Crystallographic Facets Research Articles

High frequency fatigue crack propagation behavior of a nickel-base turbine disk alloy

Fatigue crack propagation tests were conducted on the powder metallurgy nickel-base superalloy KM4 at room temperature. Two different heat treatments were investigated, one which produced a relatively coarse grain size around 55 μm, and another which produced a very fine grain size around 6 μm. Tests were conducted at 50 Hz and 1000 Hz in an advanced servohydraulic testing machine at R-ratios between 0.4 and 0.7. There was no effect of frequency on the fatigue behavior at room temperature, which is expected in this type of alloy, and this result yields confidence in the reliability of the servohydraulic fatigue testing system. The threshold stress intensity for fatigue crack propagation decreased with decreasing grain size and with increasing R-ratio, again as expected. With increasing grain size, the crack path tortuosity and the crystallographic facet size on the fracture surface both increased substantially, leading to increases in roughness-induced closure and a higher apparent threshold. The threshold ΔK values measured at 10 −10 m/cycle corresponded to essentially infinite lifetimes, as very small decreases in Δ K from the threshold values resulted in complete crack arrest, and led to difficulty in restarting the crack growth at higher Δ K levels. Finally, comparisons of the observed thresholds with existing models revealed significant discrepancies between the predicted and measured values.

In Situ TEM Heating of Nanosized ZrO2

In situ TEM heating experiments at temperatures between 820° and 860°C provide significant insights into the atomistics of neck formation during the sintering of nanoscale faceted, single‐crystal particles of ZrO2. High‐resolution TEM was used to observe the crystallography and the relative orientation in a variety of original particle‐pairs and the relative orientation of their evolving necks. Previous studies have determined that unconstrained particles can rotate during neck growth. When there is a significant amount of misalignment, the particles are not able to align completely. In these instances, the neck that forms is epitaxial with one of the particles. Additionally, in particle‐pairs with significant misalignment, the neck stops growing when it forms a crystallographic facet which is continuous with the preexisting facet of the template particle. When the two particles have close to the same orientation, the particles rotate into complete alignment and the neck that forms is consequently epitaxial with both particles. When the neck‐particle system is in complete registry, the growing neck is rounded: there is an aperiodic arrangement of steps, even on the atomic resolution scale. The experimental results are discussed in terms of the random starting state of the two particles, the equilibrium crystal shape of the particle‐neck system, and the operative kinetic mechanisms.

Fracture surface roughness and roughness-induced fatigue crack closure in Ti-2.5 wt% Cu

Abstract Fatigue crack closure and fracture surfaces in the alloy Ti-2.5 wt% Cu at low Δ K (d a /d N −8 m cycle −1 ) were examined. For a solution-annealed equiaxed microstructure as well as for a fine and a coarse lamellar microstructure, roughness-induced crack closure occurs due to serrated crack path, which influences the crack growth rates significantly. The fracture surface roughness is found to be a measure of roughness-induced crack closure in lamellar microstructures where planar slip prevails. The height distribution and tilt angle distribution of the crack path facets are the decisive factors for roughness-induced crack closure. In the solution-annealed equiaxed microstructure, ductile fracture areas by the side of crystallographic facets can act as obstacles to crack tip sliding along planar slip planes, causing less mode II displacement and thus resulting in a much lower level of crack closure.

Effect of guest–host interaction on Raman spectrum of a CO2 clathrate hydrate single crystal

The polarized Raman spectra of an artificial CO2 clathrate hydrate single crystal have been measured in order to examine the crystal-orientation dependence of the Raman spectra. Since the crystal had crystallographic facets, the orientation of the crystal was determined by using the Miller indices of the facets. When the angle θ between the polarization plane of the incident laser beam and the direction of one of the 〈110〉 axes of the single crystal varied, it was observed that the intensities of the peaks, which were caused by the Fermi resonance of the symmetric stretching mode and the overtone of the bending mode of CO2, and the O–H symmetric stretching vibration mode, varied with θ. Since the tetrakaidecahedron cage in the CO2 clathrate hydrate is distorted along the 〈100〉 axis, the variations of the scattering intensities of the CO2 have been calculated by using a simple model that assumes that the CO2 rotates on the {100} plane in the tetrakaidecahedron cage. The results obtained from the experiments are consistent with the calculations made by using this model. It has been concluded that the anisotropy of the peak intensities of the CO2 show the influence of the cage geometry on the motion of the guest molecule. The anisotropy of the O–H symmetric stretching vibration mode was interpreted with a five-body structure model. As the calculation with the model was consistent with the result obtained from the experiment, it was found that the anisotropy of the peak intensity of the O–H symmetric stretching vibration mode was related to the arrangement of the water molecules. We consider that the result indicates the influence of the motion of the guest molecule on the surrounding hydrogen-bonded network.

Dynamic Crack Propagation in Single-Crystalline Silicon

AbstractTensile tests on notched plates of single-crystalline silicon were carried out at high overloads. Cracks were forced to propagate on {110} planes in a <110> direction. The dynamics of the fracture process was measured using the potential drop technique and correlated with the fracture surface morphology. Crack propagation velocity did not exceed a terminal velocity of v = 3800 m/s, which corresponds to 83%7 of the Rayleigh wave velocity vR. Specimens fractured at low stresses exhibited crystallographic cleavage whereas a transition from mirror-like smooth regions to rougher hackle zones was observed in case of the specimens fractured at high stresses. Inspection of the mirror zone at high magnification revealed a deviation of the {110} plane onto {111} crystallographic facets.

The behaviour of cadmium monocrystals and polycrystals in acid chloride solution

The polarisation characteristics of Cd polycrystals and monocrystals, with (0001), (112̄0), and (101̄0) surfaces, were studied in 0.1 M NaCl + 0.04 M HCl (pH 1.3) and compared with thermodynamic equilibria. Anodic dissolution rates were independent of surface orientation, exhibiting Tafel slopes ( b a ) near 27 mV that were reasonably consistent with a rate controlled process based on mass transport of CdCl + from the Outer Helmholtz Plane (OHP). Other processes are likely to have played a contributory role in the anodic behaviour, because crystallographic faceting accompanied dissolution. Cathodic evolution of hydrogen exhibited mixed rate control, with a cathodic Tafel slope ( b c ) near −81 mV. Hydrogen evolution appeared to be sensitive to surface orientation, with the (101̄0) surface showing the largest deviations in behaviour.

Low-pressure OMCVD growth of AlGaAs vertical quantum wells on non-planar substrates

AlGaAs vertical quantum well structure were formed by low-pressure (20 mbar) organometallic chemical vapor deposition on V-grooved substrates. Ga segregation in the wells of these structures was evidenced by means of atomic force microscopy and photoluminescence experiments. Transmission electron microscopy cross sections resolve different branches in the well. This substructure, whose definition is enhanced in low-pressure growth, is associated with the formation of specific facets at the bottom of the grooves. The dependence of the growth rate on the crystallographic facet and the Al mole fraction is the key mechanism for the establishment of the self-limiting profile in the V-grooves.

Influence of Fracture Mode Transition on Fatigue Life of Aluminum Alloy Reinforced by SiC Particles at Room and Elevated Temperatures.

A study has been made of the fatigue fracture mechanism in A6061 T6 metal matrix composite (MMC) reinforced with 5vol.% SiC particulate at room and elevated temperatures. The main results are summarized as follows. (1) Fatigue strengh decreased with increase of ambient temperature. (2) Relationship between fatigue life and temperature followed the Arrhenius rule. Therefore the fracture mechanism was the same, irrespective of ambient temperature. (3) Fatigue cracking of MMC initlated in the aluminum matrix and crack growth was in the shear mode, because the morphology of the fractured surface showed crystallographic facets. Then, fatigue crack growth mode changed from shear to monotonic fracture mode. (4) The transition occurred at the point at which the plastic zone size of the crack tip became larger than the average SiC particle size.

Selective InAs growth by chemical beam epitaxy

Selective growth of InAs was achieved using trimethylindium and cracked arsine on GaAs(100) substrates patterned with a silicon nitride mask. True selectivity can only be achieved in a very narrow range of substrate temperature and V III ratios. Growth optimization for various substrate types and pattern coverage were studied. Excellent surface morphology and electrical properties have been achieved at these optimized growth conditions. Vertical sidewalls can be obtained for selectively grown regions oriented along the [011] direction. Regions oriented in other directions show the growth of well-defined crystallographic facets. Device quality material was grown, as demonstrated by resonant interband tunneling diodes (RITDs) with room-temperature peak-to-valley current ratios (PVCRs) comparable to those obtained from RITDs grown and processed in a conventional manner.

Microstructural development of BaTiO3powders synthesized by aqueous methods

The hydrothermal growth of perovskite BaTiO3powders has been studied by transmission electron microscopy. The growth is carried out under high alkaline conditions (pH — 14) achieved with Ba(OH)2. Anatase (TiO2) is used as a titanium source. The perovskite BaTiO3nucleates heterogeneously on anatase TiO2 particles with an epitaxial relationship of (001)TiO2‖ (001)BaTiO3and [010]TiO2‖ [010]BaTiO3. This epitaxial relationship preserves the parallel alignment of the oxygen octahedra between the structures. A mosaic misorientation between (001)TiO2and (001)BaTiO3along 〈110〉 is seen in this relationship due to the lattice mismatch between TiO2and BaTiO3. After complete conversion of the anatase to BaTiO3, the BaTiO3particles develop into {111} octahedrons with ∼10 nm {001} and {110} microfacets on the {111} faces. This evolution suggests that {111} becomes the stable crystallographic facet for BaTiO3under highly alkaline conditions.

In SituAtomic Force Microscopy of the Electrochemical Dissolution of a Copper Grain

Dynamic evolution of the submicrometer surface morphology of a copper grain undergoing electrodissolution in the “electroetching regime” has been monitored by in situ atomic force microscopy. Images obtained for a nominal current density of 400 μA cm-2 indicate rapid etching into the surface to reveal well-defined crystallographic faces. The thermodynamically most stable {111} facets develop first, forming the initial primary dissolution faces; but as dissolution progresses, they are replaced by stably dissolving {211} and {221} facets. Hence, surface morphology can either be thermodynamically or kinetically controlled. Local current density is distributed inhomogeneously at the submicrometer level, being 1 order of magnitude larger than the global average at some locations. Identical crystallographic facets do not etch at the same rate and the dissolving facets typically evolve in a complex temporal-spatial manner. This behavior may be related to nonlinear pattern formation. Images obtained for a lower current density of 20 μA cm-2 provide unequivocal evidence of a surface recrystallization phenomenon concurrent with the anodic dissolution process. The surface reordering extends up to the submicrometer length scale and leads to development of smooth facets.

Topographical Analysis on Fatigue Fracture of Aluminum Alloy Matrix Composite.

From a mesoscopic viewpoint, the effects of SiC particles on fatigue crack initiation and growth behavior in an A6061-T6 metal matrix composite (MMC) were investigated with a topographical approach employing a laser-type profile measurement microscope. The results are summarized as follows. Fatigue crack of MMC grew and stopped sporadically and crack propagation rate decreased with a decrease in the number of particles. Fatigue crack of MMC originated from the aluminum matrix and the crack growth was a shear mode. Thus, the fractured surface showed crystallographic facets. The number of facets on the fractured surface decreased with increasing number of particles, since SiC particles in the aluminum matrix were resistant to shear mode crack growth. Crack growth changed from shear mode to brittle-like monotonic fracture mode when SiC particles cracked.

Fatigue crack propagation mechanisms in an aluminium lithium alloy

Different fatigue micromechanisms have been identified by fractographic observations in an aluminium-lithium alloy tested in vacuum and in air under constant and variable amplitude loading conditions. In vacuum, large crystallographic facets are observed for all the studied test conditions. In air, for constant amplitude test conditions, the different cracking mechanisms observed are (apart from facetted crack growth) quasi intergranular cracking and cracking by formation of Herringbone patterns. Under certain variable amplidude loading conditions crystallographic facets are not observed and evidence of striation formation is found. The morphology of these striations is different from that of classical ductile striations. This change in cracking mechanism is attributed to multiple slip activity and slip homogenization operating only in air for particular loading conditions.

ELECTRON BACKSCATTERING PATTERN IDENTIFICATION OF SURFACE MORPHOLOGY OF FATIGUE CRACKS IN TA6V

Abstract— Fatigue crack growth in an α+β titanium alloy (TA6V) with three microstructural conditions is studied in high vacuum at 300°C. The faster growth rates are observed in the material with a low content of primary α phase and a fine equiaxed microstructure. Near the threshold regime, a slow crystallographic Stage I‐like propagation is observed in the three microstructural types where slip occurs along a single‐plane system which develops within the α grains. These crystallographic facets are identified using an electron backscattering pattern (EBSP) technique and were found to lie on basal planes.

Nucleation of twins by grain coalescence during the sintering of BaTiO3ceramics

Abstract As shown by means of sintering procedures and electron microscopy studies, the grains of BaTiO3 ceramics develop crystallographic facets, preferentially of {111} octahedron and {100} cube types, below and above the eutectic temperature respectively of liquid-phase sintering. This faceting, in conjunction with a high mobility or the grains and suitable driving forces, yields a certain probability of forming twins and even double twins by the coalescence of suitably oriented grains during shrinkage. Such a double twin represents a grain which has a size and growth advantage during subsequent sintering. It can therefore be considered to represent the nucleus of the known doubly twinned ‘anomalous’ grains characterized by a central twin lamella. During its preferential growth, one anomalous grain consumes some 104−106 normal grains. Owing to this large number, the probability that grain coalescence events form a twin is quite a reasonable value as theoretically estimated. With some generalization, th...

Direct epitaxial growth of (AlGa)As/GaAs quantum wires by orientation-dependent metalorganic vapour phase epitaxy

The direct growth of (AlGa)As/GaAs quantum wire structures using metalorganic vapour phase epitaxy (MOVPE) is investigated. The present concept takes advantage of the different growth rates of GaAs and (AlGa)As on (100) and {111} planes to transfer a trapezoidal GaAs starting structure with dimensions in the μm-range into a quantum wire structure in the nm-range. For stripe dimensions below 500 nm significant changes in growth behaviour (enhanced growth rates and for special cases also the formation of other crystallographic facets) are observed by using scanning electron microscopy (SEM) studies. Based on these findings (AlGa)As/GaAs quantum wire structures are realized with lateral structure sizes of 30 nm as proved by SEM. The luminescence properties of these structures, which are determined by standard and polarization-dependent photoluminescence excitation spectroscopy, are presented and discussed in comparison to the properties of quantum well structures.

Microdefects in InP crystals grown by the liquid-encapsulated Czochralski method

Microdefects observed on as-polished InP surfaces, which have rectangular shapes and crystallographic facets, have been studied in detail. These defects reveal themselves as flat-bottomed saucer shaped etch pits after Huber etching. Their density decreases as the crystal length increases. The density of small scattering centers observed by laser scattering tomography also decreases as the crystal length increases. In addition the densities of the microdefects, flat-bottomed saucer shaped etch pits and scattering centers all decrease as the time that the charge is held in the molten state before crystal growth increases. Coulometric titration analysis shows that the In concentration in the crystals decreases as the crystal length increases except for the last-to-freeze portion. Similar flat-bottomed saucer shaped etch pits were also generated by thermal decomposition of InP surfaces. These results suggest that the formation of these defects is due to a departure from stoichiometry.

Geometry and interface structure of island nuclei for GaSb buffer layers grown on (001) GaAs by metalorganic vapour phase epitaxy

Atomic force microscopy and transmission electron microscopy have been used to investigate the geometry and interface structure of island nuclei formed in the initial stages of buffer layer growth for MOVPE GaSb on (001) GaAs. There is a bimodal distribution of island sizes with a high density of small, homogeneous nuclei and a lower density of larger, secondary nuclei. The smaller islands have pronounced crystallographic facets which are consistent with those which would be expected for minimization of surface energy and lateral growth anisotropy. The secondary islands are present at junctions between primary nuclei and may have formed due to enhancement of growth rates at emergent threading segments of misfit dislocations. The lattice misfit is accommodated by a regular square arrangement of edge-type misfit dislocations but unusual strain contrast arises in HREM images due to either a “stand-off” of dislocations from the interface or a corrugated interface.

Oxidation behavior of rhenium at high temperatures

Oxidation of polycrystalline Re has been studied at temperatures from 1,500 to 1,900 C. During oxidation volatile Re-oxides were emitted in the form of smoke and resulted in dramatic surface recessions of the samples. XRD analysis indicated that ReO[sub 3] was the primary oxide present in the condensed vapor deposits. Preferential oxidation of Re, manifested by the formation of crystallographic facets, was noted on the oxidized surfaces. Etchpits and islands bounded by high-symmetry planes showing a 6-fold symmetry were formed thereon, suggesting that the kinetics of oxidation are slower on close-packed planes. It is demonstrated that surface recession rate, dR/dt, which is equivalent to weight change per unit area and time (dW/A[center dot]dt), can be used to characterize oxidation behavior. The overall surface recessions of both the PM-Re and CVD-Re generally increased with oxidation duration and temperature. The CVD-Re exhibits lower recession rates than the PM-Re in the temperature range examined, which is attributable to the stronger basal-plane texture and larger grain size of CVD-Re. Oxidation of PM-Re was observed to be anisotropic. At 1500[degree]C, oxidation rates on the direction I (rolling plane) were higher. At higher temperatures (1,700 and 1,900 C), on the other hand, an opposite result wasmore » obtained. The differential oxidation rate of the PM-Re is suggested to originate from the synergistic effects of temperature-dependent oxidation behavior and basal-plane texture that have evolved during sample processing. This hypothesis is consistent with the fact that similar activation energies were obtained for the oxidation of CVD-Re and PM-Re (I).« less

Direct measurement of size fluctuation in reverse-mesa etched quantum wire structures by the atomic force microscope

By means of atomic force microscopy, we have directly measured, for the first time, size fluctuations in InGaAs/InP quantum wire structures fabricated by e-beam lithography and reverse-mesa wet etching. Based on a statistical analysis of the measured data, we discuss the standard deviation of the lateral size of wire structures. The results clearly indicate that the size fluctuations in the wire patterns are predominantly small compared to those in the original mask patterns. This is attributed to the formation of crystallographic facet during the reverse-mesa wet etching.