Lifetime in Ti-6Al-2Sn-4Zr-6Mo at Elevated Temperature

A study is presented of elevated temperature (260 °C) fatigue lifetime variability in the shot-peened condition of the α + β titanium alloy, Ti-6Al-2Sn-4Zr-6Mo. It is shown that failures separate into two distributions: (1) governed by the nominal residual stress (RS) profile, promoting subsurface crack initiation and longer lifetimes; and (2) life-limiting behavior that is controlled by localized material-shot-peening interaction. In the residual-stress-free condition, failures occurred predominantly by surface crack initiation at the microstructural scale, on the order of 10 μm, by crystallographic facet formation in one or a few α particles or colonies. This mechanism was mitigated under the nominal shot-peening (SP) RS profile, producing failures initiating from the subsurface region by crystallographic faceting spread over a significantly larger area (equivalent diameter of about 100 to 200 μm) than in the absence of RS. Although the microstructure-scale surface-crack initiation was suppressed, the life-limiting failures under SP continued to occur by surface initiation, but through an apparently larger crack initiation size by formation of a flat, noncrystallographic fracture area (equivalent diameter of about 60 to 200 μm) at the crack origin.

Shape-Controlled Synthesis of Palladium Nanoparticles and Their SPR/SERS Properties

Morphological evolution of Pd nanoparticles was studied in a solution-phase synthesis using cetyltrimethylammonium bromide (CTAB) and CTAB/sodium citrate mixture as capping agents, respectively. The morphological diversity of Pd nanoparticles is the combined effect of different Pd twinned seeds formed in the nucleation stage and selectively enlarging one set of crystallographic facets in the growth stage, both of which can be affected by the concentrations of CTAB. Through changing the concentrations of CTAB and sodium ascorbate, Pd nanoparticles with different shapes were obtained. When citrate ions were introduced to manipulate the nucleation and growth process, star-shaped icosahedra and nanorods with pentagram cross-sections were obtained. Pd nanoparticles with different shapes have quite different surface plasmon resonance and surface-enhanced Raman scattering properties.

Microstructure variation effects on room temperature fatigue threshold and crack propagation in Udimet 720Li Ni‐base superalloy

ABSTRACTAn assessment of the effects of microstructure on room temperature fatigue threshold and crack propagation behaviour has been carried out on microstructural variants of U720Li, i.e. as‐received U720Li, U720Li‐LG (large grain variant) and U720Li‐LP (large intragranular coherent γ′ variant). Fatigue tests were carried out at room temperature using a 20 Hz sinusoidal cycling waveform at an R‐ratio = 0.1 on 12.5 mm × 12.5 mm square cross‐section SENB specimens with a 60° starter notch. U720Li‐LG showed the highest threshold ΔK (ΔKth), whilst U720Li‐LP showed the lowest ΔKth value. U720Li‐LP also showed higher crack growth rates in the near‐threshold regime and at high ΔK (although at higher ΔK levels the difference was less marked). Crack growth rates of U720Li and U720Li‐LG were relatively similar both in the near‐threshold regime and at high ΔK. The materials showed crystallographic stage I type crack growth in the near‐threshold regime, with U720Li showing distinct crystallographic facets on the fracture surface, while U720Li‐LG and U720Li‐LP showed mostly microfacets and a lower proportion of large facets. At high ΔK, crack growth in the materials becomes flat and featureless indicative of stage II type crack growth. The observed fatigue behaviour, which is an effect of the combined contributions of intrinsic and extrinsic crack growth resistances, is rationalized in terms of the microstructural characteristics of the materials. Enhanced room temperature fatigue threshold and near‐threshold long crack growth resistance are seen for materials with larger grain size and higher degree of planar slip which may be related to increased extrinsic crack growth resistance contributions from crack tip shielding and roughness‐induced crack closure. Differences in the deformation behaviour, either homogeneous or heterogeneous due to microstructural variation in this set of materials may provide approximately equivalent intrinsic crack growth resistance contributions at room temperature.

Oriented Nanocrystal Arrays of Selectable Polymorphs by Chemical Sculpture

Growing crystals with selected structure and preferred orientations on seed substrates is crucial for a wide variety of applications. Although epitaxial or textured film growth of a polymorph whose structure resembles the seed crystal structure is well-known, growing oriented nanocrystal arrays of more than one polymorph, selectable one at a time, from the same seed has not been realized. Here, we demonstrate for the first time the exclusive growth of oriented nanocrystal arrays of two titania polymorphs from a titanate crystal by chemically activating respective polymorph-mimicking crystallographic facets in the seed. The oriented titania nanocrystal arrays exhibit significantly higher photocatalytic activity than randomly oriented polymorphs. Our approach of chemically sculpting oriented nanocrystal polymorph arrays could be adapted to other materials systems to obtain novel properties.

Preparation of ZnO particle with novel nut-like morphology by ultrasonic pretreatment and its luminescence property

Novel nut-like zinc oxide crystal has been prepared by a low temperature hydrothermal method with the presence of Cu 2+ ion. It seemed that the ultrasonic pretreatment was the key factor during the preparation process. SEM observations revealed that the as-prepared ZnO crystal exhibited nut shape showing well-defined crystallographic facets. The cross-section of the ZnO crystal was hexagonal of about 800 nm–1 μm in diameter, and the aspect ratio was a little smaller than 1:1. The room temperature photoluminescence behavior of the nut-like zinc oxide crystal was much stronger than the reference samples.

Global faceting behavior of strained Ge islands on Si

The evolution of crystallographic facets of strained heteroepitaxial Ge islands on Si isinvestigated. Islands growing on Si(001), (111), (110) and (113) are bound by anequilibrium set of facets that includes only shared stable surfaces between bulk Si andGe—{105}, {113}, {15 3 23} and {111}. The formation of a stereographic map from these indicesfacilitates the prediction of Ge faceted-island shapes on any Si substrate at different stagesof growth. The analysis presented here can be applied to other heteroepitaxial islandingsystems where a finite set of shared equilibrium facets exists for the bulk starting materials.

Investigation of early growth of calcium hydroxide crystals in cement solution by soft X-ray transmission microscopy

The early growth of calcium hydroxide (CH) crystals in cement solution is investigated by soft X-ray transmission microscopy imaging. A quantitative analysis of the successively recorded images of the hydration process enabled to evaluate the supersaturation ratio of solution, growth rates, both kinetic and diffusion coefficients, and concentrations of solute molecules at \( \{ 10\overline{1} 0\} \) and {0001} facets of the CH crystals. It is concluded that the difference in obtained concentrations of solute molecules at these facets may be associated with solubility anisotropy of crystallographic facets of the CH. The interfacial energy of the CH nuclei in aqueous solution is evaluated to be 0.114 J/m2 that by an order of magnitude is smaller than the average free surface energy of this phase. The proposed theoretical approach is universal and, in potential, may be applied to any precipitating phase in a supersaturated solution.

Shape‐Controlled Synthesis of Pd Nanocrystals in Aqueous Solutions

AbstractThis article provides an overview of recent developments regarding synthesis of Pd nanocrystals with well‐controlled shapes in aqueous solutions. In a solution‐phase synthesis, the final shape taken by a nanocrystal is determined by the twin structures of seeds and the growth rates of different crystallographic facets. Here, the maneuvering of these factors in an aqueous system to achieve shape control for Pd nanocrystals is discussed. L‐ascorbic acid, citric acid, and poly(vinyl pyrrolidone) are tested for manipulating the reduction kinetics, with citric acid and Br– ions used as capping agents to selectively promote the formation of {111} and {100} facets, respectively. The distribution of single‐crystal versus multiple‐twinned seeds can be further manipulated by employing or blocking oxidative etching. The shapes obtained for the Pd nanocrystals include truncated octahedron, icosahedron, octahedron, decahedron, hexagonal and triangular plates, rectangular bar, and cube. The ability to control the shape of Pd nanocrystals provides a great opportunity to systematically investigate their catalytic, electrical, and plasmonic properties.

Fabrication of single crystalline gold nanobelts

Well dispersed, single crystalline gold nanobelts, with unique {110} crystallographic facets, were synthesized by a combined method consisting of directional solid-state transformation of an Fe–Au eutectoid and a selective phase dissolution without the presence of any surfactant. They have an average thickness of 25–30 nm, a width of 200–250 nm, and an average length of 20 µm, exhibiting thus extremely high aspect ratios of more than 1500. The obtained gold nanobelts were characterized by field emission scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, and UV-Vis spectrometry. The well-dispersed gold nanobelts can be selectively assembled onto substrates which have a significant potential for novel electrodes with a large effective surface area and a selective crystal orientation of {110}, less common for gold.

Synthesis of ZnO Microtubes by a Facile Aqueous Solution Process

Zinc oxide (ZnO) microtubes 1.0–2.5 μm in length, 300–500 nm in outer diameter, and 50–80 nm in wall thickness have been synthesized by a facile aqueous solution process, by simply dropping ammonia water into zinc chloride aqueous solution. This new method shows some advantages, such as synthesis under atmospheric pressure at a glass flask, low cost and easy control. The growth mechanism of ZnO microtubes is discussed in this paper. The different growth rates of ZnO crystal facets create the different crystallographic facets of hexagonal microtubes. And the polar structure results in the formation of ZnO microtubes for minimizing the spontaneous polarization energy. The dependence of morphologies of ZnO particles on the reaction temperature is also investigated. The scanning electron microscopy results show that the reaction temperature plays an important role in the morphologies of ZnO particles.

A general nonaqueous route to crystalline alkaline earth aluminate nanostructures

A nonaqueous route based on the solvothermal reaction of alkaline earth precursors with aluminium isopropoxide in benzyl alcohol is introduced. This simple process leads to crystalline complex nanostructures of alkaline earth aluminates, which, up to now, could only be obtained by solid state reaction at temperatures above 1100 degrees C or by sol-gel and further calcination at temperatures only slightly lower ( approximately 800 degrees C). The approach appears to be rather general since under the same reaction conditions BaAl(2)O(4), CaAl(4)O(7), and SrAl(4)O(7) could be obtained. The as-synthesized materials were characterized by X-ray diffraction, electron microscopy techniques, solid-state NMR and FT-IR spectroscopies. The reaction mechanism, which was studied as well, indicates the in-situ formation of benzoate species. These can preferentially bind to particular crystallographic facets of the aluminates via bridging bonds, thereby stabilizing the surfaces that give rise to the peculiar complex structure of the final material. In order to supplement the synthesis approach and to investigate the formation of impurity phases, pure aluminium oxide hybrid nanostructures were synthesized under similar conditions and fully characterized.

Carbon Supported CdSe Nanocrystals

Insights to the mechanism of CdSe nanoparticle attachment to carbon nanotubes following the hot injection method are discussed. It was observed that the presence of water improves the nanotube coverage while Cl containing media are responsible for the shape transformation of the nanoparticles and further attachment to the carbon lattice. The experiments also show that the mechanism taking place involves the right balance of several factors, namely, low passivated nanoparticle surface, particles with well-defined crystallographic facets, and interaction with an organics-free sp2 carbon lattice. Furthermore, this procedure can be extended to cover graphene by quantum dots.

Ultrathin β-In2S3Nanobelts: Shape-Controlled Synthesis and Optical and Photocatalytic Properties

Ultrathin β-In2S3 nanobelts have been successfully synthesized via a facile improved solvothermal route. The crystal phase, morphology, crystal lattice and composition of as-prepared products were characterized by XRD, FESEM, TEM, HRTEM and EDS, respectively. Results revealed that the as-synthesized β-In2S3 nanobelts are in cubic structure 50−90 nm in width, 13 ± 2 nm in thickness, and the overall length even up to several microns. A possible shape evolution and crystal growth mechanism was suggested, and the formation of β-In2S3 nanobelts resulted from the preferential growth along the ⟨220⟩ direction and enclosed by {202} and {022̅} crystallographic facets. The strong quantum confinement effect in UV−vis spectra and the blue emission in photoluminescence spectra imply the as-synthesized β-In2S3 nanobelts as a promising candidate for phosphor in display devices. Furthermore, the good photocatalytic effects indicate that these β-In2S3 nanobelts are likely to be applied as a new kind of photocatalyst in the future.

Ammonothermal growth of bulk GaN

Bulk GaN crystals have been realized by the basic ammonothermal method. Three-dimensional polyhedron-shaped crystals in size of approximately 5 mm and their sliced wafers demonstrated the feasibility of the basic ammonothermal method to produce GaN wafers. Clear crystallographic facets of N face and m planes were observed although Ga face was decorated with angled facets of {1 0 1¯ 1} planes. The growth rate and growth nature showed distinct anisotropy. The transmission electron microscopy revealed high quality of microstructure, whereas selective etching and X-ray diffraction revealed multiple grains, which were presumably caused by insufficient structural quality of seed crystals.

High-temperature fatigue properties of austenitic superalloys 718, A286 and 304L

High-temperature fatigue properties of austenitic superalloys 718, A286, and 304L were investigated in region between 10 2 and 10 7 cycles. The alloys 718 and A286 are precipitation-hardening type austenitic superalloys, and 304L is a solid solution-hardening type austenitic steel. The fatigue strengths of alloys 718 and A286 in high-cycle region over 10 4 cycles showed a grain size effect. The initiation sites of the fracture were the crystallographic facets corresponding to the austenitic grain size. Therefore the coarse-grain alloys showed the lower fatigue strength than the fine-grain alloys, because the fatigue strength is generally inversely proportionate to the initiation defect areas for the fatigue cracks. In low-cycle region under 10 4 cycles, on the other, the initiation sites of the fracture were the surface of the specimens. This type fracture mode is common for fatigue fracture of smoothed specimens. The ratio of the fatigue limit at 10 7 cycles to the tensile strength is extremely low for the coarse-grain alloys compared with the common case of 0.5. From these results fatigue fracture map is proposed.

Pulsed selective epitaxial growth of hexagonal GaN microprisms

Hexagonal GaN microprism structures were fabricated by pulsed selective epitaxial growth (SEG) under conditions that suppressed lateral overgrowth. Unlike previously reported pulsed SEG processes in which the precursor gas flow was modulated by valves, the approach reported here utilizes a flow geometry that produces a significant deposition rate only over the downstream portion of the 2 in wafer, which yields a pulsed deposition process when combined with substrate rotation. Hexagonal GaN microprism structures were grown on a GaN/sapphire substrate with a dielectric (SiO x ) mask of varying thickness (100, 150, and 200 nm) with circular openings ranging from 4 to 20 μm in diameter. Optimal structures were obtained with a SiO x mask thickness of 200 nm and circular openings of 6 and 8 μm. The vertical growth rate of the hexagonal prism structures was ∼50 nm/min, corresponding to the deposition of approximately six GaN bilayers (∼1.7 nm) per cycle. This pulsed deposition process has advantages over etching processes for fabricating hexagonal microprisms for optical cavity devices, including elimination of etching damage and formation of crystallographic facets with a high degree of parallelism.

Faceting of Σ3 Grain Boundaries in Al

The temperature dependence of the energy of various facets of twin GBs has been measured. For the investigation of GB faceting the Al bicrystals of 99.999% wt. purity were grown by the modified Bridgman technique. One grain in these bicrystals is semi-surrounded by another one. Bicrystals were coated with a layer of Sn–Al alloy and annealed at various temperatures. Contact angles at the junction of a GB and two solid/liquid interfaces have been measured. The ratios of GB energy to solid/liquid interface energy have been calculated. Using these data, the Wulff-Herring plots and GB phase diagrams were constructed. Three different crystallographic facets were observed for the coincidence GB. Two of them are stable at all studied temperatures, the third one becomes metastable below ~ 800K. In GBs with θ = 3° only one facet (symmetric twin GB) is stable.