Sciences et génie des matériaux
Criterion for cathodic protection of 25CD4/Inconel 182 system
This study aims to investigate the cathodic protection criterion of a galvanic system of low alloy steel 25CD4substrate /Inconel 182 austenitic stainless-steel ?ller metal couple obtained using Shield Metal Arc Welding(SMAW) process. The microstructure investigation revealed the presence of Type II boundary along the steelsubstrate/Inconel interface where high carbon content and high hardness were recorded. The electrochemicaltests evaluated in marine environment (3.5% NaCl) at room temperature revealed that the corrosion potential(E) of the interface was between the steels substrate and the Inconel 182 ?ller metal ones, On the other hand,the current density (Icorr) and corrosion rate were slightly higher in the overlaid area. In order to determine thesystem protection criterion, chronopotentiometry method was introduced. It was found that the cathodic protectioncriterion for the assembly is based on the criterion of the least noble materialwhich is the steel substrate.corr Voir les détails
Mots clés : cathodic protection, 25CD4, Inconel 182, overlay, Cronopotentiometry.
Effect of annealing temperature on the microstructureevolution, mechanical and wear behavior of NiCr–WC–CoHVOF-sprayed coatings
In the present work, the effect of annealing temperature on the microstructure, mechanical and tribological properties of NiCr–WC–Co coatings produced by the high-velocity oxy-fuel (HVOF) technique has been investigated. X-ray diffraction and scanning electron microscopy revealed the dissolution of WC into the NiCr matrix to form W2C and Cr3C2 with the annealing process. This dissolution became complete at 800 °C. The mechanical properties of the coatings were investigated using nano-indentation and Vickers fracture toughness measurements. These measurements suggested that the hardness, Young’s modulus, and fracture toughness values increased because of the newly formed carbide phases as a result of the dissolution of the WC particles.The overall properties of the coatings were found to be optimum for annealing temperatures of 800 °C. The wear mechanism appears to be abrasive in the as-sprayed coating, and it becomes a combination of an abrasive and oxidative wear with increasing the annealing temperature Voir les détails
Mots clés : HVOF;, WC coating:, Annealing;, Wear resistance;, Nano-indentation
Cu2O addition and sintering temperature dependence of structural,microstructural and dielectric properties of CaCu3Ti4O12 ceramics
This study is aimed in the replacement of commonly used (CuO) by Cu2O in the synthesis of perovskiteCaCu3Ti4O12 (CCTO) phase by the solid-state reaction method. The XRD analysis of powder calcined at 1100 ?Cand ceramics sintered at different temperatures show that the CCTO phase was well crystallized with thepresence of small quantities of additional phases. The SEM/EDS analysis of prepared pellets show that the formation of Cu2O/CuO phase occurs above 950 ?C, resulting in enhanced densifcation at 1050 ?C (>96%).However, it is found that the Cu2O-based CCTO begins to degrade around 1090 ?C. The densifcation aftersintering at 1050 ?C reaches 96%, meanwhile dielectric constant and loss tangent values are optimum in the lowfrequency range (<103 Hz); i.e. 13378 and 0.177, respectively. This favors the use of Cu2O instead of CuO inCCTO ceramics for applications at low frequencies. Voir les détails
Mots clés : Ceramics, CCTO, powder metallurgy, XRD and SEM, Dielectric properties
Synthesis of Lead-Free Ceramicsof the Perovskite Type for PiezoelectricApplications by Conventional Solid-StateReaction
Structural properties of BaTiO3, CaTiO3 and Ba0.85Ca0.15Ti0.9Zr0.1O3 prepared by conventional solid state reaction technique, at different calcinations temperatures 1100, 1150 and 1280 °C and sintering temperatures (1200and 1300 °C) are studied. These compositions were selected because of theirinteresting piezoelectric properties. To follow the decomposition process of theprecursor, a differential thermal analysis coupled with thermogravimetric analysis (ATG-ATD) was performed. Structural parameters are analyzed by X-raydiffraction (XRD) and scanning electron microscopy (SEM). The obtained resultsshowed clearly the synthesis of the perovskite phase. The diffractogram illustratesthat BCTZ symmetry is both cubic with a Pm-3 m space group and orthorhombic with a R3m space group, the calculated phase rates are respectively 10% and90%. The results allowed us to specify the effect of sintering temperatures on thestructural properties of ceramics. Voir les détails
Mots clés : Lead free ceramics, synthesis, ;, microstructure, Perovskite
Stability, rigidity and thermal vacancies evolution in Fe-Cr-Mn alloys with C and N additions: DFT and Wagner-Schottky model investigations
The pseudo-potential Density Functional Theory (PP-DFT) combined with the statistical Wagner-Schottky model, are applied to study the Fe-xMn-(26-x)Cr-5Mo-3Cu-0.6C-0.7N austenitic alloys (x 1⁄4 6, 9, 12 and 15 wt%). The obtained results show that the rigidity and the thermal vacancy behavior isvery sensitive to the system composition. The overall system stability is found to be governed by thepresence of nitrogen. Both carbon and nitrogen favor the vacancy creation at T~1200K and prevent thethermal vacancy activation at T~1600K. Interestingly, beyond 1273K, carbon atom could migrate towardthe substitution sites while nitrogen atoms do not leave their favorable octahedral sites. Voir les détails
Mots clés : Fe-Cr-Mon Alloys;, Structural Stability;, Rigidity, Thermal Vacancies, DFT, Wagner-Schottky Model
Impact of rare-earth elements on the corrosion performance of binarymagnesium alloys
The corrosion behaviour of Mg-0.3Ce, Mg-0.41Dy, Mg-0.63Gd, Mg-1.44Nd and Mg-1.43La (wt.%) alloys in3.5 wt% NaCl solution was investigated using electrochemical tests. The as-cast microstructures of theMg-RE alloys were characterized by the presence of second phases (MgxCe, Mg41Dy5 , Mg12Gd, Mg12Nd, Mg41Nd5, Mg24Nd and Mg12La) with different volume fraction and distribution. Results show that thecorrosion mechanism was altered from uniform to localized corrosion mechanism depending on thespecific RE alloying elements. The corrosion resistance of the Mg-RE alloys is increasing in the followingorder: Mg-1.43La, Mg-1.44Nd, Mg-0.3Ce, Mg-0.63Gd and Mg-0.41Dy. Accordingly, the corrosionmorphology in the best resistant Mg-0.41Dy alloy and the worst Mg-1.43La alloy were observed andcompared after 2h and 24 h of immersion using SEM-EDS, XPS and XRD analysis. The formation of theDy 2 O 3 oxide prevents the Mg-0.41Dy alloy from pitting corrosion and lead to an excellent corrosionsurface even after 24 h of immersion. Meanwhile, the presence of a high fraction of the Mg 12 La phasealong the grains boundaries in the Mg-1.43La alloy causes severe pitting corrosion by acting as anodicphase. Voir les détails
Mots clés : corrosion resistance, Chloride ion, Magnesium Alloy, Rare earth element
Effect of aging temperature on the microstructure, local-mechanical properties and wear behavior of a UNS S32750 super-duplex stainless steel
In this paper, the effect of aging temperature on the microstructure, local-mechanical properties and wear behavior of a UNS S32750 super-duplex stainless steel is investigated. A massive preferential precipitation of σ phase and Cr2N is detected at the periphery of ferrite with an increase in their extent as aging temperature increases. Simultaneously, the δ phase decomposition is accentuated with temperature and reaches its maximum at 850°C.This behavior results in an enhancement in hardness and Young’s modulus accompanied by a higher plasticity ratio. The wear resistance of the aged samples is investigated in terms of friction coefficients and wear rates against two counterparts. The wear performance follows σ phase presence, the higher σ phase extent, the higher the wear resistance. It is found that the change in wear mechanisms in the 850°C aged samples from abrasive against AISI-304L counterpart to adhesive-oxidative against Al2O3 counterpart is responsible in the increase in wear resistance. Voir les détails
Mots clés : 2507 SDSS, Aging temperature, microstructure, precipitates, Nano-indentation, Wear resistance.
Effet de déformation par pliage sur le comportement à la corrosion de l’acier duplex 2205
The effect of cold deforming by folding on the microstructure and corrosion behavior of austenoferritic stainless steel (duplex 2205) has been studied in this work. Duplex 2205 offers a good compromise between the toughness of austenitic steels and the mechanical and corrosion resistance of ferritic steels. We deformed the duplex steel by bending it at different levels (12, 25, 40 and 50%). The effect of the deformation on the microstructure and mechanical properties was investigated by the various experimental technics; microhardness; metallography, SEM. The relationship between increasing deformation and corrosion resistance was investigated by electrochemical tests. The results showed that cold working modifies the internal microstructure of both phases and that high cold forming rates have a detrimental effect on the corrosion resistance of duplex steels. Voir les détails
Mots clés : deformation, duplex, corrosion, Austenite, Ferrite, Resistant, Folding
Etude expérimentale des vitesses et d’atténuation d’un polymère semi cristallin PEHD par des méthodes ultrasonores
Semi-crystalline polymers are materials that permeate our daily lives, and are used in a wide range of applications, generally when viscoelasticity is required. Therefore, numerous works of research have been conducted these last years to study their elastic properties. Since then, we propose the use of nondestructive analysis techniques based on detecting and spreading ultra sound waves with both immersion and contact. The results obtained on the analyzed materials presents us with good estimates judged to be very satisfying compared to literature. Measures of the velocity and mitigation of longitudinal and transverse ultra sound waves have been conducted on two samples of different thickness of a semi-crystalline polymer of the type PEHD. The experiments have been conducted in ambient temperature. The objective is the characterization of this polymer using ultra sound waves. Voir les détails
Mots clés : Ultrasounds, Velocity, Attenuation, echoes, transmission, polymers.
Étude ab-initio dans le cadre de la DFT des propriétés structurales et physiques de supraconducteur à haute température critique (SHTc) de type pnictides de fer (FeSe)
In this work, we presented a study on the structural, elastic, electronic, magnetic and optical properties of iron selenide. The calculations were performed by the Pseudo-Potential method which is based on density functional formalism (DFT), using local density (LDA) and generalized gradient (GGA) approximations. The doped (GdFeSe and CeFeSe) and undoped (FeSe) structural properties such as the mesh parameters (a, b and c) and the compressibility modulus, are calculated as a function of pressure and the results found are in agreement with those advanced by the experimental and other calculations. In the study of the electronic properties of FeSe, the energy gap value calculated with the GGA approach are in agreement with those of other calculations. As far as the elastic properties are concerned, we calculated the elastic constants with the GGA approach and the found values of the latter are very close to those of the experiment. We also calculated the magnetic moment and confirmed that FeSe is a non-magnetic material. For the optical properties, we calculated the different optical properties such as the function of the reflectivity, the refractive index and the absorption coefficient. Voir les détails
Mots clés : Superconductivity, pnictures, FeSe, DFT, pseudo potential, Castep