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Articles by G.R. Ebrahimi
Total Records ( 4 ) for G.R. Ebrahimi
  H. Arabshahi and G.R. Ebrahimi
  A new simulation method has been developed and used to model mechanical properties of materials at many different length scales, from the nanoscale where an atomic description is appropriate, through a mesoscale where dislocation based descriptions may be useful to macroscopic length scales. In some materials, such as nanocrystalline metals, the range of length scales is compressed and a polycrystalline material may be simulated at the atomic scale. It is observed how the grain boundaries contribute actively to the deformation. At grain sizes below 5-10 nm deformation in the grain boundaries dominate over the traditional dislocation-based deformation mechanisms. This results in a reversal of the normal grain size dependence of the yield stress. It is shown that the material becomes softer when the grain size is reduced.
  G.R. Ebrahimi , H. Keshmiri and H. Arabshahi
  In making tubes of corrosion resistant and hardly deformed steels and alloys, the pilger rolling method is used for hot rolling of final thick-walled tubes or mother tubes of large diameters (above 300 mm) and small quantities of other size tubes when no other, more efficient tube rolling or extrusion equipment is available. To clarify individual parameters of the production process and make choice of the deformation and temperature parameters, mechanical properties of the alloy type 30Cr25Ni32Mo3 superaustenitic stainless steel at various temperatures were studied. The tests have been performed using samples taken from the forged 400 mm diameter billet to determine strength and plastic properties of the billet metal at various temperatures and its macro and microstructure. The test results will be used in the choice of optimum conditions of preheating of the billets and hot rolling of tubes. On the whole, it should be stated that as forged alloy 30Cr25Ni32Mo3 features a favorable combination of strength and plastic properties in the hot working temperature range of 1075-1200°C.
  H. Arabshahi , G.R. Ebrahimi and S. Gholafroz
  The dipolar electric field arising from the opposite displacement of the negatively and positively charged atoms provides a coupling between the electrons and the lattice which results in electron scattering. In this study, a new model of inelastic electron-phonon scattering has been carried out in which the scattering probability does not depend on macroscopic parameter. The reviewed model gives the good agreement between the theory and experiment.
  H. Arabshahi and G.R. Ebrahimi
  A Monte Carlo model that simulates the primary electron production inside the photoconductors mentioned, for a number of monoenergetic and polyenergetic x-ray spectra that cover the mammographic energies has been developed. The model simulates the primary photon interactions (photoelectric absorption, coherent and incoherent scattering) as well as the atomic deexcitations (fluorescent photon production, Auger and Coster-Kronig electron emission). In addition, a mathematical formulation has been developed for the drifting of primary electrons of a-Ge in vacuum under the influence of a capacitor’s electric field and the electron characteristics on the collecting electrode are being studied. The formulation is based on the Newton’s equations of motion and the theorem for kinetic energy change. Furthermore, a code has been developed that calculates the distribution of the electric potential inside a-Ge using an existing analytical solution, the boundary values of the case and certain numerical calculation methods. Finally, the structure and the mathematical formulation of a model that would simulate the electron interactions inside a-Ge have been developed. An existing model has been reexamined and enriched with certain theoretical considerations and simulation formalisms.
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