Papers by Keyword: Dislocation Density

Abstract: It is well known that the work hardening process of low-carbon steels is highly dependent on the movement and accumulation of dislocations in the crystal grains, which affect the stress and strain magnitudes and their distribution. The aim of this paper is to explain the importance of dislocation movement and density on the temperature, i.e. stress and strain changes during cold plastic deformation of low-carbon steels. Therefore, tests were carried out in this paper using the methods of static tensile testing, thermography, digital image correlation (DIC) and microstructural analysis. The microstructure analysis was carried out using a light and transmission electron microscope (TEM). The transmission electron microscope analysis was performed in two different modes, the TEM and scanning TEM (STEM). The results of static tensile testing, thermography and digital image correlation (DIC) are related to the microstructural changes that occur during the work hardening process of low-carbon steel. At the moment of maximum work hardening (immediately before fracture), significant grain elongation and high dislocation density of low-carbon steel were observed.

Abstract: Using Sol-Gel and impregnation processes, the GaNFe₂O₃ and GaNFe₂O₃-PPY nanocomposites were synthesized with varying concentrations of PPY 3%, 10%, and 30% by weight. Using the Sci Finder software could not trace any report in the literature for this synthesized Ga_(2x+2)NFe_2(49-x)O₃-PPY nanocomposites. The prepared gallium nitride ferrite and gallium nitride ferrite-Polypyrrole samples were subjected to structural analysis using X-ray diffraction. The X-Ray diffraction characterization confirmed Nano state formation. From the XRD spectra the dislocation density, average crystallite size, number of unit cells, and porosity were calculated and analyzed . It has been observed that with increment of unit cells and dopant concentration there is a decrease of dislocation density of gallium nitride ferrite. When the concentration of PPY is increased in gallium nitride ferrite-Polypyrrole nano composites, the dislocation density increases and the number of unit cells decreases. The porosity is increased as the concentration of PPY is increased from 3%,10% to 30% when compared to GaNFe₂O₃ nano ferrites.

Abstract: Increasing the operational reliability and durability of parts and mechanisms used to operate under conditions of intense wear, loads, high pressure and temperatures requires the protection of working surfaces with functional coatings. The E.O. Paton Institute of Electric Welding of the National Academy of Sciences of Ukraine has developed a technology and equipment for multichamber detonation spraying (MCDS) of these coatings. This paper summarizes the data of experimental studies of the structure of composite coatings of various systems (Ni–Cr–Fe–B–Si, Cr₃C₂–NiCr, WC–Co–Cr, ZrSiO₄, and Al₂O₃) for different materials. Research has established the influence of technological modes of spraying on structural and phase changes in the coating materials obtained by detonation spraying. Under different processing modes, the materials change volume fraction of phase components, microhardness, (sub)grain structure parameters, size of dispersed phases, and nature and distribution of dislocation density. The peculiarity of the structure of coatings obtained by the MCDS method is the formation of a dispersed structure, the presence of a nanoscale substructure and nanoparticles of hardening phases with a size of 10–100 nm. The formation of a nanostructural state contributes to an increase in the strength, fracture toughness, and crack resistance of coatings obtained by the MCDS method.

Abstract: The essential task of modern industry is to increase the reliability and durability of products. One of the promising ways to increase the products operational durability is the ceramics and cermets powders functional coatings application to the working surfaces by the method of high-speed multi-chamber detonation spraying. The objective of the given paper is to determine the regularities of the influence of structural-phase features in the formed material of functional cermets coatings of metal parts on their strength characteristics and crack resistance, while taking into account structural criteria that will provide the required set of strength and crack resistance properties. The method for deep and detailed analysis of the structure features, morphology and distribution of phase particles, their stoichiometric composition, substructure parameters, dislocation density in the coating material obtained due to multi-chamber detonation spraying has been developed in this paper. The research of detonation coating material was carried out using transmission electron microscopy (TEM) on JEM-200CX instrument (produced by JEOL company) with the accelerating voltage of up to 200 kV. The prospects of using the method of multi-chamber detonation spraying on various materials (steel, copper, aluminum, titanium) and alloys are shown on the basis of the research results. A number of composite coatings made of aluminum and zirconium ceramics; chromium, tungsten carbides have been obtained. It has been defined that changes occur in the ratio of the following parameters: microhardness, pore volume fraction, phase composition, distribution of dispersed phases, grain, subgrain, dislocation structures, etc, under different processing modes in the surface layers and corresponding change in the modes of detonation spraying.

Abstract: In order to get the insights about microstructural changes that occurs under the thermo-mechanical processing conditions, the physics based modelling approach is very useful. Therefore, the flow curves of alloy 718 are theoretical simulated using a dislocation density dependent constitutive model for different conditions. Presented model considers the microstructural ingredients that are immobile dislocation density, effective grain size and dislocation cell size as the variables to address the creep. The simulated flow curves show a good agreement with the experimental flow curves. The magnitude of immobile dislocation density and dislocation cell size in between 3.87× 10¹⁴ - 3.87× 10^14­ m^-2 and 8.29-8.45 μm, respectively, at the completion of the simulation. Furthermore, this approach also provides the possibility to quantify and depict the variation in each strengthening contributions.

Abstract: EUROFER97 martensitic steel is recognized in EU as the reference material for the test blanket module in ITER reactor and for structural sections subject to high radiation doses in DEMO reactor. An extended experimental campaign has been carried out with the scope of improving strength without loss of ductility. The main idea behind the present study is to reach the goal through grain refinement achieved by cold rolling and heat treatments for inducing recrystallization of the work hardened structure. A combination of five cold rolling reduction ratios (CR) (20%, 40%, 50%, 60%, 80%) and eight heat treatments in the temperature range 400-750°C (steps of 50 °C) with soaking time of 1 hour has been examined to describe the evolution of microstructure and mechanical properties. The strength of deformed samples decreases as the heat treatment temperature increases and the change is more pronounced in the samples cold-rolled with higher CR ratios. The results showed that cold rolling with CR of 80% followed by a treatment at 650 °C produces a fully recrystallized structure with sub-micrometric grains which guarantees improved yield stress and hardness than standard EUROFER97 steel, with a comparable total elongation. In conclusion, this work demonstrated the feasibility to strengthen EUROFER97 without compromising its ductility.

Abstract: This investigation estimated porosity and dislocation density in austenitic stainless steel 316LSi thin walls fabricated by Cold Metal Transfer Wire and Arc Additive Manufacturing (CMT-WAAM). Porosity density was calculated using ImageJ software. MAUD software (Materials Analysis Using Diffraction) was used to analyze the microstructural parameters and dislocation density. The density of pores and microstructural parameters of 316LSi alloy exhibit typical values of AM conditions. The porosity values oscillate between 2.80 to 3.48 %. The obtained dislocation density values are 5.0 e+12, 4.3 e+12, and 3.2 e+12 for 2.4 e+12 m^-2 for 70, 80, 90, and 140 A current source, respectively. In 316LSi thin walls, the increases in the current input in CMT-WAAM are accompanied by the very lowest decrease in the dislocation density state.

Abstract: Photoluminescence (PL) signatures of 4H-SiC bare and epitaxial wafers from a surface inspection tool have been studied. Large variations in PL black or white dot densities were confirmed for comparable crystal quality and growth process conditions. Comparison with KOH etching results confirms that both PL black and white dots are tied to discrete threading dislocations. PL spectra results suggest dislocation decoration by donor-acceptor pairs.

Abstract: Post-growth thermal processing at higher temperature generates more BPDs (basal plane dislocations). It is observed that dislocation visibility in surface inspection tool images varies significantly even at comparable dislocation densities. Combination of dislocation decoration and light absorbance from SiC matrix by point defects or dopants has been proposed as a working hypothesis to explain dislocation visibility variations.

Abstract: Analysis of structural factor influence on local internal stresses and zones of deformation localization in upper and lower bainite structures in welded joints of low-alloy steel at wet underwater welding was performed. It is established that when welding joints under the water and applying an external electromagnetic field in the metal of the heat-affected zone (HAZ), a finer-grained substructure is formed with a general decrease in the dislocations density and with their uniform distribution. Estimates of the local internal stresses level considering the dislocation density distribution in the structural zones of their localization show that their maximum level is formed in the metal of the HAZ overheating region at welding without the external electromagnetic field along the upper bainite laths boundaries. The upper bainite structure is characterized by forming localized deformation zones, where the most significant dislocation density gradients are observed. This can lower the crack resistance of welded joints. Low values of local internal stresses are characteristic of welded joints obtained in the modes applying an external electromagnetic field. This is facilitated by the overall decrease in the dislocation density and their uniform distribution in the lower bainite structural components, which provides high crack resistance of welded joints.