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.

<p>In this thesis a mixture density network has been constructed to predict steel hardenability for a given alloy composition. Throughout the work hardenability is expressed in terms of jominy profiles according to the standard jominy test. A piecewise linear description of the jominy profile has been developed to solve the problem of missing data, model identification from data based on different units and measurement uncertainty. When the underlying physical processes are complex and not well understood, as the case with hardenability modelling, mixture density networks, which are an extension of neural networks, offer a strong non-linear modelling alternative. Mixture density networks model conditional probability densities, from which it is possible to determine any statistical property. Here the model output is presented in terms of expectation values along with confidence interval. This statistical output facilitates future extension of the model towards optimisation of alloy cost. A good agreement has been obtained between the experimental and the calculated data. In order to ensure the reliability of the model in service, novelty detection of the input data is performed.</p>

In this thesis a mixture density network has been constructed to predict steel hardenability for a given alloy composition. Throughout the work hardenability is expressed in terms of jominy profiles according to the standard jominy test. A piecewise linear description of the jominy profile has been developed to solve the problem of missing data, model identification from data based on different units and measurement uncertainty. When the underlying physical processes are complex and not well understood, as the case with hardenability modelling, mixture density networks, which are an extension of neural networks, offer a strong non-linear modelling alternative. Mixture density networks model conditional probability densities, from which it is possible to determine any statistical property. Here the model output is presented in terms of expectation values along with confidence interval. This statistical output facilitates future extension of the model towards optimisation of alloy cost. A good agreement has been obtained between the experimental and the calculated data. In order to ensure the reliability of the model in service, novelty detection of the input data is performed.

Powder metallurgy (PM) steel is produced by near net shape manufacturing, which is used to fabricate alloy steels for many purposes. Designing new powder metal steels that can form a significant fraction of martensite relies on hardenability calculations developed for wrought steels. These proven tools are built upon assumptions for wrought steels that do not hold true for PM steels. One assumption is that the alloying elements are homogenized throughout the material. In admixed powder blends that are industrially sintered this is not the case. Using prealloyed powder is a solution to this issue, yet it places restrictions on alloy design and compressibility. There are tools available to computationally optimize diffusion problems, yet the complexity during the sintering of PM steel is such that a robust model has yet been produced. It is intuitive that with smaller particles of Fe sintering time can be reduced. A direct experimental investigation linking Fe-powders’ sizes and hardenability on Fe-C-Cr-Mn-Mo-Ni PM steel was subject to microstructure analysis and mechanical properties (Jominy test) for comparative analysis. Another assumption that is made for wrought steel is a consistent density of 7.87g/cm3. This is not the case for PM steel as the press and sinter method produces pores, decreasing the density. This directly affects the thermal conductivity and phase transformation of the steel. In an effort to understand how these differences affect Grossmann’s predictions of hardenability, a direct experimental investigation linking the density to hardenability was launched on prealloyed FL-4605 and FL-4605+2%Cu. Specifically the Jominy test was completed on a range of densities, as well as compared to software predictions. The chemical variations in admixed and sintered PM steel produce a unique system where one TTT diagram cannot predict the entire final microstructure. PM steel such as this is observed in industry, and can be created through incorporating larger Fe-particles such that less alloying constituents have a chance to fully alloy these regions. Since the large particles will not have the chance to be alloyed, they will not have the ability to form martensite. Since the regions between large particles will be alloyed, martensite will form, creating a hard matrix surrounding softer particles. This structure is characteristic of a metal matrix composite (MMC), and therefore should be treated as such. There are methods of MMC design that involve numerical methods of predicting strength and toughness. These methods, along with experimental data (tensile and Charpy testing) of Fe-C-Cr-Mn-Mo-Ni PM steels with ranging volume fractions of pearlitic inclusions were compared. / Thesis / Master of Applied Science (MASc)

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.

The physical properties of polymer quench bath directly affect the cooling rate of a quenched part. These properties include the type of quenchant, its temperature, concentration, and agitation level. These parameters must be controlled to optimize the quenching process in terms of alloy microstructure, properties and performance. Statistically designed experiments have been performed to investigate the effects of the process parameters (i.e. polymer concentration and agitation) on the heat transfer behavior of cast aluminum alloy A356 in aqueous solution of Aqua-Quench 260 using the CHTE quenching-agitation system. The experiments were designed using Taguchi technique and the experimental results were analyzed with Analysis of Variance (ANOVA) based on the average cooling rate. It is found that average cooling rate dramatically decreases with the increase in polymer concentration. Agitation only enhances the average cooling rate at low and medium concentration levels. From ANOVA analysis, the process parameter that affects the variation of average cooling rate most is the polymer concentration, its percentage contribution is 97%. The effects from agitation and the interaction between polymer concentration and tank agitation are insignificant. The mechanical properties of age-hardenable Al-Si-Mg alloys depend on the rate at which the alloy is cooled after the solutionizing heat treatment. A model based on the transformation kinetics is needed for the design engineer to quantify the effects of quenching rates on the as-aged properties. Quench Factor analysis was developed by Staley to describe the relationship between the cooling rate and the mechanical properties of an age-hardenable alloy. This method has been previously used to successfully predict yield strength, hardness of wrought aluminum alloys. However, the Quench Factor data for aluminum castings are still rare in the literature. In this study, the Jominy End Quench method was used to experimentally collect the time-temperature and hardness data as the inputs for Quench Factor modeling. Multiple linear regression analysis was performed on the experimental data to estimate the kinetic parameters during quenching. Time-Temperature-Property curves of cast aluminum alloy A356 were generated using the estimated kinetic parameters. Experimental verification was performed on a L5 lost foam cast engine head. The predicted hardness agreed well with that experimentally measured.

Cette étude se propose de fournir une description précise des microstructures lamellaires de deux alliages, le Zircaloy-4 et le TA6V, et de caractériser leur déformation à haute température. A cette fin, des techniques expérimentales nouvelles pour ces matériaux ont été développées : essai Jominy instrumenté, compression plane encastrée à parois mobiles, microtomographie X. Les principaux résultats soulignent le rôle de la phase alpha_GB formée aux anciens joints de grains bêta sur la sélection de variants dans le Zircaloy-4 et le TA6V. L'agencement tridimensionnel des colonies dans le TA6V est également mis en lumière pour la première fois et discuté en relation avec la genèse de la microstructure. Dans le Zircaloy-4 comprimé à haute température, plusieurs mécanismes de localisation de la déformation sont observés. L'activité du maclage à 750 °C, en particulier, est clairement mise en évidence. Une classification de ces hétérogénéités est proposée et leur influence sur la recristallisation est discutée.