Title: Investigation of vacancies in Fe-Al alloys Author: RNDr. František Lukáč Department: Department of Low Temperature Physics Supervisor: doc. Mgr. Jakub Čížek, Ph.D., Department of Low Temperature Physics, Faculty of Mathematics and Physics Abstract: Fe-Al alloys exhibit relatively high vacancy concentrations, which significantly influence the mechanical properties of these alloys. Positron annihi- lation spectroscopy was employed for investigations of vacancies in this work and the vacancy concentrations in Fe-Al alloys were determined by positron lifetime spectroscopy and measurements of positron diffusion length of implanted mono- genergetic positrons. The correlation of hardness with the vacancy concentration was characterized by determination of the vacancy hardening coefficient in Fe- Al alloys. The thermal evolutions of hardness and the vacancy concentration were compared during the annealing of the Fe-Al alloys with various composi- tions. Coincidence Doppler broadening of annihilation peak was employed for determination of the chemical composition of quenched-in vacancies in Fe-Al al- loys. Increased concentration of Al atoms in surrounding vacancies in Fe-Al alloys was related to the attractive interaction between the Al atoms and the vacancy. Quantum mechanical ab initio calculations were...

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<p>Applications of Aluminum (Al) alloys in harsh environments involving high stress and high temperatures are often hindered because of their inherently low strength and poor performance at high temperatures. The strongest commercial Al alloys reported up to date have a maximum strength less than 700 MPa. Although ultrafine grained Al alloys prepared by severe plastic deformation have higher strength, they encounter grain growth at moderate temperatures. </p> <p>This thesis focuses on adopting transition metal solutes and non-equilibrium approach to fabricate high-strength, thermally stable nanotwinned Al alloys. To understand the underlying deformation mechanisms of nanotwinned Al alloys, <i>in-situ</i> micromechanical tests, high resolution and analytical transmission microscopy and atomistic simulations were used. Our studies show that nanotwinned supersaturated Al-Fe alloys have a maximum hardness and flow stress of ~ 5.5 GPa and 1.6 GPa, respectively. The apparent directionality of the vertical incoherent twin boundaries renders plastic anisotropy and compression-tension asymmetry in the nanotwinned Al-Fe alloys, revealed by systematic <i>in-situ</i> tensile and compressive micromechanical experiments conducted from both in-plane and out-of-plane directions. Moreover, the nanotwinned Al-Fe alloys experience no apparent softening when tested at 200 °C. When selectively incorporating with one additional solute as stabilizer, the ternary nanotwinned Al alloys can preserve an exceptionally high flow stress, exceeding 2 GPa, prior to precipitous softening at an annealing temperature of > 400 °C. The thesis offers a new perspective to the design of future strong, deformable and thermally stable nanostructured Al alloys. </p>

The final thesis determined the properties of alloys formed from mixtures of powders processed by the SLM method. Powders of alloy AlSi12 and EN AW 2618 were fused in the proportion 75 wt. % AlSi12 + 25 wt. % 2618, 50 wt. % AlSi12 + 50 wt. % 2618 and 25 wt. % AlSi12 + 75 wt. % 2618. Metallographic analysis, EBSD analysis and line EDS microanalysis were made on the samples. Tensile test at room temperature and hardness were carried out to determine the mechanical properties. Fractographic analysis was performed after tensile test.

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Le soudage FSW (Friction Stir Welding) est un procédé de soudage en phase solide pressenti pour des applications de transport en générale aérospatial et naval. Malgré le nombre considérable d’études qui ont été réalisées depuis son avènement en 1991, le contrôle du procédé n’est pas encore effectif.Ce travail a consisté en une partie expérimentale visant à la génération, par un outil trigone, de joints soudés dont la microstructure a été corrélée à l’écoulement de matière pendant le procédé. La connaissance de cet écoulement de matière a permis dans la deuxième partie d’enrichir le modèle thermofluide développé en périodique pour prédire la microstructure des joints de soudure FSW, notamment les "onion rings". Finalement, l’occurrence des "onion rings" a été corrélée à la vitesse de déformation maximale atteinte par les particules de la zone soudée, prédite par le modèle. Par ailleurs, un travail d’affinement du champ de vitesse en voisinage du pion est réalisé en modélisant l’outil trigone. Ce qui permet en plus de l’interaction (entrainement) outil/matière par frottement, d’intégrer une interaction par obstacle. Cette approche devrait permettre, en perspectives de ce travail, une meilleur description thermomécanique locale et par voie de conséquence microstructurale. / Friction Stir Welding is a solid state joining process developed for transport applications as aerospace and naval. Since its introduction, a large number of investigations have been carried out but the process is not fully controlled. This work including experimental section in which welds have been generated by trigonal tool. The microstructure of these welds has been correlated with the material flow during the process. By understanding the material flow, the transient thermofluid model developed in the second section has been significantly enriched. This modeled has been developed for predicting the microstructure of the weld, especially, the "onion rings". Finally, the occurrence of "onion rings" has been correlated with the maximal strain rate reached by any particle in the weld seam, simulated by the model. However, the velocity has been refined at the vicinity of the tool through the trigonal pin modelling. This was helpful to move the material not only by friction but also by obstacle at the interaction tool/material. The above approach should enable, in this work layout, a better local thermomechanical description and consequently microstructural.

The effect of annealing on the tribological and corrosion properties of Al–12Si samples produced by selective laser melting (SLM) is evaluated via sliding and fretting wear tests and weight loss experiments and compared to the corresponding material processed by conventional casting. Sliding wear shows that the as-prepared SLM material has the least wear rate compared to the cast and heat-treated SLM samples with abrasive wear as the major wear mechanism along with oxidation. Similar trend has also been observed for the fretting wear experiments, where the as-prepared SLM sample displays the minimum wear loss. On the other hand, the acidic corrosion behavior of the as-prepared SLM material as well as of the cast samples is similar and the corrosion rate is accelerated by increasing the heat treatment temperature. This behavior is due to the microstructural changes induced by the heat treatment, where the continuous network of Si characterizing the as-prepared SLM sample transforms to isolated Si particles in the heat-treated SLM specimens. This shows that both the wear and corrosion behaviors are strongly associated with the change in microstructure of the SLM samples due to the heat-treatment process, where the size of the hard Si particles increases, and their density decreases with increasing annealing temperature.

Dans le cadre d'un projet de transport automobile propre pour la mise en place de véhicules électriques décarbonés grand public, les aimants représentent un enjeu important au niveau des moteurs électriques. Il est alors nécessaire de se passer des aimants à base de terres rares, peu écologiques et dont un seul pays détient le monopole. Deux voies sont explorées ici : la nanostructuration des hexaferrites de type M par frittage flash de type SPS (Spark Plasma Sintering), et la synthèse d'alliages Mn-Al-C par la méthode de la trempe rapide sur roue. Concernant le premier matériau, l'optimisation du protocole de synthèse a permis d'augmenter le champ coercitif d'un facteur 2. Deux outils de caractérisation des aimants ont été mis en place : le premier évalue la texture du matériau (modèle de Stoner et Wohlfarth), le second étudie les processus d'aimantation (Preisach). Pour les alliages Mn-Al-C, nous avons pu démontrer la présence d'un couplage antiferromagnétique entre les atomes de Mn des sites 1a et 1d. La diffraction de neutrons à également pu montrer que le moment magnétique du Mn du site 1d est par ailleurs très supérieur à celui du site 1a. Un broyage cryogénique suivi d'un frittage SPS à haute pression (400 MPa) a permis d'augmenter la valeur du champ coercitif de 40 %. Les résultats obtenus sont très prometteurs quant à la faisabilité d'alternatives aux aimants à base de terres rares. / In the context of green car development for mass market, development of low cost rare-earth-free magnets is clearly a milestone, because the rare-earth are expensive and their production is not ecological and a monopoly of one country. Two routes are explored herein : nanostructuration of M-type hexaferrites by flash sintering (Spark Plasma Sintering), and the synthesis of Mn-Al-C alloys by melt spinning. Concerning the first material, the optimization of the synthesis protocol allowed to increase the coercive field by a factor of 2. Two characterization tools of the magnets were set-up : the first evaluates the texture of the material (Stoner and Wohlfarth model), the second, based on Preisach model, is used to infer the magnetization processes. For Mn-Al-C alloys, we have evidenced from Weiss plots the antiferromagnetic coupling between 1a and 1d-sites Mn atoms. Neutron diffraction confirmed this result and shown that the magnetic moment of the 1d-site Mn is also much larger than that of 1a-site. Cryogenic ball milling followed by high-pressure SPS (400 MPa) increased the coercive field by 40 %. The results obtained are very promising as to the feasibility of alternatives to rare-earth magnets.

<p><a>Aluminum (Al) alloys have been widely used in </a>industry as light-weight structural materials. However, the mechanical strength of the strongest Al alloys is still much lower than most high-strength steels. This thesis aims to investigate the fabrication and mechanical behaviors of nanotwinned high-strength Al alloys and multilayers.</p> <p>Twin boundaries are special grain boundaries with mirror symmetry. Twin boundaries can generate slip discontinuity and block the transmission of dislocations, and serve as dislocation sources to accommodate plasticity. However, twinning in Al is rare due to its high stacking fault energy and low unstable stacking fault energy. In this thesis, we used multiple methods to introduce high-density twins into Al and achieve outstanding mechanical properties and thermal stability. </p> <p>Certain type of solutes can greatly increase the twin density in Al by decreasing the stacking fault energy of Al and retarding the detwinning process. Nanotwinned Al-Ni and Al-Ti binary alloys fabricated by magnetron sputtering show high strength, good deformability, and unique deformation mechanisms. Furthermore, deformation and thermal stability of binary nanotwinned Al alloys can be enhanced by adding a third or fourth solute element. </p> <p>Interfaces can facilitate twin formation in Al as well. High-density twins and stacking faults were introduced into Al by using Al/Ti layer interfaces. Nanotwinned Al/Ti multilayers have ultra-high strength, superb deformability and thermal stability. This thesis provides promising pathways to fabricate Al alloys and composites with high strength and good thermal stability.</p>