Thesis (Ph. D.)--Washington State University, December 2007. / Includes bibliographical references (p. 142-151).

Measurements are carried out for dendrite tip growth of succinonitrile-acetone alloys solidifying freely in an undercooled melt. The current experimental investigation is conducted using the equiaxed dendritic solidification experiment (EDSE). This setup allows for precise measurements of the dendrite tip velocity, radius and shape for a range of undercoolings and solute concentrations. The collected data are compared to available theories of free dendritic growth, such as the Lipton-Glicksman-Kurz and Li-Beckermann models. It is found that for dilute succinonitrile-acetone alloys, the measured dendrite tip Péclet numbers agree well with previous theories of free dendritic growth, if the effects of melt convection are taken into account. The tip selection parameter deviates significantly from the pure succinonitrile value and is inversely related to the applied undercooling. Besides, the selection parameter shows no dependence on the solute concentration. These results are consistent with phase-field simulations and preceding experimental investigations. In addition, scaling relationships for the sidebranching shape were obtained in terms of the dendritic envelope, projection area and contour length. These new scaling relations agree well with previous measurements in pure succinonitrile dendrites by Li and Beckermann.

A detailed study has been made of thermally activated glide between 373 K and 20 K for UHV annealed single crystals of two Nb based substitutional alloy systems containing 1-16 at.%Mo or 4 - 60 at.%Ta, in conjunction with a study of the deformation of UHV annealed single crystals of Nb between 4.2 K and 77 K. Whilst the addition of Ta had only a small effect on the properties of Nb as measured by activation volume and enthalpy and the temperature dependence of the flow stress, it produced a large increase in the low temperature yield stress and displaced the appearance of anomalous slip to lower temperatures, e.g. 77 K for the 10 at.%Ta alloy. Addition of Mo produced more rapid changes Whilst the 1 at.%Mo alloy behaved like the Nb-Ta alloys, the appearance of anomalous slip was depressed to 113 K. Further additions appeared to suppress anomalous slip completely and radically alter the behaviour of the alloys. The thermodynamic analysis suggested that the more concentrated Nb-Mo alloys show a change in the rate limiting step at low temperatures, cf. a peak in the activation volume - effective stress curve. No solution softening was observed in the alloys. Complex transients were found for all the alloys at and below 77 K after changes in strain rate. The yield and thermal stresses for Nb deforming by anomalous slip were independent of temperature between 77 K and 50 K, only regaining a temperature sensitivity below 50 K. The importance of this for models of anomalous slip was discussed. Complex overshoots were observed after changes in the strain rate. Those observed at and below 20 K have been explained by reference to the change in specimen temperature produced by heat generation during dislocation glide.

Over the last few decades, body-centered-cubic (bcc) beta (β) titanium alloys have largely been exploited as structural alloys owing to the richness in their microstructural features. These features, which lead to a unique combination of high specific strength and ductility, excellent hardenability, good fatigue performance, and corrosion resistance, make these alloys viable candidates for many applications, including aerospace, automobile, and orthopedic implants. The mechanical properties of these alloys strongly depend on the various phases present; which can be controlled by thermomechanical treatments and/or alloy design. The two most important and studied phases are the metastable ω phase and the stable α phase. The present study focuses on the microstructural evolution and the mechanical behavior of these two phases in a model β-Ti alloy, binary Ti-12wt. %Mo alloy, and a commercial β-Ti alloy, β-21S. Microstructures containing athermal and isothermal ω phases in the binary Ti-12wt. %Mo alloy are obtained under specific accurate temperature controlled heat treatments. The formation and the evolution of the ω-phase based microstructures are investigated in detail via various characterization techniques such as SEM, TEM, and 3D atom probe tomography. The mechanical behavior was investigated via quasi-static tensile loading; at room and elevated temperatures. The effect of β phase stability on the deformation behavior is then discussed. Similar to the Ti-12wt. %Mo, the formation and the evolution of the athermal and isothermal ω phases in the commercial β-21S alloy was studied under controlled heat treatments. The structural and compositional changes were tracked using SEM, TEM, HR-STEM, and 3D atom probe tomography (3D-APT). The presence of additional elements in the commercial alloy were noted to make a considerable difference in the evolution and morphology of the ω phase and also the mechanical behavior of the alloys. The Portevin-Le Chatelier (PLC) like effect was observed in iii this alloy at elevated temperature and this has been attributed to the shearing of the ω precipitates and the dynamic precipitation of the α phase within these channels. The formation of the stable α phase in the commercial β-21S alloy due to the influence of precursor phases, like the metastable ω phase, is investigated. It is evident from the microstructural characterization, using SEM, TEM, HR-STEM, and 3D-APT, that the ω phase does play a role on the fine scale α precipitation. The mechanical behavior of the β+α microstructure, investigated via tensile testing, shows that these alloys are ideal candidate for precipitation hardening. The exceptional strength values obtained in this alloy have been attributed to a combination of several factors.

Nous avons réalisé des calculs de structure électronique ab initio, basés sur la théorie de lafonctionnelle de la densité (DFT), pour étudier les propriétés des dislocations vis h111i dansles métaux de transition cubiques centrés (V, Nb, Ta, Mo, W et Fe). Dans tous ces éléments,le coeur facile non-dégénéré est la configuration d’énergie minimale et la configuration de coeurdissociée a une énergie très élevée, comparable ou plus élevée que celle du coeur difficile, encontradiction avec les prédictions des potentiels interatomiques. Nous avons mis en évidence destendances de groupe marquées sur l’énergie de coeur de la dislocation facile, reliées à la positiondu niveau de Fermi par rapport au minimum du pseudo-gap de la densité d’états électroniques.Notre travail fait aussi apparaitre un comportement atypique du fer, avec une énergie relativedu coeur difficile basse, proche de celle du point col entre deux coeurs faciles, conduisant à unpotentiel de Peierls plat autour de la configuration difficile, contrairement aux autres éléments.A partir de ces calculs DFT, nous avons construit le paysage énergétique à deux dimensionsdans le plan {111} (potentiel de Peierls) et nous avons étudié plusieurs propriétés relativesau glissement des dislocations, et en particulier l’énergie de formation de la paire de décrochementset la dépendance de la contrainte de Peierls en fonction de l’orientation cristalline.Nous proposons une modification simple de la loi de Schmid, qui prend en compte la trajectoirenon rectiligne de la dislocation et qui permet d’expliquer qualitativement pourquoi l’asymétriemaclage/antimaclage est moins marquée dans Fe que dans les autres métaux cubiques centrés. / We performed electronic structure ab initio calculations based on density functional theory(DFT) to study the h111i screw dislocation properties in body-centered cubic transition metals(V, Nb, Ta, Mo, W and Fe). In all investigated elements, the nondegenerate easy coreis the minimum energy configuration and the split core configuration has a high energy nearor above that of the hard core, contrary to interatomic potential predictions. A strong groupdependence of the core energy of the easy dislocation is also evidenced, related to the positionof the Fermi level with respect to the minimum of the pseudogap of the electronic density ofstates. Our work also reveals an atypical behavior in Fe, with a low relative energy at the hardcore position, close to that of the saddle configuration between easy cores, resulting in a flatPeierls potential around the hard core configuration, at variance with other elements. Fromthese DFT calculations, the two-dimensional energetic landscape in the {111} plane (Peierlspotential) is constructed and we investigated several properties of dislocation glide and in particular,the kink-pair formation enthalpy, as well as the dependence of the Peierls stress oncrystal orientation. We proposed a simple modification to the Schmid law that takes accountof the non-straight trajectory of the dislocation and that qualitatively explains why the twinning/antitwinning asymmetry is less pronounced in Fe than in other body-centered cubic metals.

Afin de mieux appréhender la plasticité des alliages métalliques, il est important de pouvoir décrire à l'échelle atomique les interactions entre dislocations et solutés et d’en déduire l’effet sur la mobilité des dislocations. Au cours de cette thèse, nous nous sommes intéressés aux métaux de transition cubiques centrés (CC), et en particulier au fer, en présence de solutés interstitiels. A l’aide de calculs en Théorie de la Fonctionnelle de la Densité (DFT), la structure de cœur de la dislocation vis de vecteur de Burgers b=½<111> a été étudiée dans le fer en présence de solutés de bore, carbone, azote et oxygène et dans les métaux CC des groupes 5 (V, Nb et Ta) et 6 (Mo, W) en présence de carbone. Nous avons mis en évidence dans le fer et les métaux du groupe 6 une reconstruction du cœur de la dislocation en présence de solutés, associée à une très forte énergie d’attraction dislocation/soluté. Un comportement différent a été observé pour le groupe 5, la configuration la plus stable pour le carbone étant un site octaédrique proche de la dislocation, sans reconstruction de cœur. Cette tendance de groupe a été reliée à la structure des mono-carbures. Les conséquences des interactions fortement attractives dans le fer en présence de carbone ont ensuite été développées. D’une part la ségrégation d’équilibre du carbone proche du cœur de la dislocation a été étudiée à l’aide de modèles en champ moyen et de simulations Monte Carlo. D’autre part, la mobilité de la dislocation décorée a été étudiée en modélisant le mécanisme de double décrochement, en lien avec des observations expérimentales en microscopie électronique à transmission / In order to improve our understanding of alloy plasticity, it is important to describe at the atomic scale the dislocation-solute interactions and their effect on the dislocation mobility. This work focuses on the body-centered cubic (BCC) transition metals, in particular Fe, in presence of interstitial solute atoms. Using Density Functional Theory (DFT) calculations, the core structure of the screw dislocation of Burgers vector b=½<111> was investigated in iron in presence of boron, carbon, nitrogen and oxygen solute atoms, and in BCC metals from group 5 (V, Nb, Ta) and 6 (Mo, W) in presence of carbon solutes. A core reconstruction was evidenced in iron and group 6 metals, along with a strong attractive dislocation-solute interaction energy. A different behavior was observed in group 5 metals, for which the most stable configuration for the carbon atom is an octahedral site in the vicinity of the dislocation, without any core reconstruction. This group tendency was linked to the structure of mono-carbides. Consequences of the strongly attractive dislocation-solute interactions in Fe(C) were then investigated. First the equilibrium segregation close to the dislocation core was studied using a mean-field model and Monte Carlo simulations. Then, the mobility of the dislocation in presence of carbon atoms was investigated by modeling the double-kink mechanism with DFT, in relation with experimental data obtained with transmission electron microscopy

The dependence of flow stress of body centered cubic metals on variables such as strain rate, temperature, strain and microstructural is a research area of continued interest. Recently, there has been renewed interest in deformation of fine grained BCC metals, which display characteristics that are different from their coarse-grained counterparts. Deformation mechanisms, strain-rate and temperature dependence, and strain hardening characteristics of fine-grained BCC metals are not well understood. The aim of this thesis is to understand the effect of strain-rate, temperature, strain and microstructure (i.e., grain size) on the mechanical response of poly¬crystalline tantalum. Among the topics addressed were constitutive modeling of flow stress, understanding the microstructural origins of strain hardening, and characterizing the effect of severe plastic deformation (SPD) on microstructure and mechanical properties. Rolling and equal-channel angular pressing (ECAP) were among the processing techniques employed. Mechanical testing was conducted over a range of temperatures and strain rates, and this was supported by a slew of microscopic characterization methods. It was found that the strain hardening response depends on microstructural evolution at different strain rates. Results indicate that the same thermally activated mechanisms operate in both as-received and processed material and this was found to be the overcoming of Peierls barriers via a double-kink mechanism. Lastly, it was found that the low strain rate sensitivity of SPD processed material was not due to fine grain size, but instead due to high internals stresses.