Papers by Keyword: Dislocation

Abstract: Data of skilled electroslag remelting of the spent brand AISI 420 steel electrodes on various technologies are provided (with rotation and without rotation of the spent electrode). Are noted, features of influence of rotation of the spent electrode on conditions of the course of process of remelting and crystallization of an ingot. Influence of a way of remelting on a complex of properties of the received ingot is analysed. The analysis of results of researches showed increase in uniformity of the studied properties at realization of technology of electroslag remelting with rotation of the spent electrode on the melting course.

Abstract: The paper describes a cause of the geometric accuracy degradation of slender metallic products during their operation, which consists in the relaxation of residual stresses accumulated while product manufacturing. The most promising technology among the existing technologies of residual stress relaxation is based on the use of vibromechanical vibrations. This technology has high performance and low energy consumption. The reason for the relaxation of residual stresses under the action of vibromechanical vibrations is the gradual accumulation of internal energy, which leads to plastic dislocation shifts upon reaching a critical level. The dislocation motion under the action of vibromechanical vibrations continues until the dislocations take a more compact layout that corresponds to the equilibrium energy state. For technological implementation of the process there was developed a method of ultrasonic stabilization of elastic plates, which serve as sensing elements in differential-pressure transducers. The experimental studies of ultrasonic stabilization are carried out. According to the results of the studies, there have been conducted tests of the microstructure of the control samples. It is established that the samples of steel 20CH13 subject to ultrasonic treatment have a more uniform structure than the samples after the factory technology of long "aging". This confirms the assumption about the structural-energy processes occurring during vibromechanical relaxation of residual stresses. The technology of ultrasonic stabilization can be recommended as a replacement to the "aging" technology on the basis of the obtained results.

Abstract: The generation and transformation of dislocations in 4H-SiC crystals grown by PVT were investigated. Experiments were carried out in two stages for more comprehensive observation on dislocation behaviors. For the first stage known as initial growth, we investigated mainly the seed and grown interface. The behavior and transition of the dislocations in grown crystal were observed along the length of the crystal at second stage. The formation of threading edge dislocations (TEDs) strongly depends on the surface morphologies related with internal temperature gradients during crystal growth. The basal plane dislocation (BPDs) and threading screw dislocation (TSDs) cause from the seed crystal and formed at the initial stage of growth were gradually decreased in number along the length of the crystal and under certain conditions such as distorted stresses, dislocations were converted into other types of dislocations.

Abstract: In this work a deep investigation of the dislocation on 4H-SiC substrate has been shown. The dislocation intersecting the surface were enhanced by KOH etching at 500 deg. C. performed on whole 6 inches substrate. A comparison between basal plane dislocations and threading screw dislocations in the substrate with the defects in the epitaxial layer (mainly stacking faults and carrots) was performed. The comparison between shows a correlation between basal plane dislocations density and stacking faults density maps.

Abstract: Graphene-aluminum (Gr/Al) composite laminated by aluminum (Al) and graphene sheets alternately has excellent mechanical properties thanks to the high strength, high Young’s modulus and the two-dimensional atomic structure of graphene. In this study, the uniaxial tensile properties of Gr/Al nano-laminated composite are studied by molecular dynamics (MD) method. It is found that the thickness of Al layer has a significant effect on the tensile strength and Yang’s modulus of the Gr/Al composite. Composite with a smaller thickness of Al layer shows better properties. Graphene not only block propagation of dislocations, but bear most of the loads, resulting in higher Young's modulus, tensile strength and failure strain of the composites than those of pure Al. The simulation of temperature-effect shows that the Gr/Al composite is difficult to arise plastic deformation at low temperature, which lead to a higher strength and modulus of the composite. In addition, the effect of graphene stacking on the properties of composites is investigated. Through tensile tests at the vertical and parallel interfaces, it is found that graphene stacking may lead to a reduced performance of the composite.

Abstract: The paper presents the results of the study of the stage of accumulation of damage and fatigue rupture of titanium alloys (using the method of acoustic emission). The main object of research was the development of a method for designing a generalized fatigue diagram characterizing the stage of fatigue damage accumulation. The studies aimed at experimental verification of the hypothesis of the stage of damage accumulation, which can be established only by the registered parameters of acoustic emission with separate analysis by types of acoustic emission sources. In contrast to the method of research, which is carried out fractographic analysis, the use of acoustic emission method can significantly reduce the amount of testing. The types of acoustic emission sources on the distribution plane of two-parameter “AE signal energy E_AE vs. frequency parameter K_f” are considered. Fatigue stages in the tests of trial alloys were determined by the activity of the AE signals emitted by different types of AE sources (dislocation, micro - and macro-cracks). A generalized diagram of fatigue developed according to the specified stages. The developed method significantly reduces the volume of fatigue tests and fractographic studies.

Abstract: A variable electric field is applied to a crystal. This field gives rise – through the piezoelectric coupling – to the variable mechanical stresses. Then the dislocations in the crystal will be driven by Peach-Koehler force and will start moving, dissipating the external field energy. Connection of the electric field energy dissipated per unit time with the internal friction is found. The case of resonant loss (Granato-Lucke model) is considered. The loss related to this mechanism to be at frequencies of megahertz range. The relaxation processes being responsible for the Bordoni and Hasiguti peaks also are considered. The use of obtained equations makes it possible to distinguish the dislocation contribution to both dielectric loss and dielectric dispersion and, therefore, to derive additional information about the crystal structure in a sufficiently simple way in terms of only one method.

Abstract: A study was carried out to determine the deform behavior in a polycrystalline Ni-based superalloy based on micropillar compression tests. Three different heat treatments of this alloy were evaluated by systematically controlling the cooling rate from the supersolvus solutioning step, in order to examine the effect of γ' microstructure on the CRSS (Critical Resolved Shear Stress). It is shown that the γ' precipitates have the marked effect on the deform behavior of micropillar, as the size of the secondary γ' in the general microstructure decreased, the CRSS were increased; SEM and TEM examination show that compressive deformation behavior of [110] oriented micropillars is dominated by dislocation plasticity. Cross slip is occurring in different slip planes, and exhibits away slip lines as well as stacking faults across γ/γ' microstructure. The γ' and carbides in this alloy effectively impeded the motion of dislocations and inhibit the strain burst behavior, caused by the running out of dislocations to the surface of micropillars,which leads to the sustained strain hardening the plastic deformation stage of micropillars.

Abstract: In order to provide an accurate prediction of the flow stress during the thermal deformation process, a physical model is established. In the model, it is assumed that the flow stress is mainly composed of short-range stress and long-range stress. The short-range stress is a friction stress needed to be overcome when dislocations pass short-range obstacles and through the lattice, and the long-range stress is athermal stress caused by the interaction of dislocation substructure. The model is established mainly based on the evolution of dislocation density which is described as a competitive process of work hardening and recovery. Meanwhile, the interaction between vacancies and dislocations is also taken into account. The effect of solutes and precipitation on stress is quantified. In addition, some experiments have been performed using two steels containing different amounts of Nb under various deformation conditions. The experimental results indicate the prediction accuracy of the model is satisfactory.

Abstract: The critical inconsistency on the small angle boundary energy between the theoretical prediction of Read-Shockley model and the experimental results was discussed. The plots of the ratio between boundary energy E and the tilt angle θ against log θ show different tendency between the EAM simulated and the experimental results. The first principles calculations on the small angle tilt boundary energy on Al ⟨100⟩ direction were performed. Still the calculation sizes of the model are limited, the obtained values are located between the EAM simulated and the experimetal results.