Etch Pit Technique Research Articles

Influence of dislocations on thermal conductivity of strontium titanate

Recently, several creative processing techniques yielded thermoelectrics with reduced thermal conductivity and, thereby, an enhanced figure or merit. These were based on engineered complex microstructures with attendant dislocation structures. In this study, we implement highly controlled mesoscopic dislocation structures into the model thermoelectric SrTiO3 in order to quantify phonon scattering at dislocations. Both single crystals and polycrystalline material have been furnished with enhanced dislocation densities increased by a factor of 150–300 by plastic deformation. Thermal conductivity was measured using laser flash analysis between room temperature and 325 °C. Etch pit techniques and ultra-high voltage electron microscopy afford quantification of dislocation density. Experimental results were compared to predictions by the Debye-Callaway model. The latter revealed that dislocation densities of 1015 m−2 would be necessary for the reduction of thermal conductivity of SrTiO3 in the investigated temperature range, which could not be realized using the plastic deformation mechanism applied.

Transmission behavior of dislocations against Σ3 twin boundaries in Si

Transmissions of dislocations across grown-in twin boundaries, i.e., Σ3 {111} boundaries, in multicrystalline silicon (mc-Si) were investigated by direct observations using the etch-pit technique and x-ray topography. Dislocations were freshly generated from a scratch by three-point bending at 800 °C. The generated screw dislocations whose Burgers vector was parallel to the twin boundaries stopped their motion at the boundaries and piled up there. This result reveals that the resistance stress of twin boundaries against screw dislocations is greater than ∼18 MPa. Alternatively, nonscrew-type dislocations were generated from the twin boundaries into the adjacent twinned grain under the stress greater than ∼15 MPa. The results are discussed in terms of cross-slip of dissociated dislocations for transmission across the twin boundaries and stress concentration by piled-up dislocations in the interactions of dislocations with grain boundaries developed in metals.

Dislocation distribution and patterning in [formula omitted]-axis Mg single crystals during early compression

Dislocation distribution and patterning during the onset of uniaxial compression of 〈a〉-axis Mg single crystals was investigated by transmission electron microscopy (TEM) and etch-pitting technique. The crystals deform by dislocation glide using basal and nonbasal slip systems. The high hardening rate Θ ≈E/50−E/30 indicates the effective forest hardening by nonbasal dislocations observed in the microstructure. Distribution of dislocation etch-pits shows the polygonized arrangement of dislocations along the basal plane. TEM observations reveal the heterogeneous structure of dislocations clustered in dense arrays surrounded by dislocation free areas. Dislocation arrangements that are building blocks of the microstructure include monopoles, dipoles, multipoles, ribbons, walls, bundles, and braids. The patterns are characterized with respect to their dislocation content and spatial distribution. They fall into the category of low energy structures, which favorite the configurations reducing the elastic energy of the arrays. The degree of interactions between basal and 〈c+a〉 slip systems is attributed to playing a key role in the formation of observed dislocation arrangements.

Effect of temper rolling and subsequent annealing on texture development and magnetic permeability of semi-processed electrical steel with 2.3 wt.% Si

Abstract The effect of temper rolling and subsequent annealing on texture development and magnetic properties of semi-processed non grain oriented electrical steel has been investigated. The result shows that 5% temper rolling and final annealing at 850°C resulted in strong α-fiber component. The etch-pit technique shows that strain induced boundary migration is responsible for new texture component augmentation. Vibrating sample magnetometery reveals that maximum and minimum magnetic permeability are related to rolling and transverse directions, respectively. Average magnetic permeability is improved to some extent as a result of temper rolling and subsequent annealing.

Indentation size effect and dislocation structure evolution in (001) oriented SrTiO3 Berkovich indentations: HR-EBSD and etch-pit analysis

In the present work, the dislocation structure evolution and indentation size effect in (001) oriented STO have been studied by using sequential polishing, chemical etch-pit technique and high-resolution electron backscattered diffraction (HR-EBSD) analysis via Berkovich nanoindentation experiments. Nanoindentation load-displacement curves show multiple pop-in events, which relate to nucleation and extension of dislocation pile-ups around the residual impression. Sequential polishing and etching revealed the three-dimensional dislocation etch-pit structure at various sub-surface depths. The dislocation densities are determined for a 5 mN and 10 mN indentation below the surface via HR-EBSD and etch-pit analysis. With HR-EBSD, the lattice rotation and thereby GND densities are determined, while the etch-pit technique revealed the total dislocation density. Based on the independently measured dislocation densities, we clearly show a depth dependent dislocation density, where both total and GND densities increase with decreasing indentation depths. The higher dislocation densities below smaller indents explain the observed size dependency of the hardness in STO, which also sheds light on the extraordinary combination of high indentation hardness and low yield strength in STO.

Controlling Mechanical Properties of Warm Extruded Commercial V-65 Alloy

V-65 (Russian grade) is commercial Aluminium alloy used in making rivets in aerospace applications as it exhibits relatively high shear strength and plasticity. Controlling properties of this alloy by the addition of second phase particles in the form Zirconium that hinders grain growth by exerting Zener drag is discussed in this work. The alloys were subjected to differential thermal analysis and peaks were observed at 4190c, indicating the dissolution of Al3Zr particles. The alloy was warm extruded at 300 & 3500c and the kinetics of Recrystallization was followed by image analysis on samples of recrystallized at 4200c for various times assuming that short duration of anneal would not dissolve Al3Zr because of diffusion coefficient of Zirconium in Aluminium is low. Detailed microstructure analysis indicated the Al3Zr particles exerted pinning pressure on grain boundaries effectively producing fine grains. These results were justified by the volume fraction analysis of grains. Transmission Electron Microscopy was also conducted and the fine particles were found to be Al3Zr particles with L1-2 crystal structure and lattice parameter 4.08 A0.The cube grains were identified by etch pit technique and it was found that addition of Zr did not alter the number of cube grains. These cube grains facilitate the alloy in deep drawing and its application in the fuselage of an aeroplane

Dislocation dynamics in SiGe alloys

The dislocation velocities and mechanical strength of bulk crystals of SixGe1−x alloys grown by the Czochralski method have been investigated by the etch pit technique and compressive deformation tests, respectively. Velocity of dislocations in the SiGe alloys of the composition range 0.004 < x < 0.08 decreases monotonically with an increase in Si content at temperature 450–700°C and under stress 3–24MPa. In contrast, velocity of dislocations in the composition range 0.92 < x < 1 first increases, then decreases and again increases with a decrease in Si content at temperature 750-850°C and under stress 3–30MPa. The velocity of dislocations was quantitatively evaluated as functions of stress and temperature. Stress-strain behaviour in the yield region of the SiGe alloys of composition 0 < x < 0.4 is similar to that of Ge at temperatures lower than about 600°C. However, the yield stress becomes temperature-insensitive at high temperatures and increases with increasing Si content. The stress-strain curves of the SiGe alloys of composition 0.95 < x < 1 are similar to those of pure Si at temperatures 800–1000°C and the yield stress increases with decreasing Si content down to x = 0.95. The yield stress of the SiGe alloys is dependent on the composition, being proportional to x(1−x), showing a maximum around x ≈ 0.5. Built-in stress fields related to local fluctuation of the alloy composition and the dynamic development of a solute atmosphere around the dislocations, may suppress the activities of dislocations and lead to the hardening of SiGe alloys.

Structure and microstructure of GaTe crystals grown by High-Pressure vertical zone melting

GaTe crystals of 20mm in diameter and up to 70mm in length have been grown from melt using the high-pressure vertical zone melting (HPVZM) method. Real defect structure has been evaluated by XRD, SEM, EPMA, optical microscopy, microhardness and transparency measurements. The phase transformation of hexagonal GaTe single crystal to monoclinic form by grinding has been detected. Such local defects as dendrites and dendrite clusters have been found at cleaved surface (0001), basal dislocations with density of 2×105cm−2 have been detected by the etch-pit technique.

Impurity effects on the generation and velocity of dislocations in Ge

The dynamic behavior of dislocations in heavily impurity (Ga, As, O) -doped Ge crystals was investigated as a function of stress and temperature by means of the etch-pit technique. The Ga and O impurities effectively suppressed dislocation generation from a surface scratch, while the As impurity did not strongly do so. Dislocations in Ga- and O-doped Ge crystals were supposed to be immobilized by the stable complexes formed through the impurity segregation and reaction. Remarkably, the O impurity, even at a low concentration of 1016 cm−3, induced the suppression of dislocation generation. The As impurity enhanced and the Ga impurity retarded dislocation velocity in motion. The O impurity had no effect on the velocity of the dislocations.

Vanadium-Doped Cadmium Manganese Telluride (Cd1−x Mn x Te) Crystals as X- and Gamma-Ray Detectors

CdMnTe offers several potential advantages over CdZnTe as a room- temperature gamma-ray detector, but many drawbacks in its growth process impede the production of large, defect-free single crystals with high electrical resistivity and high electron lifetimes. Here, we report our findings of the defects in several vanadium-doped as-grown as well as annealed Cd1−xMnxTe crystals, using etch pit techniques. We carefully selected single crystals from the raw wafer to fabricate and test as a gamma-ray detector. We describe the quality of the processed Cd1−xMnxTe surfaces, and compare them with similarly treated CdZnTe crystals. We discuss the characterization experiments aimed at clarifying the electrical properties of fabricated detectors, and evaluate their performance as gamma-ray spectrometers.

Hall-Petch Relation and Twin Boundaries in Pure Copper and Cu-Al Alloys

The present work has investigated whether the twin boundaries are to be included or not in the Hall-Petch equation for polycrystalline metals. Pure copper, Cu-3.5at%Al, Cu-6.8at%Al and Cu-14.8at%Al alloys, which have the average grain sizes from 8.4 to 176.0μm and the different ratio of annealing twins, were produced and pulled in tension at temperatures from 77 to 973K under a strain rate of 1.2 × 10-4s-1. The distribution of dislocations in the surface grains was also observed by using the etch pit technique. 0.1% proof stress including the twins for each specimen at room temperature conforms well to the Hall-Petch relation. The halves of intersecting stress σ0 derived from the straight lines are approximately equal to the critical resolved shear stress of each single crystal, i. e. CRSS, so far reported. It is found that the proportional relation between the σ0/2 value and the square root of aluminum concentration is in good agreement for the single crystals and the polycrystalline specimens. The temperature dependence on the σ0/2 values for Cu-14.8at%Al alloys is similar to the variations for the CRSS of single crystals, especially at the region of low and elevated temperatures. From the Hall-Petch parameters at 293K, the unlocking stress of edge dislocations from the solute atoms is found to be 3.5 times larger than the CRSS of single crystals. This CRSS is thought to be the unlocking stress of screw dislocations which come out onto the crystal surface. It is considered that the role of the twin boundaries is almost equivalent to the grain boundaries, because the multiplicative dislocations pile up against the twin and grain boundaries during the small deformation.

Study on the indentation size effect in CaF 2: Dislocation structure and hardness

In this study nanoindentations have been performed on a cleaved surface of a CaF 2 single crystal and the dislocation structure has been investigated by the etch pit technique using atomic force microscopy. The deformation during indentation is first purely elastic until dislocations are created observable in a pop-in in the load displacement data, as well as in a dislocation rosette around the indentation. After pop-in a relatively high hardness is observed, which gradually decreases, until at 3 μm a nearly constant hardness is found. By using sequential polishing, etching and imaging, the dislocation structure underneath indentations with indentation depths of 300 nm and 110 nm (load: 5 mN, 1 mN) is quantified. The dislocation density and radial distribution of dislocation density depend on the indentation depth, where a smaller indentation depth leads to a higher dislocation density, which is in qualitative agreement with the observed increase in hardness.

Effect of Grain Size on Fatigue Behavior of Pure Copper and Cu-Al Alloys

In order to clarify the grain size dependence on the fatigue behavior of polycrystalline metals including twins from the view point of dislocation theory, pure copper, Cu-3.5at%Al, Cu-6.8at%Al and Cu-14.8at%Al alloys having the average grain sizes ranging from 9 to 122μm were prepared by a strain-annealing method. Fatigue tests at room temperature for each specimen were performed under the constant stress amplitude over a life range of 104 to 107cycles. The fatigue properties at elevated temperatures for Cu-14.8at%Al alloys were confirmed up to 773K, and also the distribution of dislocations close to fatigue cracks was observed by using the etch pit technique. The grain size dependence on the fatigue strength for pure copper and Cu-3.5at%Al alloys disappears beyond 106cycles, whereas the fatigue lives of Cu-6.8at%Al and Cu-14.8at%Al alloys uniformly increase with grain refining over all ranges. Relationship between the fatigue strength at 107cycles and aluminum concentration for Cu-Al alloys of fcc solid solution can be approximately represented by a straight line as well as the shear stress. The fatigue strength of Cu-14.8at%Al alloys markedly decreases with increasing temperature and becomes almost half of that for room temperature at 773K. The results based on the etch pit observation indicate that the dislocations multiplied under cyclic loading pile up against the grain and twin boundaries, where large cracks along both boundaries form and propagate due to the stress concentration.

Hall-Petch Relation and Twin Boundaries in Pure Copper and Cu-Al Alloys

The present work has investigated whether the twin boundaries to be accounted or not in the Hall-Petch equation for polycrystalline metals. Pure copper, Cu-3.5at%Al, Cu-6.8at%Al and Cu-14.8at%Al alloys, which have the average grain sizes from 8.4μm to 176.0μm and different ratio of annealing twins, were produced and pulled in tension at the temperatures from 77K to 973K under a strain rate of 1.2 × 10-4s-1. The distribution of dislocations in the surface grains was also observed by using the etch pit technique. The 0.1% proof stress including twins for each specimen at room temperature well obeys to the Hall-Petch relation. The halves of frictional stress σ0 derived from the straight lines are approximately equal to the critical resolved shear stress of each single crystal, i. e. CRSS, so far reported. It is found that the proportional relation between the σ0/2 value and the square root of aluminum concentration is in good agreement for the single crystal and polycrystalline specimens. Temperature dependence on the σ0/2 values for Cu-14.8at%Al alloys is similar to the variations for CRSS of single crystal, especially at low and elevated temperature regions. The shear stress for unlocking of edge dislocations from solute atoms estimated from the Hall-Petch parameters at 293K is found to be 3.5 times larger than CRSS of single crystals, which is thought to be the unlocking stress of screw dislocations. It is considered that the role of twin boundaries is almost equivalent to the grain boundaries, because the multiplicated dislocations pile up against the twin and grain boundaries in the small deformation.

Etch Pit Observation of Dislocations in Early Deformation of Cu-Al Alloy and Pure Mg Polycrystals

In order to make clear the micro-yielding mechanisms of polycrystalline metals including twins, the movement of dislocations in the surface grains of Cu-6.8at%Al alloy and pure Mg polycrystals during the early stages of deformation was directly observed by using etch pit technique. The fresh dislocations multiply from the Frank-Read sources within the grains, and pile up against the twin and grain boundaries of two kinds of specimens. The pile-up dislocations on the primary and/or secondary slip planes are also confirmed in Cu-6.8at%Al alloys. Especially during the compressive loading for pure Mg, the occurrence of deformation twins is remarkable with an increase of strain rate, whereas the distribution of fresh dislocations tends to decrease in the surface grains. The present results suggest that the effect of twin boundaries on micro-yielding is almost equivalent for that of grain boundaries, which act as barriers to moving dislocations even in the pre-yield deformation.

Sub-Grain Size and Hall-Petch Relation in Pure Copper Single Crystals

The effect of sub-grain on the yield stress of pure copper single crystals with the [253] orientation was investigated by using the etch pit technique. The single crystal plates were successfully prepared from the seed crystals, which were produced at the melting temperature of 1473 K by the Bridgeman method. The present investigation confirmed the Hall-Petch relation concerning the effect of sub-grain boundaries on the macroscopic yielding of pure copper. The result derived from the extrapolation of the relationship of critical resolved shear stress (CRSS) and the initial dislocation density and sub-grain size is in good agreement with the evaluation in high purity copper single crystals of low dislocation density.

Thin germanium-carbon alloy layers grown directly on silicon for metal-oxide-semiconductor device applications

We report the growth and characterization of thin (&amp;lt;35nm) germanium-carbon alloy (Ge1−xCx) layers grown directly on Si by ultrahigh-vacuum chemical vapor deposition, with capacitance-voltage and leakage characteristics of the first high-κ/metal gate metal-oxide-semiconductor (MOS) capacitors fabricated on Ge1−xCx. The Ge1−xCx layers have an average C concentration of approximately 1at.% and were obtained using the reaction of CH3GeH3 and GeH4 at a deposition pressure of 5mTorr and growth temperature of 450°C. The Ge1−xCx films were characterized by secondary ion mass spectrometry, atomic force microscopy, x-ray diffraction, and cross-sectional transmission electron microscopy. A modified etch pit technique was used to calculate the threading dislocation density. The x-ray diffraction results showed that the Ge1−xCx layers were partially relaxed. The fabricated MOS capacitors exhibited well-behaved electrical characteristics, demonstrating the feasibility of Ge1−xCx layers on Si for future high-carrier-mobility MOS devices.

Size Effects in LiF Micron-Scale Single Crystals of Low Dislocation Density

AbstractThis study examines the deformation response of 20, 5, and 1 micron diameter samples fabricated by FIB-milling from bulk ultrapure LiF single crystals. The bulk crystals have a very low initial dislocation density as revealed by an etch-pit technique. Two types of &lt;001&gt; microsamples were deformed preferentially by single slip when compressed inside a nanoindentation system. Similar to previously studied FCC-derivative metals, LiF microsamples demonstrate dramatic strengthening achieving the engineering flow stress σ ≈ 650 MPa in 1-μm samples. The stress-diameter dependence obeys a power law, σ ∼ D−m, where m ≈ 0.8. Stochastic variation of flow stress, fast intermittent deformation events (“avalanches”) and highly localized slip bands after avalanches - all characteristic of size effects in metals, are also observed in LiF. Possible dislocation mechanisms of the observed size effects are discussed.

Nitrogen effects on generation and velocity of dislocations in Czochralski-grown silicon

The dynamic properties of individual dislocations in nitrogen (N)-doped Czochralski-grown Si crystals with concentrations up to 6×1015cm−3 were investigated at temperatures of 650–950°C using the etch pit technique and compared with such characteristics of N-free Si crystals. The velocity of dislocations in motion in N-doped crystals was revealed to be unaffected by N doping. It was found that the generation of dislocations from a surface scratch was suppressed in N-doped Si and that the critical stress for dislocation generation increased with N concentration, which is interpreted as being due to dislocation immobilization caused by impurity segregation. N doping is concluded to be effective in the promotion of precipitation of oxygen impurity resulting in immobilization of dislocations.

Dislocation–impurity interaction in Czochralski-grown Si heavily doped with B and Ge

Dynamic interaction between dislocations and impurities in Si is investigated by using X-ray topography and the etch-pit technique from the viewpoint of Czochralski growth of Si crystals. Dislocation generation is effectively suppressed by doping of B and co-doping of B and Ge, which is originating in the immobilization of dislocations by preferential segregation of impurities. Dislocation velocity in motion is enhanced by increasing the concentration in B impurities. Change of the lattice parameter due to the impurity doping leads to the generation of misfit dislocations at the seed/crystal interface.