Papers by Keyword: Subgrains

Abstract: To prepare bulk single-crystal REBCO superconductors by the new single-direction growth method (SDMG: Single-Direction Melt-Growth), it is necessary to produce a large-area seed of high quality, for example, based on EuBCO. Since the samples prepared by the SDMG method copy the structure of the seed, for the production of large-area seeds it is necessary to optimize the time-temperature regime in order to grow seeds with a suitable structure and composition and minimize structural defects (limiting the amount of subgrains and others). A higher growth rate was used in comparison with the standard growth rates used to produce EuBCO seeds of larger dimensions. The increased growth rate in the crystal growth window reduces the outflow of the melt from the sample, and thus it is possible to achieve a single-crystal sample in the entire volume of the precursor. The samples were produced at different growth rates: 1; 2; 3 and 5 °C/h. The microstructure of the samples was studied by polarized light microscopy and scanning electron microscopy. The size and distribution of Eu211 particles in the sample volume and the subgrain structure were studied on the fabricated samples.

Abstract: Microstructural design of a new generation of 9%Cr steels for fossil power plants is considered. It was shown that microstructural stability of 9%Cr steels impairs their creep resistance. Two types of restoration processes can occur in the heat resistance steels under creep conditions: (i) normal grain growth and (ii) dynamic recovery. The first process associates with the migration of high-angle boundaries (HAGB) of blocks of tempered martensite lath structure (TMLS). However, their migration is negligible even during creep deformation. Boundaries of packets and prior austenite boundaries (PAB) are effectively pinned by precipitations of M₂₃C₆ and Laves phase Fe₂(W,N). The second process consists of transformation of lath boundaries to subboundaries and their subsequent migration (subgrain coarsening) under creep. Under aging the migration of low-angle boundaries (LAGB) is retarded by uniformly distributed nanoscale M(C,N) dispersoids and particles of M₂₃C₆ precipitated on these boundaries under tempering. Under creep the dissolution of M₂₃C₆ carbides located along LAGBs and coagulation of uniformly distributed M(C,N) carbonitrides facilitates LAGB migration. It was shown that the normal grain growth is not important for deterioration of creep strength. Conversion of the lath boundaries into subgrain boundaries strongly decreases creep rate. In contrast, continuous subgrain coarsening is the main process restricting the ability of the 9%Cr steel for long-range service under creep conditions. Tertiary creep is attained due to the occurrence of subgrain coarsening.

Abstract: The material constitutive behaviour and microstructure of Aluminium alloys 6082 and 7449 were studied with the Gleeble hot compression test. The novel aspect of the work is that the testing was done at high strain-rates and at temperatures within 5 K of the solidus. The results indicated that the strength was maintained up to near solidus temperatures, with no dramatic strength reduction being observed. There was however, a distinct change in the slope observed with the 7449 results around 720 K. The experimental results were then fit to the Zener-Holloman equation, which describes the relationship between the material flow stress, temperature and strain-rates. The material microstructure of the hot compression test samples was analysed, and the averaged grain size was calculated to compare with friction stir weld nuggets. This will be used to infer the processing conditions that exist in the dynamically recrystallized weld nugget. Finally, a simple model was used to understand how processing conditions affected the deformation behaviour.

Abstract: This work aims to investigate the influence of hot deformation on dynamic recrystallization(DRX) behavior of 7050 aluminum alloy by means of physical simulation method. The hot compression tests were carried out on Gleeble 1500 machine in temperature range of 250-450°C and strain rate range of 0.01–10 s-1. Transmission electronic microscopy (TEM) was employed to observe and analyze the microstructure and DRX behavior in different deformation conditions. The results show that the effects of deformation temperature and strain rate on microstructural evolution of the alloy are remarkable. When temperature is lower than 350°C, only the dynamical recovery( DRV) occurs and typical sub-grains appear. In the range of 350-400°C, the incomplete DRX occurs and the recrystallized grain size increases with increasing temperature. The complete DRX occurs at 450°C and the fine equiaxied grains with high-angle boundaries develop resultantly. The DRX grain size increases as strain rate decreases. The desirable microstructure and properties can be obtained by optimizing the forging process parameters.

Abstract: The relation of the internal stress and the parameters of the heterogeneous dislocation structure was suggested in the form of the classical Taylor formula relating the internal stress to the total dislocation density stored in the subgrain interior and in the subgrain boundaries. The other formula combines linearly the stress contribution generated by network dislocations and the stress contribution of the subgrain structure semiempirically related to the subgrain size. The formulas can evaluate the ratio of internal stress components due to sub-boundaries and free dislocations.

Abstract: The synchrotron based X-ray diffraction method “High angular resolution 3DXRD” is briefly introduced. The technique enables the investigation of individual dislocation free regions in a dislocation structure, in-situ within the bulk. Results on the strain distribution within a single grain in a copper sample deformed in tension to 2%, and kept under load, are presented. It is found that the dislocation free regions of the dislocation structure on average are subjected to compressive strain with respect to the mean (tensile) strain in the grain. Results on the dynamics of individual dislocation free regions during straining are further reviewed, with special focus on the observation of intermittent behaviour.

Abstract: A multiscale model on dislocation patterning of cell structure and subgrain for polycrystal is newly developed on the basis of reaction-diffusion theory. A FD simulation for dislocation patterning and a FE one for crystal deformation are simultaneously carried out for a FCC polycrystal at large strain. Reflecting stress value on stress-effect coefficients, it is numerically predicted that the evolution of dislocation pattern in a polycrystal is different in response to the stress condition of each grain.

Abstract: Specimens of commercial purity aluminum were subjected to a strain path change test during high temperature deformation. Specimens were deformed at 4000 C and strain rate of 0.1 s-1 up to various strains of 0.2, 0.5, and 1. Then in a strain path change test, specimens were first deformed to a strain of 0.5, and subsequently deformed to strains of 0.2 and 0. In order to further the understanding of the deformation mechanisms in aluminum, the subgrain sizes and misorientations were characterized in detail by comparative studies using optical microscopy in polarized light (POM), orientation imaging microscopy (OIM/SEM) and transmission electron microscopy (TEM). The analysis revealed that while subgrain size is relatively insensitive to strain, overall misorientations increased with increasing strain. These analyses confirmed a strong bimodal distribution of boundaries during deformation coupled with a low fraction of medium angle boundaries. The results contribute to the understanding that dynamic recovery in aluminum maintains subboundaries with low misorientation but as grains elongate and more subgrain become adjacent to grain boundaries the fraction of high angle boundaries rises.

Abstract: The microstructure in heavily deformed metals can be characterized as a complex “mixture” of low and high angle boundaries. By careful annealing of such cold deformed conditions, ultra-fine grained materials can be obtained. This phenomenon has been known for long and utilised in the production of special aluminium sheet qualities, and has received new interest with the emergence of the equal channel angular pressing (ECAP) technique. This work reviews the mechanical properties resulting from plastic deformation and annealing of aluminium, looking at alloys which prior to annealing was subjected to both severe plastic deformation (ECAP) and more conventional deformation by cold rolling. The effect of the resulting microstructures on the subsequent work hardening properties are model, applying the new microstructural metal plasticity model (MMP-model) developed in Trondheim over the last decade.

Abstract: The paper presents the results of research concerning the influence of hot deformation parameters on the structure and substructure as well as the plastic properties of a Fe–Ni austenitic alloy. The research was performed on a torsion plastometer in the range of temperatures of 900÷1150°C, at a strain rate 0.1 and 1.0 s-1. Plastic flow curves have been drawn up and the interrelations have been determined between the process parameters and the recrystallized grain size, inhomogeneity and shape. Functional relations between the Zener-Hollomon parameter and the mean grain size after dynamic recrystallization have been developed and the hot deformation activation energy has been estimated. The examination of substructure on TEM allowed the calculation of structural parameters: the average subgrain area and the mean dislocation density. A detailed investigation has shown that the substructure is inhomogeneous, consists of dense dislocation walls, subgrains and recrystallized regions.