Understanding microstructural evolution in Friction Stir Welding (FSW) of steels is essential in order to understand and optimize the process. Ferritic steels undergo an allotropic phase transformation. This makes microstructural evolution study very challenging. An approach based on Electron Backscattered Diffraction (EBSD) and phase transformation orientation relationships is introduced to reconstruct pre-transformed grain structure and texture. Reconstructed pre-transformed and post-transformed grain structures and textures were investigated in order to understand microstructural evolution. Texture results show that there is evidence of shear deformation as well as recrystallization in the reconstructed prior austenite. Room temperature ferrite exhibits well-defined shear deformation texture components. Shear deformation texture in the room temperature microstructure implies that FSW imposes deformation during and after the phase transformation. Prior austenite grain boundary analysis shows that variant selection is governed by interfacial energy. Variants that have near ideal BCC/FCC misorientation relative to their neighboring austenite and near zero misorientation relative to neighboring ferrite are selected. Selection of coinciding variants in transformed prior austenite Σ3 boundaries supports the interfacial-energy-controlled variant selection mechanism.

Le Zircaloy-4 utilisé comme matériau de gainage des combustibles nucléaires est trempé β puis filé sur aiguille dans le haut domaine α. La microstructure de trempe, qui conditionne les opérations de mise en forme ultérieures, se présente sous deux formes : vannerie ou colonies de lamelles parallèles. Ces dernières se fragmentent difficilement lorsqu’elles sont normales à l’effort de compression. La thèse étudie trois aspects de ce phénomène. Le premier concerne les conditions de trempe : temps d’homogénéisation dans le domaine β et vitesse de refroidissement. Une adaptation au Zircaloy-4 de l’essai Jominy montre que ces deux paramètres ont une influence décisive sur la taille des colonies (par l’intermédiaire de la taille des grains β) et sur l’épaisseur des lamelles. Le second présente des essais de compression selon trois directions orthogonales. La troisième passe fragmente les colonies qui ont résisté aux deux autres et affine sensiblement la microstructure. A 750°C en particulier, un cycle de trois passes permet d’obtenir des grains de 30 µm ; mais les meilleurs résultats sont obtenus à 650°C (grains de 17 µm) et à grande vitesse de déformation (grains de 10 µm).Dans le troisième, un modèle de plasticité cristalline tridimensionnel implémenté dans le code d’éléments finis ABAQUS simule le comportement des lamelles sous l’effet de la contrainte. Il prend en compte leur orientation cristallographique en plus de leur morphologie. Dans la plupart des cas, les lamelles s’incurvent dès le début de la déformation macroscopique du matériau, ce qui induit des localisations de la déformation. / Zircaloy-4 used for fuel cladding in nuclear plants is quenched from the β range and then extruded and rolled in the upper α range. At the start of this mechanical process, the alloy possesses a lamellar, Widmanstätten microstructure. This one, which is critical for the subsequent forming process, appears under two forms: basket weave and colonies of parallel lamellas. These are difficult to break when they are normal to the compressive load. The thesis studies three aspects of this phenomenon. The first concerns the quenching conditions: homogenization time in the β range and cooling rate. An adaptation of the Jominy test to Zircaloy-4 shows that these two parameters have a decisive influence on the size of the colonies (via the β grain size) and the thickness of the lamellas. The second presents compression tests under three orthogonal directions. Results show that the third pass breaks the colonies that resisted to the previous attempts and refine noticeably the microstructure. In particular at 750°C, three passes are sufficient to obtain grains of 30 µm, but the best results are obtained at 650°C (grains of 17 µm) and at high strain rate (grains of 10 µm). Thirdly, a three-dimensional crystal plasticity model is implemented in the finite elements code ABAQUS to simulate the behaviour of lamellas under stress. It takes into account their crystallographic orientation in addition to their morphology. In most cases, the lamellas bend at the onset of the macroscopic deformation, which induces localization phenomena.

During loss-of-coolant accidents (LOCA) and reactivity-initiated accidents (RIA), nuclear fuel rods experience high heating rates that change the microstructure and properties of zirconium cladding materials, which are in forms of stress-relieved, like cold-worked (CW) or recrystallised (RX) microstructure. The present study aimed to determine how different fast heating rates and starting microstructures affect the kinetics of phase transformation, the transformation textures and eventually the mechanical response in the dual-phase region. The LOCA/RIA cycles from heating at 8 to 100C/s to alpha+beta or above beta transus temperature were achieved via resistive heating in an electro-thermal-mechanical tester. Synchrotron X-ray diffraction (SXRD) and electrical resistivity measurements showed that the approach curves of CW Zircaloy-4 shift to higher temperature at faster constant heating rates and change to a new approach curve when changing rates. 2-second holding at two-phase temperature produces identical phase fractions as equilibrium. These observations are consistent with the diffusional character of the phase trans- formation. Heated at 100oCs1, RX samples transform with 2D beta-growth while CW ones show simultaneous beta-nucleation and growth. The difference arises because the fast heating rate helps preserve low-angle grain boundaries (GB) in the CW microstructure up to phase transformation temperature, increasing beta nucleation sites and prevent beta-growth. This gives rise to different textures of RX and CW materials before transformation, producing different textures, which are weak in both cases. However, this difference is enhanced during grain growth and transformation on cooling. Thus, the RX material shows strong final alpha texture with 0002 maxima aligned in TD and tilted 20deg from ND towards TD while the CW reveals an essentially random one. In both RX and CW materials, variant selection does not occur during transformation on heating. During beta-grain growth, although there is variability in beta-textures measured by SXRD and EBSD beta reconstruction, it is clear that variant selection occurs, leading to strengthening of the beta texture. During transformation on cooling, variant selection occurs early in nucleation of the alpha phase from the shared 110 beta GB in the RX condition. The flow stresses of CW Zircaloy-4 in the two-phase regime at a given temperature depend on the heating rates, despite having the same phase fractions. Heated at a slower rate, the material shows an upper yield stress followed by softening behaviour while that heated faster has a smaller yield stress followed by a high work-hardening rate and then stable flowing stresses. The evolution of diffraction elastic strains and intensity suggest the upper yield stress and softening are due to stress-induced transformation of the harder alpha grains into large and isolated softer beta grains. In contrast, the sample heated faster develops an almost continuous film of beta grains along the GB of unrecrystallised alpha-grains which results in early beta-yielding and coherent deformation of the two phases, leading to constant flow stresses. The findings will improve the accuracy of inputs from phase fractions, microstructure and texture of zirconium claddings when modelling LOCA/RIA. A crystal plasticity model should consider the effects of heating rates and cold-work, which are often ignored. The link between deformation, fast heating rates and microstructure evolution might be relevant to other processes like additive layer manufacturing and even forging in the two-phase region.

Šiame baigiamajame darbe buvo nagrinėjami aukšto pravažumo (4x4) šarvuoti taktiniai automobiliai ir jų specifika. Apžvelgti, apskaičiuoti ir palyginti automobilių rodikliai. Nagrinėjamas G. Bekker metodas automobilių mobilumui nustatyti. Naudojantis BDTM metodika apskaičiuojami automobilių mobilumo parametrai. Nagrinėjamas prioritetų skirstymo ir parinkimo (MPSP) metodas. Šio metodo pagrindu sudaroma metodika, su kuria galima palyginti su kitais ir išrinkti optimalų automobilį, atsižvelgiant į prioritetines automobilio charakteristikas, bei ekspertų vertinimus. / In this final thesis gives an overview of the high utility (4x4) armored tactical vehicles and their specifications. Reviewed, calculated and compared the vehicles parameters. Was examined G. Bekker equations and method of determination vehicles mobility. Calculated vehicle mobility parameters using the BDTM methodology. Examined the modificated allocation of priorities and selection (MPSP) method. Developed high-utility (4x4) tactical vehicle selection method that allows you to compare with others and choose the optimum car, depending on the vehicle's performance, and expert assessment. This method is based on the method of MPSP and gives an opportunity to evaluate the high utility (4x4) tactical vehicle to different partial criteria and their meaning of importance.

Additive Manufacturing (AM) is an innovative manufacturing process which offers near-net shape fabrication of complex components, directly from CAD models, without dies or substantial machining, resulting in a reduction in lead-time, waste, and cost. For example, the buy-to-fly ratio for a titanium component machined from forged billet is typically 10-20:1 compared to 5-7:1 when manufactured by AM. However, the production rates for most AM processes are relatively slow and AM is consequently largely of interest to the aerospace, automotive and biomedical industries. In addition, the solidification conditions in AM with the Ti alloy commonly lead to undesirable coarse columnar primary β grain structures in components. The present research is focused on developing a fundamental understanding of the influence of the processing conditions on microstructure and texture evolution and their resulting effect on the mechanical properties during additive manufacturing with a Ti6Al4V alloy, using three different techniques, namely; 1) Selective laser melting (SLM) process, 2) Electron beam selective melting (EBSM) process and, 3) Wire arc additive manufacturing (WAAM) process. The most important finding in this work was that all the AM processes produced columnar β-grain structures which grow by epitaxial re-growth up through each melted layer. By thermal modelling using TS4D (Thermal Simulation in 4 Dimensions), it has been shown that the melt pool size increased and the cooling rate decreased from SLM to EBSM and to the WAAM process. The prior β grain size also increased with melt pool size from a finer size in the SLM to a moderate size in EBSM and to huge grains in WAAM that can be seen by eye. However, despite the large difference in power density between the processes, they all had similar G/R (thermal gradient/growth rate) ratios, which were predicted to lie in the columnar growth region in the solidification diagram. The EBSM process showed a pronounced local heterogeneity in the microstructure in local transition areas, when there was a change in geometry; for e.g. change in wall thickness, thin to thick capping section, cross-over’s, V-transitions, etc. By reconstruction of the high temperature β microstructure, it has been shown that all the AM platforms showed primary columnar β grains with a <001>β.