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The high crystal quality and low dislocation densities of aluminum nitride (AlN) grown on flat and nano-patterned sapphire substrate that are synthesized by the metal-organic chemical vapor deposition (MOCVD) method are essential for the realization of high-efficiency deep ultraviolet light-emitting diodes. The micro-strains of 0.18 × 10−3 cm−2 for flat substrate AlN and 0.11 × 10−3 cm−2 for nano-patterned substrate AlN are obtained by X-ray diffractometer (XRD). The screw and edge dislocation densities of samples are determined by XRD and transmission electron microscope (TEM), and the results indicate that the nano-patterned substrates are effective in reducing the threading dislocation density. The mechanism of the variation of the threading dislocation in AlN films grown on flat and nano-patterned substrates is investigated comparatively. The etch pit density (EPD) determined by preferential chemical etching is about 1.04 × 108 cm−2 for AlN grown on a nano-patterned substrate, which is slightly smaller than the results obtained by XRD and TEM investigation. Three types of etch pits with different sizes are all revealed on the AlN surface using the hot KOH etching method.

The fatigue behaviour of notched and unnotched specimens produced by additively manufactured Inconel 718 were analysed in the as‐built and heat‐treated conditions. The surfaces display high roughness and defects acting as fatigue initiation sites. In the as‐built condition, fine subgrains were found, while in in the heat‐treated state, the subgrains were removed and the dislocation density recovered. SN‐curves are predicted based on tensile properties, hardness and defects obtained by fractography, using the urn:x-wiley:ffe:media:ffe13300:ffe13300-math-0001‐method. © 2020 The Authors. Fatigue & Fracture of Engineering Materials & Structures published by John Wiley & Sons Ltd This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

This paper presents the results of the study of the effect of irradiation temperature on structural and optical distortions and deformations, as well as the strength properties of BeO ceramics as a result of irradiation with Ar8+ and Xe22+ ions at a radiation dose of 5 × 1013 cm-2. The choice of radiation dose is due to the effect of overlapping defective areas arising along the trajectories of ions in ceramics, which makes it possible to model radiation damage caused by the effect of accumulation as a result of cascade collisions and overlapping damaged areas. The temperature range of 300–1000 K was chosen to simulate different operating conditions, as well as the possibility of simulating partial annealing of defects during irradiation at high temperatures. During the research, it was established that high-temperature radiation reduces influence of size of electronic and nuclear power losses of ions of Ar8+ and Xe22+ with energy of 70 MeV and 231 MeV, respectively, on extent of radiation damage of ceramics of BeO. Irradiation at a temperature of 1000 K results in an equal 14% change in dislocation density for these particles, a comparable decrease in the yield intensity of optically stimulated luminescence by 5% and 15%, as well as microhardness by 25% and 30%, respectively. © 2021, The Author(s), under exclusive licence to Springer Science+Business Media, LLC, part of Springer Nature. This research was funded by the Science Committee of the Ministry of Education and Science of the Republic of Kazakhstan (No. AP08855828).

We describe the analysis of in-situ HT-XRD data of a dual phase stainless steel exposed to a complex thermal cycle of heating, holding and cooling. For the conditions used only low quality diffraction data could be collected. Peak positions, peak areas and peak broadening are modeled by the Rietveld method. The low signal-to noise ratio and the presence of artificial peaks due to tube tails complicate the data evaluation. In a first attempt the parameters are refined by a local optimization procedure (e.g. Levenberg-Marquardt). However, this procedure fails by being caught in one of several local minima. Next, a Bayesian approach with a Markov Chain Monte Carlo (MCMC) algorithm is used as a global optimization procedure to refine the simulated Rietveld diffractograms. Accurate estimates of the evolution of the phase fractions and dislocation densities in martensite and austenite during all stages of the thermal cycle are obtained by this MCMC algorithm. While an approach based on multivariate second order Taylor series completely underestimates the error, the uncertainties in the model parameters could be estimated appropriately from histograms obtained by the MCMC method.

The analysis of line broadening in X-ray and neutron diffraction patterns using profile functions constructed on the basis of well-established physical principles and TEM observations of lattice defects has proven to be a powerful tool for characterizing microstructures in crystalline materials. These principles are applied in the convolutional multiple-whole-profile (CMWP) procedure to determine dislocation densities, crystallite size, stacking fault and twin boundary densities, and intergranular strains. The different lattice defect contributions to line broadening are separated by considering the hkl dependence of strain anisotropy, planar defect broadening and peak shifts, and the defect dependent profile shapes. The Levenberg&ndash, Marquardt (LM) peak fitting procedure can be used successfully to determine crystal defect types and densities as long as the diffraction patterns are relatively simple. However, in more complicated cases like hexagonal materials or multiple-phase patterns, using the LM procedure alone may cause uncertainties. Here, we extended the CMWP procedure by including a Monte Carlo statistical method where the LM and a Monte Carlo algorithm were combined in an alternating manner. The updated CMWP procedure eliminated uncertainties and provided global optimized parameters of the microstructure in good correlation with electron microscopy methods.

Particulate nanocomposites with the base of ZA27 alloy were synthesized using an innovative route, which includes mechanical milling and compocasting. Scrap from the matrix alloy and ceramic nanoreinforcements were mechanically milled using the ball-milling technique, which led to the formation of composite microparticles. The use of these particles in the compocasting process provided better wettability of ceramic nanoreinforcements in the semi-solid metal matrix, which resulted in a relatively good dispersion of the nanoreinforcements in nanocomposite castings. The presence of nanoreinforcements led to the grain refinement in the matrix of nanocomposites. The mechanical properties of the synthesized nanocomposites are improved and compared with the properties of the metal matrix. The observed increase in the hardness of nanocomposites with Al2O3 nanoreinforcements (20–30 nm) was 6.5% to 10.8%, while the yield strength of these nanocomposites has increased by 12.2% to 23.2%. The hardness and compressive yield strength of the nanocomposites with Al2O3 nanoparticles (100 nm) increased by 1.7% to 8.0% and 2.3% to 8.3%, respectively. The increase in hardness of the nanocomposites with SiC nanoparticles (50 nm) was 11.5% to 20.6%, while the increase in the yield strength was 15.6% to 24.5%. The greatest contribution to the overall strengthening in the synthesized nanocomposites is the result of increased dislocation density due to the difference in coefficients of thermal expansion for the matrix alloy and nanoreinforcements.

Martensitic steels are widely used structural materials with outstanding mechanical properties. Their high strength is provided by the non-diffusional phase transformation of fcc g into thin lamellar bcc plates during fast cooling. Coherency strains between the fcc and bcc lamellae induce large dislocation densities in the range of 1016 m&minus, 2, well above the densities attainable by conventional plastic deformation. Using high resolution X-ray line profile analysis, scanning electron microscopy, and hardness tests we show that during tensile deformation when the active Burgers vectors are within the lath plane the lath-packets work soften. On the contrary, when the active Burgers vectors are oblique to the lath-plane the lath-packets work harden. The softening and hardening processes in the differently oriented lath-packets produce a composite of hard and soft components on the length scale of lath-packet size. The stress&ndash, strain response of the alloy is discussed in terms of the different mean free paths and the different annihilation lengths of dislocations in the softened and hardened lath-packets. The relatively good ductility is shown to be produced by the composite microstructure induced by plastic strain.

Line profile analysis of X-ray and neutron diffraction patterns is a powerful tool for determining the microstructure of crystalline materials. The Convolutional-Multiple-Whole-Profile (CMWP) procedure is based on physical profile functions for dislocations, domain size, stacking faults and twin boundaries. Order dependence, strain anisotropy, hkl dependent broadening of planar defects and peak shape are used to separate the effect of different lattice defect types. The Marquardt-Levenberg (ML) numerical optimization procedure has been used successfully to determine crystal defect types and densities. However, in more complex cases like hexagonal materials or multiple phases the ML procedure alone reveals uncertainties. In a new approach the ML and a Monte-Carlo statistical method are combined in an alternative manner. The new CMWP procedure eliminates uncertainties and provides globally optimized parameters of the microstructure.

This investigation provides a mechanistic background to mechanical property extractions performed from single-crystal microhardness measurements. The analysis concerns both spherical and sharp (pyramidal) indentations. We show that the uniaxial stress-strain curves inferred from hardness measurements in single crystalline units of material are coincidental with those attained under true uniaxial loadings along specific crystalline orientations. Landmark hardness relations that were originally developed for power-law strain hardening polycrystalline aggregates are extended to single-crystal indentations. Mechanical property extractions in crystals violating power-law hardening because of a marked critical resolved shear stress and/or extreme strain hardening saturation are subsequently addressed. The analysis is pertinent to the assessment of multiple-glide deformation stage-II and strain hardening saturation stage-III of cubic single crystals from indentation experiments. (C) 2015 Elsevier Ltd. All rights reserved.

The grain size dependency of lattice parameter during high-energy ball milling of W has been investigated. The lattice parameter varies non-monotonically with grain size during milling with a lattice contraction initially followed by an expansion. The lattice parameters were calculated in view of the non-equilibrium grain boundary structure that evolved during milling using excess free volume and the interfacial stresses at the grain boundaries. The calculated lattice parameters closely match the experimentally observed values. � 2014 Acta Materialia Inc.

Copper–indium–gallium–disulphide (CuInGaS2) is a promising absorber material for thin film photovoltaic. In this paper, CuInGaS2 (CIGS) thin films have been prepared by chemical spray pyrolysis method onto glass substrates at ambient atmosphere. Structural, morphological, optical and electrical properties of CuInGaS2 films were analysed by X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), UV–Vis spectrophotometer and Hall Effect measurement, respectively. The films exhibited single phase chalcopyrite structure. The strain and dislocation density decreased with increase of spray time. The grain size of the films increased from 4.45 to 9.01 nm with increase of spray time. The Raman spectrum indicated the presence of the principal chalcopyrite peak at 295 cm $$^{-1}$$ . The optical properties of the synthesized films have been carried out through the measurement of the absorbance spectrum. The optical band gap was estimated by the absorption spectrum fitting (ASF) method. For each sample, the width of the band tail ( $$E_\mathrm{Tail}$$ ) of CuInGaS2 thin films was determined. The resistivity ( $$\rho$$ ), conductivity ( $$\sigma$$ ), mobility ( $$\mu$$ ), carrier concentration and conduction type of the films were determined using Hall Effect measurements. The interesting optical properties of CuInGaS2 make them an attractive material for photovoltaic devices.

Microstructural properties of Ce1-x GdxO2-δ (x = 0 to 0.3) thin films prepared by pulsed laser deposition technique were studied. The thin films were deposited on Si(100) substrate at a substrate temperature of 973 K at the oxygen partial pressure of 0.2 Pa using KrF excimer laser with energy of 220 mJ. The prepared thin films were characterized by X-ray diffraction, Raman spectroscopy and atomic force microscopy. X-ray diffraction analysis confirmed the polycrystalline nature of the thin films. Crystallite size, strain and dislocation density were calculated. The Raman studies revealed the formation of Ce-O with the systematic variation of peak intensity and full width half maxima depending on concentration of gadolinium dopant. The thickness of the films was estimated using Talystep profiler. The surface roughness was estiamted based on AFM.

In this paper, we study structural and morphological properties of metal-organic chemical vapour deposition-grown InGaN/GaN light emitting diode (LED) structures with different indium (In) content by means of high-resolution X-ray diffraction, atomic force microscopy (AFM), Fourier transform infrared spectroscopy (FTIR), photoluminescence (PL) and current-voltage characteristic (I-V). We have found out that the tilt and twist angles, lateral and vertical coherence lengths of mosaic blocks, grain size, screw and edge dislocation densities of GaN and InGaN layers, and surface roughness monotonically vary with In content. Mosaic defects obtained due to temperature using reciprocal lattice space map has revealed optimized growth temperature for active InGaN layer of MQW LED. It has been observed in this growth temperature that according to AFM result, LED structure has high crystal dimension, and is rough whereas according to PL and FTIR results, bandgap energy shifted to blue, and energy peak half-width decreased at high values. According to I-V measurements, it was observed that LED reacted against light at optimized temperature. In conclusion, we have seen that InGaN MQW structure's structural, optical and electrical results supported one another. © 2014 Springer Science+Business Media New York.

In this study, Al/Al2O3/B4C nano-composites were fabricated via the accumulative roll bonding (ARB) process. The grain refinement of the Al/Al2O3/B4C nano-composite strips during the ARB process was studied. Microstructural characterizations of the fabricated composites after 2, 5, and 9 cycles were performed by transmission electron microscopy (TEM). The results showed that the composite sample, after 9 cycles, was filled with homogenously distributed ultra fine grains with an average grain size of 230 nm. The findings also revealed that the increase in the dislocation density due to the presence of the nano-sized particles resulted in the grain refinement of the specimens. It was also found that the grain refinement is accelerated by the presence of the refinement particles. © 2013 Elsevier Inc. All rights reserved.

The InxGa1-xN epitaxial layers, with indium (x) concentration changes between 0.16 and 1.00 (InN), were grown on GaN template/(0001) Al2O3 substrate by metal organic chemical vapour deposition. The indium content (x), lattice parameters and strain values in the InGaN layers were calculated from the reciprocal lattice mapping around symmetric (0002) and asymmetric (10-15) reflection of the GaN and InGaN layers. The characteristics of mosaic structures, such as lateral and vertical coherence lengths, tilt and twist angle and heterogeneous strain and dislocation densities (edge and screw dislocations) of the InGaN epilayers and GaN template layers were investigated by using high-resolution X-ray diffraction (HR-XRD) measurements. With a combination of Williamson-Hall (W-H) measurements and the fitting of twist angles, it was found that the indium content in the InGaN epilayers did not strongly effect the mosaic structures' parameters, lateral and vertical coherence lengths, tilt and twist angle, or heterogeneous strain of the InGaN epilayers. © 2013 Springer Science+Business Media New York.

WOS: 000394352600010, High-resistive GaN (> 10(8) a"broken vertical bar cm) layers have been grown with different buffer structures on 6H-SiC substrate using metalorganic chemical vapor deposition reactor. Different combination of the GaN/AlN super lattice, low temperature AlN, high temperature AlN and AlxGa1-xN (x ae 0.67) layers were used in the buffer structures. The growth parameters of the buffer layers were optimized for obtaining a high-resistive GaN epilayer. The mosaic structure parameters, such as lateral and vertical coherence lengths, tilt and twist angle (and heterogeneous strain), and dislocation densities (edge and screw dislocations) of the high-resistive GaN epilayers have been investigated using x-ray diffraction measurements. In addition, the residual stress behaviors in the high-resistive GaN epilayers were determined using both x-ray diffraction and Raman measurements. It was found that the buffer structures between the HR-GaN and SiC substrate have been found to have significant effect on the surface morphology and the mosaic structures parameters. On the other hand, both XRD and Raman results confirmed that there is low residual stress in the high-resistive GaN epilayers grown on different buffer structures., European Union; TUBITAK [105E066, 105A005, 106E198, 106A017]; Turkish Academy of Sciences, This work is supported by the European Union under the projects EU-METAMORPHOSE, EU-PHOREMOST, EU-PHOME, and EU-ECONAM, and TUBITAK under Project Numbers 105E066, 105A005, 106E198, and 106A017. One of the authors (E.O.) also acknowledges partial support from the Turkish Academy of Sciences.

The effect of the Low Energy High Current Pulsed Electron Beam (LEHCPEB) and High Power Pulsed Ion Beam (HPPIB) surface treatment on the dislocation density in iron target has been numerically investigated. At typical technological irradiation parameters the major factors for modification of dislocation density are the thermo-stresses in the heat affected zone. Irradiation regimes, within the process of which there is no melting, generate one maximum of dislocation density, at the same time the regimes resulting in melting produce two maximums of dislocation density: one of them is localized inside the layer which has crystallized from melt, and the other one at the melt-solid interface. The increase of enclosed energy density and the reduction of pulse duration make the excited stress wave even more significant for the dislocation modification. This wave causes hardening of the material in deeper layers which may eventually result in the influence on the whole volume of the target. © 2012 Elsevier B.V.

This paper focuses on the development of a plasticity model to describe high rate deformations of metals. Modeling of target mechanical response is performed in frames of continuum mechanics. Plastic flow is described as the result of an over barrier dislocation sliding in specific slip planes. Computations of shock wave propagation in fcc, bcc and hcp metals modeling in comparison with shock wave experiments are performed to verify the model. The model predicts yield strength increase on elastic precursor in aluminum monocrystal and titanium of high purity at high temperatures. The action on a copper target of the electron beams with energy density (the total energy incident on an unit area during an irradiation pulse) 8.6 J cm-2 and varied pulse duration has been investigated. At the considered irradiation regime the target remains in a solid state (maximal temperature is 710 K) and shear stresses can reach values of about 0.72 GPa. Depth distribution of dislocation density after irradiation has a maximum that is localized on a distance of 10 μm from the irradiated surface and the maximum dislocation density is about 6 × 109 cm-2 in the target. The shortening of the exposure time to 1 ns leads to the increase of the dislocation density. Further reduction of exposure time has a weak effect on the dislocation density because the shear stresses reach a limit. © 2011 Elsevier Ltd. All rights reserved.

Ultra-short powerful electron beam is a suitable tool for producing of high rate deformation in substance. In paper we present a new model of high rate fracture and use this model for numerical investigation of fracture of copper target at irradiation by sub-nanosecond electron beam. In this model, fracture is considered as a time-dependent process of nucleation and growth of opening mode cracks. The nucleation and growth rates are controlled by specific free energy of crack surface which is sole fitted parameter. Plastic deformations, both in cracks vicinity and total in substance, are described in frames of dislocation theory. For verification of the model, we performed simulations of spall fracture at plate impact and at irradiation by high-current electron beam with pulse duration of tens of nanoseconds, and reasonable agreement with experimental data has been demonstrated. Simulations of the sub-nanosecond electron beam action on target indicate that spall fracture of the irradiated target surface is possible. This fracture takes place at the enclosed energy density slightly below the value, which is sufficient for melting of irradiated substance. Fracture threshold energy density does not depend on the origin dislocation density and it increases with the increase of pulse duration. As a result, at long pulse durations (more than ten nanoseconds) the substance melts before fracture. © 2011 Elsevier Ltd.

Grain size refinement by severe surface plastic deformation is one way of improving the surface properties. This paper describes the microstructural evolution due to severe surface plastic deformation by oil jet peening in aluminium alloy, AA6063-T6. Detail characterization of the treated surfaces using X-ray diffraction analysis and transmission electron microscopy revealed the formation of submicron size grains at and near the surface. The nozzle-traveling velocity decides the peening intensity and coverage and affects the surface properties. The specimen peened at low nozzle-traveling velocity exhibited an ultrafine grain size (?210. nm) with high surface hardness (?0.88. GPa), compressive residual stress (-102 � 7. MPa) and dislocation density. The hardness is high at the surface and the depth of hardened layer is ?400?m. Formation of high-density dislocations and associated grain refinement resulted in increased surface hardness. Presence of surface modified layer will be beneficial in improving the fatigue and tribo behavior. � 2010 Elsevier Ltd.

We demonstrate a promising approach for the monolithic integration of Ge-based nanoelectronics and nanophotonics with Silicon: the selective deposition of Ge on Si by Multiple Hydrogen Annealing for Heteroepitaxy (MHAH). Very high quality Ge layers can be selectively integrated on Si CMOS platform with this technique. We confirm the reduction of dislocation density in Ge layers using transmission electron microscope (TEM) analysis and Schottky diode electrical behavior. In addition, the analysis of the growth directions and the geometrical shape of the resulting films based on the growth conditions provide further insight to the selective Ge growth mechanism At 400C, the growth in the direction is dominant while directional facets are observed giving the film a characteristic trapezoidal shape. This is primarily due to the relatively slow growth rate along the direction at these conditions, and it can be explained by surface migration (Fig 1(a)). As the deposition time is increased, the layer forms into a pyramid-like structure. Once the full pyramid is formed, the film thickness does not increase significantly with further growth time. This can explain the difference in the film thicknesses grown in varying dimensions of SiO2 windows. However, in Figure 1(b), a different shape is observed when films are grown at 600C due to high growth rates in both the direction (94 nm/min) and the direction (480 nm/min). This shape is attributed to restricted surface migration by an excessive number of nucleation centers[1]. We analyzed the threading dislocations in the grown films. In Figure 2, it is clearly shown that most of dislocations are at the Ge/Si interface and the surface of the film has low defect density due to the hydrogen annealing[2]. The threading dislocations A, B, and C show that they change direction and terminate at the surface normal to the facet that occurred due to the selective growth. Therefore the density of dislocations is reduced due to combined mechanism of hydrogen annealing and the selective growth. The current-voltage characteristic of the Schottky diode is also an indication of material quality[3]. Figure 3(b) shows the metal semiconductor(MS) (Ti-Ge) Schottky diode I-V characteristics with 400 C and 500 C multistep growth on a10μm x10μm selective area followed by hydrogen annealing. The I-V characteristic of the Ti-Ge junction shows a Schottky diode behavior with decent rectification which is another indication of the low defect density of the selective MHAH-Ge substrate. At -1V, a diode leakage current 10.15nA is observed. In conclusion, high quality Ge can be selectively grown on Si by MHAH with a SiO2 masking pattern. Two different growth mechanisms were observed at 400C and 600C resulting from relative growth rates in and directions. The hydrogen annealing and the selective growth can be used to reduce the dislocation density and the surface roughness. (a) (b)

Two AlInN/AlN/GaN heterostructures with 280-nm- and 400-nm-thick AlN buffer grown on sapphire substrates by metal-organic chemical vapor deposition (MOCVD) have been investigated by x-ray diffraction (XRD), atomic force microscopy (AFM), photoluminescence (PL) and Hall-effect measurements. The symmetric (0002) plane with respect to the asymmetric (10 $$\bar{1}$$ 2) plane in the 280-nm-thick AlN buffer has a higher crystal quality, as opposed to the 400-nm-thick buffer. The thinner buffer improves the crystallinity of both (0002) and (10 $$\bar{1}$$ 2) planes in the GaN layers, it also provides a sizeable reduction in dislocation density of GaN. Furthermore, the lower buffer thickness leads to a good quality surface with an rms roughness of 0.30 nm and a dark spot density of 4.0 × 108 cm−2. The optical and transport properties of the AlInN/AlN/GaN structure with the relatively thin buffer are compatible with the enhancement in its structural quality, as verified by XRD and AFM results.

International audience; During the deformation of low carbon steel by cold-rolling, dislocations are created and stored in grains depending on local crystallographic orientation, deformation, and deformation gradient. Orientation dependent dislocation densities have been estimated from the broadening of X-ray diffraction lines measured on a synchrotron beamline. Different cold-rolling levels (from 30% to 95% thickness reduction) have been considered. It is shown that the present measurements are consistent with the hypothesis of the sole consideration of screw dislocations for the analysis of the data. The presented evolutions show that the dislocation density first increases within the α fiber (={hkl}) and then within the γ fiber (={111}). A comparison with EBSD measurements is done and confirms that the storage of dislocations during the deformation process is orientation dependent and that this dependence is correlated to the cold-rolling level. If we assume that this dislocation density acts as a driving force during recrystallization, these observations can explain the fact that the recrystallization mechanisms are generally different after moderate or large strains.

cited By 8; International audience; Spark plasma sintering of a blend of powders with nanometer and micrometer sized particles yielded to a composite-like nickel microstructure consisting of ultrafine-grained (UFG) and coarse-grained (CG) volumes with the fractions of 36% and 64%, respectively. Microstructure evolution and nanohardness distributions of specimens submitted to impact loading at various velocities between 12 and 50ms-1 were determined. At a velocity of 12ms-1, cracks were formed in the UFG regions but they were stopped by the CG entities. Higher velocities resulted in crack-free microstructures and considerable grain fragmentation within CG regions. X-ray line profile analysis investigations showed a decrease of mean crystallite size from ~104 (initial state) to ~41nm (highest velocity). The dislocation density first increased up to 20ms-1 then it decreased considerably with increasing impact velocity, indicating recovery in the microstructure due to the conversion of plastic work into heat. Accordingly, the average nanohardness decreased with increasing the velocity from 20 to 31ms-1. No difference between the microstructures impacted at 31 and 50ms-1 was observed.

This paper presents a study of microstructure, mechanical properties and corrosion behaviour of a near alpha Ti-5Ta-1.8Nb alloy subjected to cryo-rolling. Grain size decreased with increase in cryo-rolling strain (?cr), from about 5?m in the annealed sample down to ~20nm at a ?cr=2.3. Dislocation density initially increased steeply up to ?cr=0.69 and then decreased with increasing strain up to 2.3. The decrease in the dislocation density was attributed to the formation and interaction of pile-ups resulting in annihilation of dislocations. Hall-Petch analysis showed two distinct slopes, region I, with a rapid increase in stress and region II with a negligible increase in stress with decrease in grain size. This was explained in terms of changes in dislocation density and grain size. Fractography studies revealed dimples and microvoids in the coarse grained alloy but the nanocrystalline alloy revealed a mixture of shear bands at the surface and smaller ductile dimples at the centre. Evaluation of the corrosion behaviour in boiling and concentrated nitric acid in three phases (liquid, condensate and vapour) of the nanocrystalline samples revealed almost no deterioration of the corrosion resistance of the Ti alloy when compared to the solution annealed alloy. � 2014 Elsevier B.V.

Al 0.31Ga 0.69N/AlN/GaN/In xGa 1-xN/GaN heterostructures grown with the metal-organic chemical vapor deposition (MOCVD) technique with different In xGa 1-xN back-barriers with In mole fractions of 0.05 ≤ x ≤ 0.14 were investigated by using XRD measurements. Screw, edge, and total dislocations, In mole fraction of back-barriers, Al mole fraction, and the thicknesses of front-barriers and lattice parameters were calculated. Mixed state dislocations with both edge and screw type dislocations were observed. The effects of the In mole fraction difference in the back-barrier and the effect of the thickness of front-barrier on crystal quality are discussed. With the increasing In mole fraction, an increasing dislocation trend is observed that may be due to the growth temperature difference between ultrathin In xGa 1-xN back-barrier and the surrounding layers. © 2012 Elsevier B.V. All rights reserved.

Atomistic calculations were employed in order to calculate the interaction energy of an edge dislocation with different point defects. The bias factor was calculated by applying a finite element method on the interaction energy landscapes obtained from the atomistic calculations. A comparison of the calculated bias factor with a model based on elasticity theory reveals around 30% discrepancy under conditions representative for electron irradiation at 600 degrees C. Possible reasons are discussed. The bias factor dependence on dislocation density and ambient temperature is presented and discussed. QC 20130718 Generation IV reactor research and development (GENIUS)

Ultrafine-grained materials produced by different severe plastic deformation methods show very high strengths but their tensile ductility is often very low. In the present work, we demonstrate an approach for retaining high strength while recovering ductility in a Cu-3 at.% Ag alloy through cold rolling and short-time annealing. X-ray line profile analysis of cold-rolled and annealed samples reveals the development of a heterogeneous solute atom distribution due to the dissolution of nanosized Ag particles in some regions of the matrix. In regions with higher solute (Ag) content, the high dislocation density present following rolling is stabilized, while in other volumes the dislocation density is decreased. High-resolution scanning electron microscopy confirms the presence of regions of varying Ag content in the matrix. Microstructure analysis of the rolled and annealed samples revealed bimodal grain size, dislocation density and solute Ag distributions as well as nanosized Ag precipitation. The as-rolled samples exhibit high tensile strengths of ?600-700 MPa with negligible uniform elongation (?1%). After short-time annealing the strength decreases only slightly to ?550-620 MPa with significant improvement in uniform elongation (from 1 to 10%); this is mainly attributed to the bimodal microstructure. � 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Nanocrystalline Ni thin films have been produced by direct current electrodeposition with different additives and current density in order to obtain 〈100〉, 〈111〉 and 〈211〉 major fiber textures. The dislocation density, the Burgers vector population and the coherently scattering domain size distribution are determined by high-resolution X-ray diffraction line profile analysis. The substructure parameters are correlated with the strength of the films by using the combined Taylor and Hall–Petch relations. The convolutional multiple whole profile method is used to obtain the substructure parameters in the different coexisting texture components. A strong variation of the dislocation density is observed as a function of the deposition conditions.

Present study endeavors to establish the physical basis of an unprecedented trend in scatter, observed in nonlinear ultrasonic (NLU) parameter, associated with varying degree of crystallographic orientation change across crystallites in a polycrystalline material. It is shown that this scatter arises due to anisotropy in harmonics generation as a result of orientation change of slip systems in polycrystals with respect to the wave propagation direction. A near ? titanium alloy has been taken as a model alloy to demonstrate this effect of crystallographic orientation change vis-�-vis change in the orientation of slip systems. Scale of crystal orientation change is shown to have a strong correlation with the degree of scatter in NLU measurements. Further, the study establishes the dominating effect of the scale of crystalline orientation change on harmonics generation as compared to variation in other microstructural parameters such as dislocation density, interface structure etc. Frequency distribution analysis of the scatter indicates that the distribution depends on the colony size which exhibits a linear correlation with standard deviation value. The dislocation string vibration model has been extended for harmonics generation in polycrystalline aggregates to explain the trend in the scatter during measurement of NLU parameter in the material. � 2012 American Institute of Physics.

International audience; The paper describes a new model of discontinuous dynamic recrystallization (DDRX) which can operate in constant or variable thermomechanical conditions. The model considers the elementary physical phenomena at the grain scale such as strain hardening, recovery, grain boundary migration, and nucleation. The microstructure is represented through a set of representative grains defined by their size and dislocation density. It is linked to a constitutive law giving access to the polycrystal flow stress. Interaction between representative grains and the surrounding material is idealized using a two-site approach whereby two homogeneous equivalent media with different dislocation densities are considered. Topological information is incorporated into the model by prescribing the relative weight of these two equivalent media as a function of their volume fractions. This procedure allows accounting for the well-known necklace structures. The model is applied to the prediction of DDRX in 304 L stainless steel, with parameters identified using an inverse methodology based on a genetic algorithm. Results show good agreement with experimental data at different temperatures and strain rates, predicting recrystallization kinetics, recrystallized grain size and stress-strain curve. Parameters identified with one initial grain size lead to accurate results for another initial grain size without introducing any additional parameter.

Friction stir processing was carried out on commercially pure aluminium, and a detailed microstructural characterisation was performed by electron backscattered diffraction and transmission electron microscopy. Friction stir processing resulted in significant grain refinement with narrow grain size distribution. The microstructure showed fine and equiaxed grains, with some ultrafine grains being also observed. Electron backscattered diffraction studies showed majority of the boundaries to be high angle, confirming the occurrence of dynamic recrystallisation (DRX). Transmission electron microscopy observations revealed dislocation arrangement into subgrain boundaries, grains having different dislocation densities and in different stages/degrees of recovery. Electron backscattered diffraction analysis also revealed a progressive transformation of sub-grain boundaries into high angle grain boundaries. A multimechanism of dynamic recovery, continuous DRX and discontinuous DRX seems to be operating during the process. The microstructure is not affected by changing the rotation speed from 640 to 800 rev min -1, except that the grain size was marginally larger for higher rotational speed. � 2011 Institute of Materials, Minerals and Mining.

An AlN buffer layer and a thick-GaN layer for high-electron-mobility transistors (HEMTs) were grown on sapphire substrate by metal-organic chemical vapor deposition (MOCVD). The structural and morphological properties of the layers were investigated by high resolution X-ray diffraction (HRXRD) and atomic force microscopy (AFM) techniques. The optical quality of the thick-GaN layer was also evaluated in detail by a photoluminescence (PL) measurement. It was found that the AlN buffer layer possesses high crystal quality and an atomically flat surface with a root-mean-square (rms) roughness of 0.16 nm. The screw-and edge-type dislocation densities of the thick-GaN layer were determined as 5.4 x 10(7) and 5.0 x 10(9) cm(-2) by means of the mosaic crystal model, respectively. It was observed that the GaN layer has a smooth surface with an rms of 0.84 nm. Furthermore, the dark spot density of the GaN surface was estimated as 6.5 x 10(8) cm(-2) over a scan area of 4 mu m(2). Turkish State Planning Organization [2001K120590]; TUBITAKTurkiye Bilimsel ve Teknolojik Arastirma Kurumu (TUBITAK) [105E066, 105A005, 106E198, 106A017]; Turkish Academy of SciencesTurkish Academy of Sciences This work was supported by the Turkish State Planning Organization (Project No. 2001K120590). This work is also supported by TUBITAK under Project Nos. 105E066, 105A005, 106E198, and 106A017. One of the authors (E. O.) also acknowledges partial support from the Turkish Academy of Sciences.

X-ray diffraction line profile analysis is a powerful and convenient method to investigate the microstructural characteristics of plastically deformed materials. The selection of a suitable and reliable analysis method is critical to derive correct microstructural/material property values. Different techniques of the X-ray peak profile analysis have been used to study the microstructural parameters of ball milled iron powders viz., coherent domain size, dislocation density and nature of dislocations. The classical and modified Williamson-Hall technique and momentum method are used to derive microstructural information. The stored energy estimates were obtained using differential scanning calorimetry while the morphological details are studied with a scanning electron microscope. The domain size decreases to about 270 nm and dislocation density increases to 3.6 � 1015 m-2 after 20 h of milling. SEM results supplements the XRD results. Stored energy of deformation also increases to 171 J/g after 20 h of milling. � 2010 Elsevier B.V. All rights reserved.

International audience; In order to investigate the impact of microstructures and deformation mechanisms on the ductility of materials, the criterion based on bifurcation theory first proposed by Rice is applied to elastic-plastic tangent moduli derived from a large strain micromechanical model combined with a self-consistent scale transition scheme. This approach takes into account several microstructural aspects for polycrystalline aggregates: initial and induced textures, dislocation densities, softening mechanisms so that the behavior during complex loading paths can be accurately described. Based on this formulation, Forming Limit Diagrams (FLDs) are derived and compared with a reference model for multiphase steels involving linear and complex loading paths. Furthermore, the effect of various physical and microstructural parameters on the ductility limit of a single-phase steel is qualitatively studied with the aim of helping in the design of new materials.

Uniaxial tensile tests were performed to examine the influence of the precipitation state on the yield strength and work-hardening behavior of AA2219 for different aging treatments. The microstructural observations in four aging treatments (viz. natural aging, underaging, peak aging, and overaging) were made through transmission electron microscopy (TEM) to understand the type of phase or intermediate stages of the phase present (Guinier-Preston (GP) zones, ??, ??, and ?). To characterize the work-hardening behavior, the analysis of the experimental results has focused on two parameters, viz. the initial work-hardening rate ?max (d?/d?) and the slope (d?/d?) of the ?-? plot, which is related to the rate of dynamic recovery. The initial work-hardening rate (?max) first drops as aging proceeds and then increases significantly upon overaging. The large increase in ?max is also associated with a concomitant increase in the slope (d?/d?) of the ?-? curve. The material constants in the differential equation for the dislocation density are evaluated and flow stress vs plastic strain curves are generated using the flow stress contributions from the solid-solution, dislocation, and precipitation hardening. The model predictions are found to be in excellent agreement with the experimental data for a range of precipitation states from underaged (UA) to overaged (OA) conditions. Curves of flow stress due to dislocation hardening with the plastic strain were also generated in the presence of shearable and nonshearable precipitates. � 2009 The Minerals, Metals & Materials Society and ASM International.

In order to investigate the impact of microstructures and deformation mechanisms on the ductility of materials, the criterion first proposed by Rice is applied to elastic–plastic tangent moduli derived from a large strain micromechanical model combined with a self-consistent scale-transition technique. This approach takes into account several microstructural aspects for polycrystalline aggregates: initial and induced textures, dislocation densities as well as softening mechanisms such that the behavior during complex loading paths can be accurately described.In order to significantly reduce the computing time, a new method drawn from viscoplastic formulations is introduced so that the slip system activity can be efficiently determined. The different aspects of the single crystal hardening (self and latent hardening, dislocation storage and annihilation, mean free path, etc.) are taken into account both by the introduction of dislocation densities per slip system as internal variables and the corresponding evolution equations. Comparisons are made with experimental results for single and dual-phase steels involving linear and complex loading paths. Rice’s criterion is then coupled and applied to this constitutive model in order to determine the ellipticity loss of the polycrystalline tangent modulus. This criterion, which does not need any additional “fitting” parameter, is used to build Ellipticity Limit Diagrams (ELDs).; International audience; In order to investigate the impact of microstructures and deformation mechanisms on the ductility of materials, the criterion first proposed by Rice is applied to elastic–plastic tangent moduli derived from a large strain micromechanical model combined with a self-consistent scale-transition technique. This approach takes into account several microstructural aspects for polycrystalline aggregates: initial and induced textures, dislocation densities as well as softening mechanisms such that the behavior during complex loading paths can be accurately described.In order to significantly reduce the computing time, a new method drawn from viscoplastic formulations is introduced so that the slip system activity can be efficiently determined. The different aspects of the single crystal hardening (self and latent hardening, dislocation storage and annihilation, mean free path, etc.) are taken into account both by the introduction of dislocation densities per slip system as internal variables and the corresponding evolution equations. Comparisons are made with experimental results for single and dual-phase steels involving linear and complex loading paths. Rice’s criterion is then coupled and applied to this constitutive model in order to determine the ellipticity loss of the polycrystalline tangent modulus. This criterion, which does not need any additional “fitting” parameter, is used to build Ellipticity Limit Diagrams (ELDs).

Publié suite à une conférence invitée au congrès : Symposium on Texture and Microstructure Analysis of Functionally Graded Materials, Cracow, POLAND, OCT 03-07, 2004 Pas de DOI : http://www.imim.pl/files/archiwum/Vol1_2005/art.nr.6.pdf; International audience; Recent X-ray methods for determining dislocation densities in texture components of deformed metals are described. They are based on the Wilkens' model of a restrictedly random distribution of dislocations and new methods for separating out the respective broadening contributions due to strain and domain size. By appropriate choice of the goniometer angles of a diffractometer one can obtain peaks from selected texture components in a deformed metal. The basic principles for the profile analysis are described and the method then applied, using both laboratory equipment and high energy Synchrotron radiation, to the determination of the orientation dependency of stored energy in cold rolled iron and hot rolled Al-Mg. It is also shown how the results of a Taylor type crystal plasticity calculation can be used to understand anisotropic peak broadening.

Two commercially produced Mo base (“TZM”) alloys are compared. The relation between creep rate and stress is reported for both alloys at 1423 K as a function of the sample location in the as-received 50 mm diameter bar. The data can be fitted to a power law. Stress exponents of 11, 15 and 14 are found for the periphery and the center of the powder-metallurgically prepared TZM alloy and for the center of the vacuum-melted TZM alloy, respectively. Electron microscopical observations after deformation suggest that the capture of dislocations by particles plays a major role in the deformation of these alloys.

Soare and Curtin (Acta Mater. 2008; 56:4091-4101, 4046-4061) have recently developed a model of dynamic strain aging in solute-strengthened alloys. Their constitutive law describes time-dependent solute strengthening using rate equations that can be calibrated using atomistic simulations. In this paper, their material model is incorporated into a continuum finite element simulation, with a view to completing a multi-scale method for predicting the formability of solute-strengthened alloys. The Soare-Curtin model is first re-formulated as a state-variable constitutive law, which is suitable for finite element computations. An efficient numerical procedure is then developed to track the strength distribution of aging mobile and forest dislocations in the solid during deformation. The method is tested by simulating the behavior of a 3D aluminum-magnesium alloy tensile specimen subjected to uniaxial loading at constant nominal strain rate. The model predicts the influence of strain rate on the steady-state flow stress of Al-Mg alloys, but no Portevin-Le Chatelier bands or serrated flow were observed in any of our simulations, and the influence of strain rate on tensile ductility is not predicted correctly. The reasons for this behavior and possible resolutions are discussed. Copyright (C) 2010 John Wiley & Sons, Ltd.

In this paper, the behaviour of the tantalum subjected to a complex range of thermomechanical solicitations is studied and modelled. The objective of this work is the prediction of microstructure evolution of a tantalum part that is cold flow-formed, then stress-relieved and softened by heat treatments. Flow forming is a cold chipless process that elongates and thins the wall of a tubular part by applying a free rotating roller on the wall of the rotating part. (See Figure 1) This process leads to large strain up to 5 and one material point may be deformed at a fluctuating strain rate. To model this process, it is thus recommended to use a constitutive law that takes into account the history of the material. Microstructure evolution is controlled through the whole process by monitoring the state variable, that is, the dislocation density, within a FEM code. The 3D FEM software FORGE2007® is used to model the entire process. The code is enhanced with physical constitutive laws relative to the tantalum and derived from works of Klepaczko and Buy et al. [1,2,3]. The formalism of Klepaczko and Buy et al. is useful since it is based on laws regulating physical mechanisms of dislocations multiplication, annihilation and kinetics of glide depending on strain rate and thermal activation. Static recovery is modelled subsequently with an incremental approach where the softening is related to the dislocation density evolution. A set of thermomechanical experiments and treatments are done to identify the constitutive law and associated microstructure evolution. Compression, dynamic and static torsion tests covering a wide range of strain rates are used to fit the mechanical constitutive law and the dislocation density evolution law both inspired from Klepaczko and Buy et al. works. The same samples are then annealed with variable temperatures and times. Microstructure of annealed samples is characterized by means of micro-hardness. They give information to evaluate the dislocation density evolution. The data is used to model static recovery.0.