Papers by Author: Mourad Keddam

Abstract: In this work, a simulation of the growth kinetics of layers on AISI 1018 steel was done by means of a kinetic model. This model considers a solid diffusion of boron into a semi-infinite medium where the boron solubility in the Fe phase depends on the process temperature. An expression of the parabolic growth constant was then obtained through an application of the mass balance equation at the (/substrate) interface. The present model was validated by the experimental data available in the reference work (I. Campos-Silva et al: Kovove Mater. Vol.47 (2009), p.1-9). A good concordance was observed between the experimental parabolic growth constants and the predicted ones by the model for an upper limit of boron in the phase equal to 8.91 wt.% ( as a fitting parameter of the model). In addition, the generated weight gain was estimated at the surface of the borided AISI 1018 steel as a function of the upper limit of boron in the phase and the temperature.

Abstract: In the present work, a thermodynamic study was carried out in order to analyze the thermodynamic stability of the and phases in equilibrium with the phase using the calculation of phase diagrams (Calphad) formalism. The two phases and are modelled as substitutional and interstitial solid solutions of boron. The expressions of the chemical potentials of B and Fe are derived in both phases to perform the thermodynamic calculations. A comparison is made between the results provided by the substitutional and interstitial models and good agreement is observed between these two models.

Abstract: The 32CrMoV13 low alloy steel was gas nitrided at 550°C, for three time durations (6.5, 13 and 20 h) and under a variable nitriding potential (1, 2.2 and 6 atm-0.5). The generated nitride layers were characterized by SEM observations, XRD and GDOS analyses as well as microhardness testing. The XRD analysis indicates that the compound layer was composed of and iron nitrides and CrN phase. The surface hardness (inside the compound layer) was found to be dependent on the nitriding potential value, its value increases as rises. It was shown by GDOS analysis that the upper and lower nitrogen concentrations at the (compound layer / diffusion zone) interface are approximatively: 4 and 0.88 wt. % N, respectively.

Abstract: Diffusion of Uranium in compacted clay was studied through the non-steady state diffusion method for the safety assessment of the radioactive waste in the storage and disposal practices. Since the permeability of the compacted clay is very low, the main mechanism for radionuclide transport is governed by the diffusion phenomenon. The diffusion process of uranium in compacted clay as a porous medium has been modeled by taking into account the effect of sorption. An increase in temperature, caused by the radiation from the canisters, takes place in the repository. Then, the effect of temperature on textural and structural properties of clay and coefficient diffusion has been examined. First, we examined the structure changes of the clay pellets under the influence of the heat in the temperature range 25–100°C. These changes are studied in the region of mesopores and macropores using the method of the mercury porosimetry. Second, the effect of temperature on the diffusion behavior of uranium was simulated. In these simulations, the experimental values of apparent diffusion coefficients of uranium were used to determine the concentration profiles versus time, depth and temperature. As conclusion, it was shown that the temperature plays an important role in the uranium transport through the compacted clay.

Abstract: The growth kinetics of Fe2B layers formed at the surface of AISI 1018 was simulated. The paste-boriding (with a paste thickness of 4 mm) was applied to produce the Fe2B phase at the material surface; considering four temperatures (1123, 1173, 1223 and 1273 K) for 2, 4, 5, 6 and 8 h. The suggested model was based on the mass balance equation at the (Fe2B /substrate) interface. As a fitting parameter of the model, the surface boron concentration (12.16 wt. %B) was obtained in order to predict with a good agreement the experimental parabolic growth constants at the (Fe2B /substrate) interface derived from the literature. An expression of the parabolic growth constant at the (Fe2B /substrate) interface was obtained as a function of the two parameters: and . In addition, a relationship of the Fe2B layer thickness was also deduced that showed a good concordance with the experimental results from the literature.

Abstract: The effect of compaction of bentonite on the diffusion behavior of uranium was studied for the safety assessment of radioactive waste (storage and disposal practices). Since the permeability of the compacted clay is very low, the main mechanism for radionuclide transport is governed by the diffusion phenomenon. The diffusion process of uranium in compacted clay as a porous medium has been modeled by Fick's second law taking into account the effect of sorption and considering the non-steady state. The diffusion coefficients and profiles concentration values were calculated by a computational method using a numerical program based on the Newton-Raphson algorithm. In this simulation, the experimental values were used to determine the uranium concentration profiles versus depth of the clay pellet. It was concluded that the dry density of the compacted clay and the aqueous solution properties (pH and ionic strength of background electrolyte) played an important role in the uranium transport through compacted clays.

Abstract: A simulation of the growth kinetics of iron boride forming on AISI 1018 carbon steel was done on the basis of a kinetic model. This model including the effect of the incubation time during the formation of iron boride, was applied in order to evaluate the kinetic constant at the ( ) interface, the layer thickness and the mass gain depending on the paste-boriding parameters such as time, temperature and boron potential reflected by the corresponding value of the surface boron content. The simulation results were found to be in a good agreement with the experimental data derived from the literature.

Abstract: This work deals with a study of the nitriding potential effect on development of the compound layer during the gas nitriding of Armco Fe samples. The gas nitriding experiments were performed in an atmosphere of partially dissociated gas ammonia (NH3) at 520 °C under a nitriding potential varying from 0.25 to 3.5 atm-0.5 during 2 h. Through this experimental work including XRD analysis, optical and SEM observations of the cross-sections of the treated samples, it is shown that the microstructural nature of the compound layer depends upon the nitriding potential value. By use of the inverse problem based on a diffusion model previously published, it was possible to estimate the diffusion coefficient of N in ' iron nitride as a function of the applied nitriding potential. XRD analysis has shown that the compound layer was composed of iron nitride. A linear semi-logarithmic relationship relating the nitriding potential to the diffusion coefficient of nitrogen in iron nitride was also derived.

Abstract: In the present work, a kinetic study based on a diffusion model was performed by use of both kinetics and thermodynamic data as input parameters, it was possible to evaluate the kinetic constant at each phase interface for a biphase configuration FeB and Fe2B grown over the surfaces of Armco Fe and Fe-Cr binary alloys at 0.5 and 4wt. %Cr by powder- pack boriding. The simulated values of the kinetics constants by the model were compared to those found in the literature and a good agreement was observed. For the Fe-4wt. %Cr alloy, it was found by simulation that the layer thickness ratio between the FeB and Fe2B phases is very sensitive to the increase of temperature and surface boron content.

Abstract: In this work, an approach of reactive nitrogen diffusion is presented and applied to the iron gas nitriding process. A kinetic model based on Fick's laws is used to simulate the layer growth kinetics of a biphase configuration composed of ε and γ’ iron nitrides grown on the pure iron substrate. This diffusional approach, under certain assumptions, reveals the influence of the nitriding potential on the layer growth kinetics during the gas nitriding of pure iron. Some simulation results are presented and discussed.