Defects and Diffusion, Theory and Simulation

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Authors: Manjeera Mantina, Long Qing Chen, Zi Kui Liu
Abstract: A simplified approach to predicting diffusion coefficients directly from first-principles is proposed. In this approach, the atomic jump frequencies are calculated through the Eyring’s reaction rate theory while the temperature dependence of diffusion coefficients are accounted using phonon theory within the quasi-harmonic approximation. The procedure can be applied to both self-diffusion and impurity diffusion coefficients and different crystal systems. Applications to self-diffusion coefficients in fcc Cu, bcc Mo, hcp Mg and impurity diffusion coefficients of Li in fcc Al, W in bcc Mo and Cd in hcp Mg show agreement with experimental measurements.
Authors: Mohammed Hadj Meliani, G. Pluvinage, J. Capelle
Abstract: The purpose of this work is to assess a gouge defect in a pipe submitted to internal pressure. To do that a method is used which is based upon a failure assessment diagram and, more precisely, upon a Modified Notch Failure Assessment Diagram (NMFAD) which has been proposed as a mesofracture approach. The safety factor has been determined under conservative conditions; i.e for a X52 pipe steel having a relatively low fracture toughness and a severe gouge defect with high aspect ratio and high constraint. In addition, a mesofracture approach to the fracture toughness transferability problem has been proposed. The crack (K-T) methodology has been modifed to create the ( –Teff) two-parameter fracture resistance criterion.
Authors: Fabian Ferrano, Marco Speich, Wolfgang Rimkus, Markus Merkel, Andreas Öchsner
Abstract: This paper investigates the mechanical properties of a new type of hollow sphere structure. For this new type, the sphere shell is perforated by several holes in order to open up the inner sphere volume and surface. The mechanical behaviour of perforated sphere structures under large deformations and strains in a primitive cubic arrangement is numerically evaluated by using the finite element method for different hole diameters and different joining techniques.
Authors: K.C. Liao
Abstract: A porous panel with a small crack subjected to mixed-mode loads under yielding and plane stress conditions is examined here. The recently proposed modified Gurson’s yield criterion is adopted to account for the non-linear behavior of a porous panel with normal anisotropy but planar isotropy. Trends in the constant J contour, based upon a finite element analysis, are correlated with those of constant fatigue life contours under mixed-mode I and II loading conditions. The effects of both the anisotropy parameter and the porosity upon the J integral are then demonstrated. Simulation results show that the transverse strain dominates constant J contours on the -plane as the anisotropy parameter increases under the mixed-mode loads. However, constant J contours are less sensitive to the transverse strain even if a relatively larger void volume fraction appears in the panel.
Authors: Ehsan Mahdavi, Mahmoud Mosavi Mashhadi
Abstract: An analytical procedure is developed to design and predict the behavior of a pressure vessel. If a pressure vessel contains hydrogen, it is difficult to predict what will happen in the future. In this study, this is accounted for and the stress intensity factor for mode-Ι is calculated because the main factor controlling mass diffusion, as a driving force, is related to the stress in this mode. Also, it is known that the stress intensity factor depends upon concentration. The main challenge in hydrogen embrittlement is the prediction of crack growth and the estimation of lifetime for a pressure vessel. This paper investigates the effect of hydrogen diffusion upon crack in a pressure vessel by using numerical finite-element simulations. The fracture behavior of the alloy as related to hydrogen embrittlement was also studied. The computational simulations involved sequentially-coupled stress and mass-diffusion concentrations at the crack tip. Although there have been various previous works in this area, most of them have been experimental estimates of hydrogen diffusion. In this paper, we calculate the stress intensity factor by using the finite-element method (FEM) and use mathematical analysis simultaneously. The analytical method alone could not be used because the mass diffusion has special characteristics. That is, the treatment of diffusion is different at each step. We conducted finite-element modeling simulations of the intergranular fracture of alloy X-750 due to hydrogen embrittlement. Sequentially coupled stress and mass diffusion determinations were carried out in order to determine crack tip stresses and hydrogen diffusivity in the crack-tip region. Good qualitative agreement between the FEM modeling and the analysis was observed.
Authors: Mohammad Ebrahim Zeynali
Abstract: The dehydrogenation of diethylbenzene to divinylbenzene is a catalytic reaction. The catalyst for the dehydrogenation was prepared by co-precipitation of iron and chromium hydroxide from nitrate solution, followed by doping with potassium carbonate and drying. To make available the internal surface area of the catalyst for the reactant, the pores must be of the proper sizes to allow the reactant to diffuse and penetrate inside the catalyst pellets. The prepared catalyst was considered as a model for investigating the role of diffusion in catalyst design. In this study, different mechanisms of diffusion, such as Knudsen and bulk, were investigated for the case of diethylbenzene diffusion into the catalyst and it was concluded that the pore sizes should be in a range that permits transitional diffusion (both Knudsen and bulk diffusion). The catalyst grain size can be controlled and varied by acting on parameters such as the speed and time of mixing, type of alkali, temperature and pH. Particle size distribution experiments were conducted for different types of alkali and speeds of mixing in order to characterize the catalyst. The effects of the grain size, formed during co-precipitation, upon the pore size distribution of the catalyst pellet which affects the effective diffusivity were discussed. The pore size distribution of the model catalyst was obtained and the effective diffusivities were calculated by numerical integration of the Johanson-Stewart equation.
Authors: Xue Feng Wang, Shao Yi Wu, Yue Xia Hu, Pei Xu
Abstract: The axial shift and the spin Hamiltonian parameters (zero-field splitting D, the g factors and the hyperfine structure constants A// and A) for Mn2+ in a CdS crystal are studied theoretically in this work. The calculations are carried out by using the perturbation formulae of these parameters for a 3d5 ion under trigonally distorted tetrahedra based upon the cluster approach, where the ligand orbital and spin-orbit coupling contributions are taken into account in a uniform way. From the studies, the impurity Mn2+ is found not to occupy the host Cd2+ site exactly but to experience a small outward shift of 0.018 Å away from the ligand triangle along the C3 axis. The above impurity axial shift leads to a much smaller trigonal distortion than the host Cd2+ site in CdS. The calculated spin Hamiltonian parameters are in reasonable agreement with the experimental data.
Authors: A.A. Ibrahim
Abstract: Lead sulfide (PbS) thin films were prepared by thermal evaporation onto glass substrates from PbS powder. The structure and DC electrical properties of evaporated PbS thin film sandwich structures with thicknesses (d) up to 600 nm have been investigated. X-ray diffraction studies showed that the films were crystalline, with a preferred orientation in the [111] direction. Capacitance measurements indicated that the films had a relative permittivity of 5.7. Room-temperature current density-voltage (J–V) characteristics revealed ohmic conduction below a transition voltage (Vt) and a power–law dependence with an exponent of ≈ 2 at higher voltages. This behaviour was interpreted in terms of space–charge limited conductivity controlled by an exponential distribution of traps below the conduction band edge. Further evidence for this conduction process was provided by a linear dependence of Vt upon d2. Analysis of the results yielded a room temperature electron concentration no of ≈ (3.9 – 5.4) x 109 m-3.
Authors: B.B. Jha, Barada Kanta Mishra, S.N. Ojha
Abstract: Acoustic emission (AE) signals, obtained during the isothermal oxidation of 2.25Cr-1Mo steel at 773 K, 873 K, 973 K and 1073 K, have been analyzed. The results indicated that the rate of occurrence of AE events and consequently the total number of AE events generated during isothermal oxidation at these temperatures increased with an increase in the oxidation rate. Variation in the temperature of oxidation did not show any variation in the root mean square (RMS) level of the AE signals. The b-parameters obtained from a logarithmic cumulative amplitude distribution plot indicated that the strength of the AE signals did not change during isothermal oxidation carried out at these temperatures. Different event rates, and consequently the difference in the total number of AE events generated during isothermal oxidation at these temperatures, are indicative of the increased rate of energy release associated with the growth of oxide layers formed at higher temperatures. The rate of energy release has been found to be higher for higher temperatures of oxidation.
Authors: Wen Bang Gong, Gang Yu Xiang
Abstract: In this paper, a formula for the calculation of the carbon content during the austenitizing of cast iron was deduced, considering the effect of silicon content upon the heat-treatment parameter. According to this formula, the carbon content of the austenite at a certain austenization temperature for a cast iron with given components can be easily calculated, and the austenization temperature required to give the expected carbon content in the austenite can also be determined. Moreover, according to the relationship between the austenization temperature Tx and the associated carbon content Cax,, and considering the effect of the silicon content, a diagram showing Cax, Tx and the silicon content during the austenitizing of cast iron was prepared.

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