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On the evolution of microstructure, texture and corrosion behavior of ahot-rolled and annealed AZ31 alloy

Samia Tighiouaret, Abdelkader Hanna, Hiba Azzeddine, Lyacine RABAHI, Achour Dakhouche, François Brisset, Anne-Laure Helbert, Thierry Baudin, Djamel Bradai  (2021)

The microstructure and texture evolution of an AZ31 alloy were investigated after hot rolling and subsequentannealing using electron backscatter diffraction (EBSD). First, the alloy was hot-rolled at 350 ? C up to low,medium and high strain (20, 50 and 85% of thickness reduction, respectively). The alloy samples where thenannealed at 350 ? C for 2, 10 and 60 min. The effect of strain level and annealing on corrosion behavior inseawater was also evaluated using electrochemical tests. At low strain, the microstructure was characterised bythe absence of twinning, mainly due to the prior thermo-mechanical history of the as-received alloy. However,various modes of twinning were observed at medium strain. At high strain, the dynamic recrystallization processresulted in a microstructure with a typical basal texture. The results demonstrate that twins are responsible forthe deviation of {0002} basal poles from normal towards the transversal direction. Annealing at 350 ? C for up to60 min led to normal grain growth in all the samples. In medium and highly strained samples, the deformationtexture was retained, while the low strain sample underwent noticeable changes due to the absence of dynamicrecrystallization. A synergetic effect of grain refinement and texture weakening was responsible for the alloy’senhanced corrosion resistance.AZ31 alloyCorrosionDynamic recrystallizationStatic recrystallizationRolling Voir les détails

Mots clés : AZ31 alloy, corrosion, Dynamic recrystallization, Static recrystallisation, rolling

Physical properties investigation of Fe1−xAlx(x≤50%-at) alloys using DFT and Wagner-Schottky model

I Berrached 1, M Gallouze 1, L Rouaiguia 1, L Rabahi 1, T Grosdidier 2, M Drir A Kellou  (2020)

Atomistic modeling based on the Density Functional Theory (DFT) is used to study thestructural, magnetic, electronic and mechanical properties of Fe1−xAlx alloys (x≤50%-at) with and without B, C and N additions over the selected range of Al atomic concentration. It is shownthat a singularity around x Al ∼ 40%-at is observed for the lattice parameters while the magneticmoment decreases uniformly without unexpected trend. The enthalpies of formation indicatethat the presence of B stabilizes the system for Al concentration in the range of 25%-at Voir les détails

Mots clés : iron aluminides, Magnetic Properties, elastics proprieties, Electronic structures, DFT calculations, thermal defects, Wagner-Schottky Model

Impact of rare-earth elements on the corrosion performance of binarymagnesium alloys

Hiba Azzeddine, Abdelkader Hanna, Achour Dakhouche, Lyacine RABAHI, Nico Scharnagl, Milan Dopita, François Brisset, Anne-Laure Helbert, Thierry Baudin  (2020)

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

A systematic DFT study of (Ti3/2RE1/2)AlC alloys: A new database foradjustable mechanical and electronic properties

C. Meftah, N. Iles, L. Rabahi, M. Gallouze, H.I. Feraoun, M. Drir  (2022)

In this study, ab initio calculations based on Pseudo-Potential Density Functional Theory (PP-DFT) method arecarried out in order to highlight the partial substitution effect of Rare Earth (RE) elements in the well-known 211-MAX phase of Ti2AlC. The considered elements are Y, Sc and RE = La, Ce, Pr, Nd, Sm, Eu, Gd leading to (Ti3/2RE1/2)AlC alloys. According to the obtained results, the (Ti3/2RE1/2)AlC alloys are significantly less compressibleunder uniaxial stress along x and z axes. They exhibit high resistance to shearing along <001> direction. Inaddition, the calculated heat capacity for (Ti3/2RE1/2)AlC alloys increases with respect to the temperature, amaximum is found in the temperature range 200–300 K. Localized states occur in (Ti3/2RE1/2)AlC alloys due tothe f states filling of the rare earth elements. The magnetic moment of (Ti3/2RE1/2)AlC compounds increasesaccording to 4f n (n=2 for Ce to n=7 for Gd) filling. Our findings provide a theoretical database for new tunableproperties of (Ti3/2RE1/2)AlC alloys. Voir les détails

Mots clés : DFT, Rare earth elements, (Ti3/2Re1/2)AlC, mechanical properties, Localized States, Magnetic moment

Stability, rigidity and thermal vacancies evolution in Fe-Cr-Mn alloys with C and N additions: DFT and Wagner-Schottky model investigations

A. Boudiaf, L. Rabahi, L. Rouaïguia, L. Adnane, A. Kellou  (2020)

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

On the corrosion behaviour of as-cast and heat-treated Mg-RE alloys in 0.9% NaCl solution

Abdelkader HANNA 1, Lyacine RABAHI 2, Mohamed Amine SOUALILI 3, Achour DAKHOUCHE 4, Djamel BRADAI 5, and Hiba AZZEDDINE  (2020)

The microstructure and corrosion behaviour of as-cast and heat-treated Mg-1.44Ndand Mg-1.43Ce (wt.%) alloys in 0.9% NaCl (wt.%) solution were investigated usingelectrochemical tests, X-ray diffraction (XRD) and scanning electron microscopy,combined with energy-dispersive X-ray spectroscopy (SEM-EDS). The as-castmicrostructure of both alloys revealed the presence of second phases. Heat treatmentat 535°C for 6 h led to a more uniform distribution of the second phases in Mg-1.44Ndalloy and their dissolution along the grains boundaries in the Mg-1.43Ce alloy. As aresult, the corrosion resistance was improved in the heat-treated alloys. Accordingly,the corrosion resistance values for the heat-treated alloys were much higher than thoseof the as-cast alloys, indicating that the heat-treated alloys were less susceptible to thecorrosion. Also, the heat-treated Mg-1.43Ce alloy seems to have very good corrosionresistance (26890 Ω cm 2 ) compared to the Mg-1.44Nd alloy (6156 Ω·cm 2 ) bypreventing pitting corrosion along the grains boundaries. The corrosion product wasmade up mainly of magnesium hydroxide Mg(OH) 2 and magnesium oxide MgO andmore uniform corrosion morphology were found in the heat-treated alloys. Voir les détails

Mots clés : Corrosion resistance Heat treatment Magnesium alloy NaCl solution Rare earth

Microstructural and Mechanical Properties of Welding and Thermal Spraying Coatings on Ductile Cast Iron

A.KELLAI, S.KAHLA, S.DEHIMI, B.Babes  (2021)

The subject of this work is to evaluate the influence and adhesion degree of different coating layers deposited on a ductile cast iron substrate by two different methods, thermal spraying and welding with and without use of an interlayer. Microstructures of different zones and interfaces of coated specimens are investigated using optical microscope and scanning electron microscope SEM. Also, the mechanical behavior was evaluated by tensile test. It is found that when stainless steel thermal spraying coating onto the ductile cast iron substrate, the use of the nickel-based interlayer Ni allowed us to mitigate the disadvantages of cracking at the interface. This is due to the mechanical effect of nickel plasticity. In the case of coating by welding, the use of nickel-based buttering ENi-CI allowed us to reduce the diffusion of graphite to stainless steel, resulting in a reduction in the formation of harder alloy carbides. Finally, the mechanicals tests in particular the tensile test shows that the coating by welding is effective but causes a structural hardening; on the other hand the coating realized by thermal spraying does not really present sufficient adhesion. Voir les détails

Mots clés : Ductile Cast Iron, mechanical properties, microstructure, Thermal Spraying, Welding Coating

Dissimilar welding of aluminum alloys 2024 T3 and 7075 T6 by TIG process with double tungsten electrodes

L.Kaba, Mohammed Elamine Djeghlal, Seddik OUALLAM, Sami KAHLA  (2022)

The aim of this work is to study the metallurgical and mechanical properties of dissimilar assemblies of 2024 T3 and 7075 T6 structural hardening aluminum alloy by the TIG twine electrode arc welding process. It will include a weld performed according to optimized welding parameters followed by a study of the macroscopic and microscopic evolution of the dissimilar assembly (2024-7075) using optical and scanning electron microscopy (SEM); in addition, the phase compositions were analyzed with an energy dispersive spectrometer (EDS). Tensile and microhardness tests were performed. The tensile fracture was observed by SEM. This paper suggests that when the double tungsten electrode TIG welding is used, a stable arc has been formed with a good bead appearance. The heat dissipated by the arc generates several zones (molten zone (WZ), bonding zones (LZ), heat-affected zones (HAZ)) with different microstructures or precipitates of the type θ (Al2 Cu), S (Al2 Cu, Mg) and η (Mg Zn2), S (Al2 Cu Mg) are formed in the heat-affected zone (HAZ) of base metals 2024 and 7075 respectively. The microhardness is lower in the molten zone and higher in the heat-affected zone of 7075 T6 alloy, which cried out an embrittlement and a 44% and 37% drop in the tensile strength of 7075 T6 and 2024 T3 base metals respectively. Voir les détails

Mots clés : 2024 and 7075 aluminum alloy, Aluminum with structural hardening, microstructure, Double electrode TIG processing, Dissimilar welding

Maximum power extraction framework using robust fractional-order feedback linearization control and GM-CPSO for PMSG-based WECS

S.KAHLA, M.Bechouat, T.AMIEUR, M.Sedraoui, B.Babes, N.Hamouda  (2020)

The most important issue in the use of wind energy conversion systems is to ensure maximum power extraction in terms of efficiency. Therefore, maximum power point tracking algorithms are as important as the maximum power point tracking controller. In this study, maximum power extraction frameworks operating the state-of-the-art optimization methods are presented for permanent magnet synchronous generator–based wind energy conversion system. These frameworks consist of a Gauss map–based chaotic particle swarm optimization and a hybrid maximum power point tracking approach that combines feedback linearization technique with fractional-order calculus. The feedback linearization control strategy can fully decouple and linearize the original state variables of the nonlinear system and thus provide an optimal controller crossing wide-range operating conditions. The objective is to maintain the tip speed ratio at its optimal value, which implies the use of a rotational speed loop. The method is based on the feedback linearization technique and the fractional control theory. Gauss map–based chaotic particle swarm optimization, which is a remarkable and recent optimization technique, is utilized to achieve optimum coefficients to efficiently ensure the maximum power point tracking operation in here. A simulation study is carried out on a 3-kW wind energy conversion system to show the effectiveness of the proposed control scheme. Voir les détails

Mots clés : Permanent Magnet Synchronous generator, Maximum Power Point Tracking, Feedback linearization control, fractional-order theory, Gauss map–based chaotic particle swarm optimization

A new robust tilt-PID controller based upon an automatic selection of adjustable fractional weights for permanent magnet synchronous motor drive control

T.AMIEUR, M.Bechouat, M.Sedraoui, S.KAHLA, H.Guessoum  (2021)

This paper focuses on achieving a good trade-off between performance and robustness for a class of uncertainty models including unstructured multiplicative uncertainties. In robust control, the simultaneous improvement of the two secure margins for nominal performances and robust stability using a standard controller structure represents two contradictory objectives and guaranteeing simultaneously of these goals represents therefore a major challenge for most researchers. In this context, a robust tilt-proportional integral derivative (T-PID) controller synthesized with an automatic selection of adjustable fractional weights (AFWs) is discussed in our work. Their parameters are optimized through solving a weighted-mixed sensitivity problem using an optimization tool which is based on the genetic algorithm. This problem is formulated from performance and robustness requirements where a fitness function is accordingly determined. Furthermore, thus its search space is built according to some guidelines for ensuring an automatic selection of adequate AFWs. The proposed constrained optimization problem is initialized by using arbitrary T-PID speed controller as well as through initial fixed integer weights (FIWs) which were chosen previously by the designer. To highlight the proposed control strategy, the synthesized robust T-PID speed controller is applied on the permanent magnet synchronous motor. Their performance and robustness are compared to those provided by an integer-order PID (IO-PID) and two conventional fractional-order PID (FO-PID) controllers. This comparison reveals superiority of the proposed robust T-PID controller over the remaining controllers in terms of robustness with reduced control energy. Voir les détails

Mots clés : Weighted-mixed sensitivity problem, Tilt-proportional integral derivative controller, Fractional-order FO-PID speed controller, Permanent magnet synchronous motor