Papers by Author: S. Jakani

Abstract: The aim of this study was to create a nano-structured coating using Plasma Thermal Spraying (PTS). This process consists in introducing pre-agglomerated nanosized particles in a high-temperature and high-velocity gas jet and projected them onto the substrate to form, layer by layer, a nanostructured coating. In order to retain nanometer grain sizes in the deposited coating through specific PTS technologies, a thermal field and velocity distribution in the plasma jet are analytically calculated. A finite element analysis is employed to calculate the thermal field evolution inside the agglomerated particles and the thermal induced internal stress distribution is determined. The parameters determined by the theoretical analysis are used for experimental coatings. The average crystallite size of nano-hydroxyapatite powder was 90nm. After deposit via Plasma Thermal Spraying (PTS) process and followed by a 2 hours heat treatment to reduce amorphous fraction, the experimental deposited coating shows that it retains the nanometer crystallite sizes. The substructure of nanocrystals was evaluated at about 120nm in size. Such a nanocoating may play the role of nucleation site to bone, allowing a faster stabilization of the implant.

Abstract: The crystallographic texture of electrolytic tough pitch copper has been investigated by neutron diffraction after deformation by cold wire-drawing (reduction of area between 51 and 94 %) and after static recrystallization. The deformation texture characterized by a strong <111> fiber is reinforced with increasing strain, while the volume fraction of <100> fiber is reduced. In turn, we show that the <100> fiber is strongly reinforced after recrystallization when intensity of the <100> maxima increases with the level of deformation. Since the <111> fiber disappears first during annealing, the static recrystallization has been followed “in situ” by measurements of the diffracted intensity evolution in the center of the {111} pole figure. From these experimental data and taking into account the Arrhenius equation, the activation energy of the recrystallization process has been determined for each deformation rate.

Abstract: In order to simulate the recrystallization process, Monte Carlo modelling has been applied to the case of wire-drawn copper deformed to a moderate strain. The complete experimental set of data was taken mainly from Electron Back Scattered Diffraction measurements in a Scanning Electron Microscope. Several nucleation hypothesis have been introduced and tested into the model. It has been shown that nucleation taking into account the sites associated with the highest stored energy and highest local misorientation leads to the best results in terms of recrystallization microstructure and texture. An important number of new orientations - that come only from annealing twinning - are not reproduced with the model, indicating the major role of this particular mechanism during the recrystallization process.

Abstract: Cold-drawn Electrolytic Tough Pitch copper wires have been investigated with Neutron Diffraction and Electron Back Scattered Diffraction. The drawn copper wires (38% reduction in area) develop major <111> and minor <001> fiber textures. It appears that the texture intensity of the reinforcements within the <111> and <001> fibers is more pronounced in the center and intermediate part of the wires. During the first annealing time, it is found that the recrystallization kinetics is enhanced when oxygen content is increased. The recrystallized fraction within the intermediate zone of the wire is two times larger in the cathode with higher oxygen content. The mechanisms at the origin of this acceleration of recrystallization kinetics will be discussed taking into account the presence of Cu2O type oxides.

Abstract: Wire-drawn Electrolytic Tough Pitch copper deformed at moderate strain has been investigated with Electron Back Scattered Diffraction in a Scanning Electronic Microscope in order to evaluate the recrystallization mechanisms at the meso-scale. Experimentally, it has been shown that the static recrystallization takes place first in the highly deformed and misoriented areas, in the intermediate regions of the wire. The grains related to the <100> fiber nucleate and grow first in these regions, but some other orientations (including the <111> oriented grains) - that have a combined nucleation/growth potential - develop in second time. The annealing twinning is active from the beginning of the recrystallization and tends to randomize the final recrystallization texture.

Abstract: Wire-drawn copper has been investigated with Electron Back Scattered Diffraction technique in a Scanning Electron Microscope after deformation by wire-drawing. In this paper, we show how to get qualitative informations about the deformation inhomogeneities related to the stored energy distribution, from the analysis of the quality index. Furthermore, the microstructural analysis in the wire diameter is completed using the quality index distribution approach. A relation between diameter of the wire and stored energy distribution is then qualitatively set. In order to validate the proposed method, the EBSD data are compared with the stored energy values obtained from neutron diffraction measurements.

Abstract: The analysis of the microstructure deformation and the static recovery were investigated by transmission electron microscopy (TEM) observations, after cold drawing in copper. The observed microstructures according to the orientation of grains are composed of dislocation cells, deformation bands and dense dislocation walls. In <001> grains, the cells are equiaxed, regular and surrounded by sharp walls, whereas in the <111> grains the cell creation is also in progress. During the annealing, the microstructure of <001> grains evolves to a stable configuration composed of very thin walls and coalesced cells that correspond to the first recrystallization nuclei.