Dislocation Density Measurements Research Articles

Dislocation density measurement by electron channeling contrast imaging in a scanning electron microscope

We have measured the average dislocation density by electron channeling contrast imaging (ECCI) in a scanning electron microscope under controlled diffraction conditions in a Fe–3 wt.% Si alloy tensile deformed to a macroscopic stress of 500 MPa. Under optimal diffraction conditions, ECCI provides an average dislocation density close to that obtained by bright-field transmission electron microscopy. This result confirms that ECCI is a powerful technique for determining dislocation densities in deformed bulk metals.

Growth and characterization of multicrystalline silicon ingots by directional solidification for solar cell applications

This study is focussed on the growth of multicrystalline silicon ingots with large grains by controlling the silicon melt cooling rate to initiate dendritic nucleation in the initial stage of the solidification. Two ingots were grown with different undercooling rates and compared with a reference ingot grown by standard cooling conditions. All ingots were grown in a lab scale directional solidification system. The wafers cut from all three ingots have been characterized for resistivity, minority carrier lifetime and dislocation density measurements by four point probe, quasi steady state photo conductance and PV Scan, respectively. The wafers were converted into solar cells and their electrical parameters have been measured. The cells fabricated from ingot 2 show slightly higher efficiencies in comparison with ingot 1 and the reference one. The present cooling rate was not enough to initiate the dendrite nucleation in the beginning of the solidification. Hence, there is no significant difference was observed in the crystal quality of the grown ingots 1 and 2.

Evaluation of Dislocation Accumulation and Work Hardening in Ferritic Steels Containing Plastically Deformable Soft Particles

The work hardening behavior and the change in dislocation density during tensile deformation were investigated in ferritic steels containing hard carbide/oxide particles or soft Cu particles with different volume fraction and particle diameter. The magnitude of work hardening were significantly different between both types of steels. In the hard particle dispersion steels, the results of dislocation density measurements agreed well with the theory estimating GN dislocation density proposed by Ashby. On the other hand, in soft Cu particle dispersion steels, the increase in dislocation density during tensile deformation was relatively small compared with the hard particle dispersion steels, resulting in the smaller work hardening rate. This is due to the plastic deformation of Cu particle itself leading to the partial accommodation of the shear strain given in the existence area of particle. A quantitative evaluation of accumulation of GN dislocations was attempted in the soft Cu particle dispersion steels by modifying Ashby's theory using ‘particle plastic accommodation parameter’. With this modification, it was found the work hardening behavior in the early stage of tensile deformation of ferritic steels could be systematically explained by the accumulation of GN dislocations regardless of the stiffness of dispersion particles.

Techniques and Applications of the Simulated Pattern Adaptation of Wilkinson’s Method for Advanced Microstructure Analysis and Characterization of Plastic Deformation

Electron Backscatter Diffraction (EBSD) based Orientation Imaging Microscopy (OIM) is used routinely at ~500 materials laboratories worldwide for the characterization and development of diverse crystalline materials. Statistically significant data sets (~107 individual EBSD measurements) can be collected and analyzed within time periods of acceptable beam stability (~105s). However, limitations in angular and spatial resolution have motivated a continued search for more robust EBSD-based methods. Herein is a gathered presentation of advanced techniques in use, intended as a guide to researchers in selecting the most appropriate method for their work. Wilkinson’s method has been shown to increase angular resolution nearly two orders of magnitude to ±0.006°, facilitating measurement of elastic strain, lattice curvature, and dislocation density. A simulated pattern adaptation of Wilkinson’s method extends these measurement capabilities to polycrystalline materials, by avoiding the need for an experimental strain free reference pattern. The angular resolution limit obtained is ~0.04°. Accurate pattern center calibration, essential to the high resolution methods, is accomplished by parallelization of band edges projected onto a sphere centered at the interaction volume. FFT powered cross-correlation functions improve the spatial resolution near grain boundaries and correct for measurement inaccuracies induced by overlapping patterns. To corroborate these claims, exemplary results taken from a wedge-indented nickel single crystal, cold-worked copper polycrystal, and rolled nickel polycrystal are shown.

Grain Size Dependence of the Flow Stress of TWIP Steel

The effect of grain size on the flow stress in TWinning Induced Plasticity (TWIP) steel was investigated via the X-ray diffraction (XRD) measurements of dislocation density. The results indicated that the hardening behavior of fine grained samples (mean grain sizes in the range of 2.1-3.8μm) can be described as typical dislocation interactions. However in coarse grained samples (mean grain sizes in the range of 4.7-38.5μm) where extensive mechanical twinning occurs, another strengthening mechanism is required. Consequently, the effect of grain size on the flow stress parameters of the proposed equation was considered and it was found that in the fine grained samples, the Holloman analysis can describe the hardening behavior. However, in coarse grained samples, a second hardening term due to the strengthening effect of mechanical twin boundaries needs to be added to the Holloman equation.

Two- and Three-Dimensional EBSD Measurement of Dislocation Density in Deformed Structures

Electron backscatter diffraction (EBSD) techniques have been used to measure the dislocation density tensor for various materials. Orientation data are typically obtained over a planar array of measurement positions and the minimum dislocation content required to produce the observed lattice curvature is calculated as the geometrically necessary (or excess) dislocation density. The present work shows a comparison of measurements in two-dimensions and three-dimensions using a dual beam instrument (focused ion beam, electron beam) to obtain the data. The two-dimensional estimate is obviously lower than that obtained from three-dimensional data since the 2D analysis necessarily assumes that the third dimension has no curvature in the lattice. Effects of the free-surface on EBSD measurements are discussed in conjunction with comparisons against X-ray microdiffraction experiments and a discrete dislocation dynamics model. It is observed that the EBSD measurements are sensitive to free-surface effects that may yield dislocation density observations that are not consistent with that of the bulk material.

Flow stress analysis of TWIP steel via the XRD measurement of dislocation density

In this study, the rate of dislocation accumulation in the tensile strained twinning induced plasticity (TWIP) steel was calculated via the X-ray diffraction (XRD) measurements and compared with other fcc metals and alloys. The results indicated that the XRD technique is an alternative method to estimate the dislocation density. Moreover, flow stress analysis of Fe–31Mn–3Al–3Si TWIP steel with the grain size of about 18 μm indicated that, beside a direct effect of the dislocation interactions on the flow stress, another strengthening mechanism is also required to describe the flow behavior. For this reason, the strengthening contribution due to the formation of mechanical twins was considered as a reduction of dislocation mean free path. Interestingly, the estimated flow stress equation consisting of the strengthening effects of both dislocation interactions and dynamic microstructure refinement due to mechanical twinning (i.e., the dynamic Hall–Petch effect) are in good agreement with the experimental data and equation proposed by Ludwigson for low SFE materials.

In-plane anisotropic transport in the 2DEG with a strong spin–orbit coupling in In 0.75Ga 0.25As/In 0.75Al 0.25As hetero-junctions

In-plane anisotropic transport, including the anisotropies of mobility ( μ e) as well as of Rashba spin–orbit (SO) coupling constant ( α) in inverted In 0.75Ga 0.25As/In 0.75Al 0.25As hetero-junction two-dimensional electron gas (2DEG) are observed and discussed. As large as 30% difference between the mobilities, μ e [1¯ 1 0] and μ e [1 1 0], was confirmed and up to 10% anisotropy of α was likely observed. Since there remains almost no strain at the 2DEG interface, the mobility anisotropy may be attributed to the anisotropies of undulation period of surface morphology and of mosaicity, the direct measure of the threading dislocation density. The anisotropy observed in α is, however, not acceptable because the parameter is determined from the isotropic 2DEG densities estimated from magneto-resistance (MR) measurements. So that, the accuracy of the analysis adopted for determining α is pointed out and the necessity of alternative analysis method is suggested.

High-quality metamorphic compositionally graded InGaAs buffers

We have investigated the use of a continuous, linear grading scheme for compositionally graded (metamorphic) In x Ga 1− x As buffers on GaAs grown using MOCVD, which can be used as virtual substrates for optical emitters operating at wavelengths >1.2 μm. Graded buffer quality, as quantified by threading dislocation density (TDD) measurements, was investigated for a range of different graded buffer designs and growth parameters. The best graded buffers obtained had TDD<9.5×10 4 cm −2, at a final composition of x=0.346. MOCVD reactor configuration was found to play a key role in obtaining the best graded buffers. Photoluminescence (PL) measurements were carried out on doped and undoped quantum-well separate confinement heterostructures (QW-SCH) that were re-grown on these buffers. The results showed that these buffers can serve as high-quality strain-relaxed templates for optoelectronic devices operating in the 1.2–1.5 μm wavelength region, and it is expected that with further refinement, high-quality virtual substrates can be made that will allow operation even beyond 1.6 μm.

X-Ray and Neutron Diffraction Measurements of Dislocation Density and Subgrain Size in a Friction-Stir-Welded Aluminum Alloy

The dislocation density and subgrain size were determined in the base material and friction-stir welds of 6061-T6 aluminum alloy. High-resolution X-ray diffraction measurement was performed in the base material. The result of the line profile analysis of the X-ray diffraction peak shows that the dislocation density is about 4.5 × 1014 m−2 and the subgrain size is about 200 nm. Meanwhile, neutron diffraction measurements have been performed to observe the diffraction peaks during friction-stir welding (FSW). The deep penetration capability of the neutron enables us to measure the peaks from the midplane of the Al plate underneath the tool shoulder of the friction-stir welds. The peak broadening analysis result using the Williamson–Hall method shows the dislocation density of about 3.2 × 1015 m−2 and subgrain size of about 160 nm. The significant increase of the dislocation density is likely due to the severe plastic deformation during FSW. This study provides an insight into understanding the transient behavior of the microstructure under severe thermomechanical deformation.

Grain structure and dislocation density measurements in a friction-stir welded aluminum alloy using X-ray peak profile analysis

The dislocation density and grain structure of a friction-stir welded 6061-T6 aluminum alloy were determined as a function of distance from the weld centerline using high-resolution micro-beam X-ray diffraction. The results of the X-ray peak profile analysis show that the dislocation density is about 1.2 × 10 14 m −2 inside and 4.8 × 10 14 m −2 outside of the weld region. The average subgrain size is about 180 nm in both regions. Compared to the base material, the dislocation density was significantly decreased in the dynamic recrystallized zone of the friction-stir welds, which is in good correlation with the TEM observations. The influence of the dislocation density on the strain hardening behavior during tensile deformation is also discussed.

110 中性子回折による集合組織と応力分布、粒径、転位密度の同時測定(超塑性と材料および造形法)

110 中性子回折による集合組織と応力分布、粒径、転位密度の同時測定(超塑性と材料および造形法)

Sub‐Surface Damage in Ground and Annealed Alumina and Alumina–Silicon Carbide Nanocomposites

High‐resolution, grazing incidence, X‐ray powder diffraction has been used to probe the strain dispersion (microstrain) and diffracting, domain size as a function of depth in alumina and alumina–silicon carbide nanocomposite. The microstrain was high, and of similar magnitude, below ground surfaces in both materials but declined much more rapidly with distance from the surface in the pure alumina than in the nanocomposite. The depth over which significant microstrain was observed was in good agreement with previous measurement of dislocation density by transmission electron microscopy. In neither case was there a systematic trend in diffracting domain size with depth. The strain dispersion at the surface was reduced to that in the bulk of the nanocomposite on annealing, the value being almost constant with depth while the diffracting domain size changed significantly, resulting in a linear decrease as the surface was approached. We suggest that the increase in the strength of the nanocomposite is associated both with a macrostrain oriented normal to the grinding direction and the greater depth of propagation of the microstrain. Further strength increase on annealing must be associated with the observed reduction of both these types of strain.

Measuring stacking fault densities in shock-compressed FCC crystals using in situ x-raydiffraction

A method is presented of in situ measurements of stacking fault densities in shockedface-centred-cubic (FCC) crystals using x-ray diffraction. Using results from both thesecond and fourth diffraction orders, wherein shifts in the Bragg peaks due to faulting areaccounted for, we calculated fault densities present in a molecular dynamics (MD)simulation of shocked single crystal of copper. The results are in good quantitativeagreement with dislocation density measurements inferred directly from the MDsimulation. The x-ray diffraction method thus presents a real possibility for experimentaldetermination in real time of dislocation densities in crystals during shock wave passage.

High depth nondestructive stress measurements on thick steel alloys

Stress measurements were performed using accelerator-based γ-ray induced positron annihilation spectroscopy technique, which allows probing of defects at high depths in thick materials up to several centimeters. Induced stresses due to tensile, fatigue, cold work, and bending tests were investigated in steel alloys of about 1-cm thickness. The measurements showed the dependence of the line-shape parameter of the annihilation peak S on the induced deformation in the four tests. They also revealed an interesting behavior for the change of S parameter with tensile deformation, related to the engineering stress-strain curve of the material. Transmission electron microscopy measurements of dislocation density in cold work deformation suggested that the saturation of positron annihilation parameters often observed in cold work data is not due to compete positron trapping at defects. It was also shown that the S parameter has a weak sensitivity and quickly saturates in fatigue test when compared with the other mechanical tests, which was interpreted as being due to the brittle nature of fatigue failure and the very little plastic deformation associated with it.

Change in Ultrasonic Parameters and Dislocation Structures during Fatigue Process of Aluminum Alloy under High Stress Amplitude

Aluminum alloy (A2024-T3) specimens were used for a fatigue testing by subjecting them to a stress amplitude of 150 MPa. At different numbers of the fatigue cycles, specimens were removed from the fatigue tester, and then subjected to the ultrasonic measurement, the dislocation density measurement, the hardness testing, and so on. From the Fourier spectrum of the bottom echo, the peak intensity (PI), the peak frequency (PF) and the average gradient of the transfer function (AGTF) were obtained. The dislocation density was obtained by the X-ray diffraction analysis. AGTF, the dislocation density and the hardness decreased at the initial stage, and then gradually increased with the increasing number of the fatigue cycles. PI showed a tendency to increase as fatigue cycles increased, but no change occurred in PF. The change of ultrasonic parameters in the fatigue process was quantitatively discussed according to Granato and Lücke dislocation-string model. Then the in-process ultrasonic measurement was carried out in the fatigue testing of the aluminum alloy by using the water bag to obtain ultrasonic parameters. PI and AGTF rapidly increased at the initial stage, and then gradually increased with increasing number of the fatigue cycles. These results suggest that the dislocation density steeply increased at the initial stage of the fatigue process, and then they gradually increased.

Shear deformation experiments of forsterite at 11 GPa - 1400C in the multianvil apparatus

Synthetic forsterite samples were shear-deformed at 11 GPa, 1400°C in the multianvil apparatus. The deformation microstructures have been characterised by SEM, EBSD, X-ray diffraction peak broadening and strain anisotropy analysis, and TEM. Different time durations have been characterised with a view to follow the evolution of strain and stress in high-pressure deformation experiments. A high density of [001] dislocations is introduced during pressurization at room temperature although no significant macroscopic shear or crystal preferred orientations are induced at this stage. The deviatoric stress is probably on the order of 1.5 GPa. Heating at 1400°C leads to a rapid decrease of the density of these dislocations. The shear deformation at high-temperature leads to measurable strain and development of crystal preferred orientations after one hour. Stress and strain-rate continue to decrease with time, such that eight hour experiments exhibit microstructures where recovery is apparent. At this stage, the stress level is estimated at ca. 100 MPa from dislocation density measurements. Crystal preferred orientations and TEM characterisation show that glide of [001] dislocations on (100) or (010) is the dominant deformation mechanism. Further investigation is needed to determine whether inhibition of [100] glide in these experiments is due to the role of water or whether a physical effect of pressure is also contributing.

Evidence of scuffing initiation by adiabatic shear instability

A mechanism for the initiation of scuffing based on adiabatic shear instability was assessed experimentally through the measurement of dislocation densities by X-ray diffraction. Scuffing was predicted to occur when the rate of local thermal softening exceeded that of work hardening in the tribological contact. Since these rates were dependent on microstructure, the theory was tested by performing scuffing tests on SAE 4340 steel subjected to five different heat treatments. The scuffing resistances were consistent with the theory, and thermal events during the tests and the resulting microstructural morphologies were consistent with a locally adiabatic environment. The dislocation densities measured in samples from tests stopped before, during, and after scuffing were consistent with the theory. Use of the Advanced Photon Source (APS) at Argonne National Laboratory was supported by the U.S. Department of Energy under Contract W-31-109-ENG-38.

Dislocation, annealing and quenching effects on the microindentation hardness of Bi 2Te 3 and Bi 2Te 2.9Se .1 single crystals

Stoichiometric crystals of Bi 2Te 3 and Bi 2Te 2.9Se .1 have been grown from the melt by the horizontal zone melting (HZM) technique. X-ray powder diffraction and energy dispersive X-ray analysis could prove that the grown crystals are stoichiometric with lattice constants a=0.4374 nm, c=3.044 nm for Bi 2Te 3 and a=0.4374 nm, c=3.038 nm for Bi 2Te 2.9Se .1. Dislocation density measurements are carried out by etch pit technique and are observed by scanning electron and optical micrograph. The mechanical strength of the as-grown, quenched and annealed crystals is assessed by the Vickers hardness measurements.

Dual-band infrared detectors made on high-quality HgCdTe epilayers grown by molecular beam epitaxy on CdZnTe or CdTe/Ge substrates

In this paper, we present all the successive steps for realizing dual-band infrared detectors operating in the mid-wavelength infrared (MWIR) band. High crystalline quality HgCdTe multilayer stacks have been grown by molecular beam epitaxy (MBE) on CdZnTe and CdTe/Ge substrates. Material characterization in the light of high-resolution x-ray diffraction (HRXRD) results and dislocation density measurements are exposed in detail. These characterizations show some striking differences between structures grown on the two kinds of substrates. Device processing and readout circuit for 128×128 focal-plane array (FPA) fabrication are described. The electro-optical characteristics of the devices show that devices grown on Ge match those grown on CdZnTe substrates in terms of responsivity, noise measurements, and operability.