Papers by Keyword: Dislocation Structure

Abstract: Two fatigued materials with f.c.c. lattice, i.e. pure polycrystalline copper and austenitic Sanicro 25 stainless steel, were subjected to the study of the persistent slip markings (PSMs) developed on the surface of the suitably oriented grains. They were observed using scanning electron microscopy (SEM) and thin surface FIB lamellae were prepared and studied by transmission electron microscopy (TEM). The aim was to correlate the specimen surface profile with the underlying internal dislocation structure. The localization of the intensive cyclic slip into persistent slip bands (PSBs) of the material was observed and associated with the PSMs on the specimen surface. Extrusions, intrusions and the dislocation structure appertaining to them were analysed, documented and discussed in relation to the models of fatigue crack initiation.

Abstract: Load-controlled experiments were conducted to study the influence of mean stress on the fatigue behavior of 316L austenitic stainless steel at the temperature of 288°C in air and light water reactor (LWR) conditions. Water environment was characterized by high-purity, neutral water with 150 ppb dissolved hydrogen. The internal dislocation structures of the material were investigated by means of transmission electron microscopy (TEM). The formation of dislocation structures for different loading conditions and different mean stresses was assessed and discussed in relation to the cyclic stress-strain response of the material as well as the effects of non-zero mean stress conditions. All findings were considered to discuss the fatigue softening/hardening behavior and the influence of mean stress on the fatigue life of material in the LWR environment.

Abstract: Microstructure stability of the directionally solidified Ni base IN792 superalloy has been investigated by Mechanical Spectroscopy (MS), i.e. internal friction (IF) and dynamic modulus measurements. Repeated IF test runs from room temperature to 1173 K have been carried out on the same samples and a Q^-1 maximum has been always observed above 700 K. Its position does not depend on the resonance frequency. After each run the values of modulus and Q^-1 at room temperature change indicating that a progressive irreversible transformation occurs. Damping phenomena have been attributed to the rearrangement of dislocation structures in disordered matrix which modifies dislocation density and average distance of pinning points. The results are supported by X-ray diffraction (XRD) and transmission electron microscopy (TEM) observations.

Abstract: A three dimensional grain mapping technique for polycrystalline materials, called X-ray diffraction contrast tomography (DCT), was developed at SPring-8, which is the brightest synchrotron radiation facility in Japan. The developed technique was applied to a commercially pure iron and austenitic stainless steel. The shape and location of grains could be determined by DCT using the apparatus in a beam line of SPring-8. To evaluate the dislocation structure in fatigue, the total misorientation of individual grains was measured by DCT. The average value of the total misorientation over one sample was increased with the number of cycles. In a grain, the change of the total misorientation was largest for primary slip plane. For austenitic stainless steel (fcc), the change of the total misorientation in fatigue was larger for planes with larger Schmid factor, while it was not depended on the Schmid factor for commercially pure iron (bcc). This different behavior must come from planer slip in fcc structure and wavy slip in bcc structure.

Abstract: The study on the evolution of the dislocation structure (DS) parameters with strain of solid solutions of Cu-0.5 at.% Al at different test temperatures has been carried out. It has been shown that in substructures with a disordered type of DS (disorder), the disordered mixtures with non-disoriented cells, as well as mixtures of non-disoriented and disoriented cellular structures, the entropy density increases with strain. It has been shown that formation of the cellular substructure corresponds to a diffuse kinetic phase transition of the 1-st kind in the DS. A jump-like decrease in entropy accompanying this phase transition is associated with the annihilation of dislocations in cellular walls and formation of excess dislocation density.

Abstract: The dislocation structure and mechanical properties of Ni₃Fe single crystals with the short range order were investigated. The laws of the dislocation structure evolution of single crystals at different deformation axis orientations were analysed. Correlation of the dislocation structure evolution with the deformation stages for the single crystals with the deformation axis orientations [001], [11], [011] and [1.8.12] was established. The role of the slipping plane numbers in the substructure evolution was revealed.

Abstract: In the literature it is proven that thermal vacancies have a great influence on the mechanism of hardening of Fe-Al alloys. Moreover, in these alloys, we observed a long-range ordering, which can significantly affect the mechanical and physical properties and their stability. In this paper, influence of low-temperature annealing on elimination of excess vacancies was investigated. TEM observation of annealed specimens for the alloys with 28 and 38 at.% aluminum have helped elucidate the phenomena responsible for vacancies elimination due to the occurrence of particular interactions between point and linear structure defects. It was shown that the aluminum content influences significantly changes in defects structure. The alloy with 28 at.% aluminum has mainly superdislocations in the structure, while in alloy with 38 at.% aluminum, mainly unit dislocations and high-energy dislocation configurations, like dislocation loops, dislocation dipoles, and dislocation jogs, were observed. The results suggest that different defect types may control the diffusion process during low-temperature annealing and that it is affected by alloy composition.

Abstract: We developed the transmission electron microscopy (TEM) sample preparation technique for the low dislocation density of 4H-SiC by combining the KOH+Na₂O₂ (KN) etching and the focused ion beam (FIB) microsampling technique. The dislocation under sea-shell pit was then characterized by large-angle convergent-beam electron diffraction (LACBED). It is demonstrated that this method is powerful for evaluating Burgers vectors of dislocations. Burgers vector of the measured basal plane dislocation (BPD) is determined to be b=1/3[-12-10]. Two-beam bright-field (TBBF) imaging identified the rotating direction of the threading screw dislocation (TSD) is counter-clockwise.

Abstract: Polycrystalline Al-1wt%Mg-0.27wt%Sc alloys bearing Al₃Sc particles with different average sizes of 4 and 11nm in diameter have been cyclically deformed at 423K under various constant stress amplitudes, and the relationship between fatigue characteristics and microstructure of the alloy has been investigated. The specimen bearing 11 nm particles exhibited a cyclic hardening to saturation, while in specimens with the small particles a cyclic softening was observed after initial hardening. In the specimen with large particles, dislocations were uniformly distributed under all applied stress amplitudes, whereas the specimens bearing small particles, in which cyclic softening occurred exhibited clearly developed slip bands. The cyclic softening for the latter specimen was explained by particle shearing within the strongly strained slip bands. The width of precipitate free zones (PFZs) has been found to be one of the factors affecting the fatigue life of the specimens at 423K. The two-step aging decreases the width of PFZs, resulting in increase in the fatigue life.

Abstract: Development of microstructure and texture in alloy Al5052 deformed at room and cryogenic temperatures in an equal channel angular pressing (ECAP) die has been investigated. Billets were deformed using the Bc route up to 14 passes. Billets pressed at room temperature showed almost a 4 fold increase in the yield strength, which increased to about 320 MPa from about 90 MPa, while it increased to about 230 MPa in billets deformed at cryogenic temperature. At two deformation temperatures, grains were refined at more or less similar size reduction rate as a function of number of passes. However, for a given amount of strain, billets pressed at the two temperatures showed subtle differences in their microstructures and texture.