Papers by Author: Hamid Garmestani

Abstract: Three Ti-6Al-4V plate materials produced by powder metallurgy technique, included pre-alloyed hydride-dehydride (HDH) plate rolled to 75% reduction in thickness, and two blended elemental (BE) powder plates rolled to 75% and 87% reduction were evaluated. The objective of this study was to determine differences in microstructure and toughness between the pre-alloyed HDH and BE Ti-6Al-4V materials processed to the same product form. Heat treatments were performed below the beta transus temperature at 982, 871, 760, and 732°C (1800, 1600, 1400, and 1350°F) for 1, 2, and 4 hours in order to determine differences in heat treating response, and above the beta transus at 1076°C (1970°F) to determine the transformation temperature. The samples were evaluated by optical microscopy and scanning electron microscopy. Charpy impact testing was performed in order to determine differences in the energy absorbed during fracture. Pole figures (0002) of selected conditions were also performed in order to determine any differences in texture between the various conditions.

Abstract: the effects of turning on the gradients of micro-texture and grain size of Al alloy 7075 processed by turning is studied. Analytical models based on Boussinesq equation and stress/force transformation, predict that the equivalent von-Mises stress penetrate almost 1 mm below the machined surface with a maximum of 1400 MPa. The induced temperature below the surface is and reaches up to 240°C. As a result of deformation below the machined surface, the grains below the surface show a preferred grain shape orientation different from the grains in the central region. The normalized population of (121) plane increases at the machined surface compared to the central part of the sample. Orientation distribution functions reveal that at the central part of the sample, the material is mostly randomly oriented and the closest texture component is the recrystallized texture. On the other hand, the strongest texture attribute at the machined surface is β-fiber torsion. The average grain size shows a 10% decreases moving away from center while below the machined surface there is a 50% increase in the average grain size. This can be due to the grain growth caused by higher temperatures right below the machining tool. The reported results provide a toolset to determine the final properties of the material after machining in addition to providing means to more accurately describing the machining mechanics.

Abstract: Titanium alloys are used in a wide variety of aerospace, energy, industrial and biomedical applications, among other reasons, due to their superior properties. These properties are highly dependent of materials microstructure, i.e. texture, dislocation density and slip system activity. Therefore, in order to be able to design materials with certain properties it is crucial an understanding of the deformation process in terms of microstructural parameters. Microstructure evolution in warm rolled commercially pure titanium was investigated by means of X-ray diffraction and modeled with a crystal plasticity self-consistent scheme. Texture measurements and peak profile analysis were used to characterize the deformation texture and evaluate the relative activity of the various slips systems activated during the deformation process. The peak profile analysis data and the self-consistent predictions of texture evolution showed a good agreement with the experimental deformation texture evolution.

Abstract: Magnetic annealing at 17T, 800°C, for 15 minutes was conducted to evaluate the effect of the field on the recrystallized microstructure of Fe-3.25%Si samples. According to X-ray results, the main texture components were found not to be affected by the field. The volume fraction of eta fiber was slightly increased and the gamma fiber, an unfavorable texture component for materials with magnetic application, was not decreased when compared to samples annealed without magnetic field.

Abstract: Magnetic annealing at five different magnitudes of field was conducted to evaluate the effect of the field on the recrystallized microstructure of Fe-0.75%Si samples. At higher fields the retardation during recrystallization is compensated by the magnetic filed driving force that causes an increase in the grain boundary mobility of grains that have a certain relationship with the direction of the field

Abstract: Using a processing path model based on the conservation principle in the orientation space explicit solutions can be formed linking any final (desired) microstructure to a given initial state for polycrystalline materials. The model uses texture coefficients in spherical harmonics expansion to as materials descriptors to represent the texture state of polycrystalline materials. In this work, the effect of increasing the maximum number of texture coefficients used in the series expansion (represented by Lmax) on the prediction of texture and its accuracy is fully studied.