Papers by Keyword: Stored Energy

Abstract: Using XRD method it was revealed that in the stress-strain state of Al nanopowder lattice a non-significant amount of energy was stored (~0.385 J/g). Nevertheless, according to the data obtained by differential thermal analysis (DTA) the total amount of stored energy in the nanopowder was 348 J/g. The estimated value might be caused by the significant contribution of nanoparticles surface energy, which cannot be detected by means of XRD method. However, the method proposed in the paper can be applied to estimate changes in the structural and energy states of the lattice for nanoparticles or another micro-and nanopowders.

Abstract: The influence of the electron beam irradiation on the parameters of aluminum nanopowder oxidation by heating in air was studied. It was found that the oxidation starts at the temperature in the range from 410° C to 460° C and independent on the radiation dose. The degree of oxidation varied from 44.4 % to 58.3 % and its dependence on the radiation dose was not established. The heat energy release occurred in two stages: at the first stage (up to ~ 660° C) in general the increase of the thermal effect was observed. At the second oxidation stage of irradiated aluminum nanopowder the growth of the thermal effect also observed. The peak of heat effect achieved by irradiation (45.0 kGy absorbed dose) was 2576 J/g higher than the thermal effect for non-irradiated aluminum nanopowder. The energy stored is an additional motivating factor in the synthesis of composite materials, intermetallic compounds, hydrogen producing reactions and synthesis of various kinds.

Abstract: This work is devoted to the development of a constitutive model for the simulation of dissipated and stored energy evolutions under irreversible (plastic) deformation of metals. The efficiency of the model was demonstrated by the calculation of the energy balance in 304(L) austenitic steel under quasistatic deformation. The results of the numerical simulation are in a good agreement with the experimental data. The additional theoretical result of the study has established (based on the numerical and experimental data) a correlation between energy storage rate and rate of strain hardening.

Abstract: Quasi-brittle materials are those where the addition of specific microstructural features such as porosity can lead to departure from linear elastic behaviour prior to maximum force, followed by graceful failure. A simple example of a quasi-brittle material is reticulated vitreous carbon foam; an open-cell structure consisting of brittle ligaments connected in a three-dimensional array. Tensile testing measurements have been made on foams with various pore and ligament dimensions; force - displacement combined with acoustic monitoring together with the evaluation of the associated elastic moduli and fracture strengths. These tests give insights into the mechanisms of quasi-brittle failure, and the results are explored using simple considerations of elastic energy storage throughout process zones.

Abstract: Commercially pure, dilute aluminium alloys, such as AA1050, dynamically generate sub-grains during cold rolling. If AA1050 is rolled at cryogenic temperatures (liquid nitrogen), this large decrease in temperature minimises the occurrence of dynamic recovery during rolling. The result is that the material has a large amount of stored energy and a high dislocation density, thereby giving it a high strength. This research looks at the recovery and recrystallization processes during annealing after cryo-rolling, and compares the formation of sub-grains and recrystallized grains to those where rolling was performed at room temperature. The type of dislocation structure that forms during the rolling process directly affects the evolution of the microstructure post-deformation. Owing to the extreme temperatures of cryo-rolling, the dislocation structure cannot undergo dynamic recovery. Instead, a distinct cell-like structure forms, with dense dislocation walls that are high energy. During subsequent annealing, the driving force for recrystallization is increased with a decrease in the rolling temperature, with the cryo-rolled material having a greater number of nucleation seed and consequently, a fine grained recrystallized microstructure.

Abstract: The effects of net driving force for migration of high angle grain boundaries were emphasized beside many other factors which could influence the process of texture formation during recrystallization annealing of 95% cold rolled pure aluminum sheets. The net driving force consists basically of stored energy. However, it could be reduced by recovery, boundary drag, solute drag and Zener drag in different extents, in which only boundary drag is mis-orientation dependent. It was indicated that both oriented nucleation and oriented growth have obvious influence on recrystallization texture, and how far they influence the texture depends also on the level of net driving force when the grain growth starts during annealing. Oriented growth, which is induced by the differences in boundary drag of differently oriented grains, and the corresponding texture formation, could be observed easily when the recrystallization proceeds under relative higher solute drag and Zener drag in commercial purity aluminum. The oriented nucleation process prevails during recrystallization of sufficiently recovered high purity aluminum with very low solute drag and Zener drag, after which strong cube texture forms. In this case the oriented growth indicates limited effect. Both the oriented growth and oriented nucleation will fail if high purity deformation matrix without clear solute drag and Zener drag has not experienced an obvious recovery before recrystallization grain growth, since extremely high net driving force leads to very small critical nucleus size and multiplicity of growing grains, which results in randomization of recrystallization texture.

Abstract: Stored energy in deformed metals plays an important role during the annealing process by providing the initial driving force for recovery and recrystallization. Many direct or indirect measurement and calculation methods have been used to evaluate the amount and distribution of the stored energy in the past decades. The advent of relatively new analytical techniques such as Electron Back-Scattered Diffraction (EBSD) has permitted the development of mathematical models such as Sub-grain Method, Image Quality (IQ) Method and Taylor Factor Method etc., these new techniques have permitted a much better understanding of the annealing behavior of cold rolled steels. The sub-grain method based on the level of sub-grain structure is used in our study to quantify the stored energy distribution prior to and its evolution during the batch annealing process of cold rolled HSLA steels. Orientation dependent stored energy distribution maps at different annealing stages have been constructed and analyzed. The results of this study show that the stored energy increases with cold rolling reduction ratio and its distribution through the thickness of the steel sample is not uniform due to the inherit inhomogeneous deformation process. The stored energy was continuously consumed during annealing. The amount of γ-fiber was relatively lower than the α-fiber in the specific steel sample, which can have a strong effect on the available driving force for recovery and recrystallization. Hence other structural factors such as precipitation and/or solute drag might become more important in controlling the kinetic behavior of the steel during annealing.

Abstract: Polycrystalline Ni (99.5 %) has been deformed to an ultra-high strain of ε_vM=100 (ε_vM, von Mises strain) by high pressure torsion (HPT) at room temperature. The deformed sample is nanostructured with an average boundary spacing of 90 nm, a high density of dislocations of >10¹⁵m^-2 and a large fraction of high angle boundaries (>15^o) 68% as determined by transmission electron microscopy and 80% as determined by electron backscatter diffraction. The thermal behavior of this nanostructued sample has been investigated by isochronal annealing for 1h at temperatures from 100 to 600°C, and the evolution of the structural parameters (boundary spacing, average boundary misorientation angle and the fraction of high angle boundaries), crystallographic texture and hardness have been determined. Based on microstructural parameters the stored energy in the deformed state has been estimated to be 24 MPa. The isochronal annealing leads to a hardness drop in three stages: a relatively small decrease at low temperatures (recovery) followed by a rapid decrease at intermediate temperatures (discontinuous recrystallization) and a slow decrease at high temperatures (grain growth). Due to the presence of a small amount of impurity elements, the recovery and recrystallization are strongly retarded in comparison with Ni of high purity (99.967%). This finding emphasizes the importance of alloying in delaying the process of recovery and recrystallization, which enables a tailoring of the microstructure and properties through an optimized annealing treatment.

Abstract: t has been demonstrated in previous work that a two-step annealing treatment, including a low-temperature, long-time annealing and a subsequent high-temperature annealing, is a promising route to control the microstructure of a heavily deformed metal. In the present study, structural parameters are quantified such as boundary spacing, misorientation angle and dislocation density for 99.99% aluminium deformed by accumulative roll-bonding to a strain of 4.8. Two different annealing processes have been applied; (i) one-step annealing for 0.5 h at 100-400°C and (ii) two-step annealing for 6 h at 175°C followed by 0.5 h annealing at 200-600°C, where the former treatment leads to discontinuous recrystallization and the latter to uniform structural coarsening. This behavior has been analyzed in terms of the relative change during annealing of energy stored as elastic energy in the dislocation structure and as boundary energy in the high-angle boundaries.

Abstract: The article deals with the problems of strengthening and cold resistance increase of steels through combining equal channel angular pressing (ECAP) with thermal processing (TP). Dependence of cold resistance on the state of the material structure determined by regimes of equal channel angular pressing and thermal processing is shown. Dependence of specific work of plastic deformations, stored and dissipated energy of steel on its plasticity is stated.