Multiple strain path changes during forming lead to complex geometrically necessary dislocation (GND) development in strain gradient fields, inducing internal stresses that contribute to the Bauschinger effect, residual stresses, and springback which alters the final geometrical shape of the part. In order to analyze and design improved processing routes, models must capture the evolution of these internal stresses. However, most models capture the effects of these stresses via phenomenological approaches that require calibration to each new material and strain path. The development of models that capture the underlying physics at the sub-grain level is underway but requires in-depth studies of dislocation behavior (at the relevant meso length scale) in order to guide and validate them. The novel experimental campaign central to this thesis aims to tackle this problem by capturing unprecedented data of dislocation activity for several sheet metals during multiple strain path deformation. The resultant insights provide a new window into multi-path forming of metals, while also aiding the development and validation of two crystal plasticity (CP) models by collaborators at the University of New Hampshire (UNH). The models incorporate internal stresses at the grain and sub-grain levels, respectively. The hardening response due to strain path change during forming of AA6016-T4 was studied at the macro- and micro-level via combined experiments and an elasto-plastic self-consistent (EPSC) model. The experiments demonstrated that possible recombination and/or redirection of dislocations onto different slip systems under strain path change allowed for a gradual elasto-plastic transition, in comparison to a much sharper response upon continued deformation under the same strain path due to buildup and immediate activation of backstresses. The phenomenological backstress law of the EPSC model underpredicted the yield stress response for the strain path change deformations, possibly due to missing sub-grain GND development and an accurate description of associated backstresses. A more detailed experimental study of multi-path deformation for the AA6016-T4 was required in order to guide development of a strain gradient elasto-visco plasticity self-consistent model (SG-EVPSC); the model includes sub-grain strain gradient fields, and related internal stress fields. Total dislocation and GND density were tracked at various points of the deformation, and a complete 3D statistical volume element was characterized, to enable accurate modeling of the microstructure. The tests revealed a relatively lower yield stress response following strain path change, presumably aided by lower latent hardening than self hardening; the tests then showed a rapid accumulation of dislocations on the newly activated slip systems resulting in much higher final dislocation density without affecting the ductility of the pre-strained material. Interestingly, GND development was dominated by the precipitates instead of grain boundaries. These observations are vital for an accurate forming prediction from CPFEA models. Finally, optimized forming conditions of continuous bending under tension produced a ratcheting strain path resulting in a gradual GND development and a more complete retained austenite transformation in quenched-&-partitioned- and TRIP-assisted bainitic ferritic-1180 steels increasing their ductility by at least 360%.

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Ce travail porte sur l’étude de mésophases lamellaires lyotropes qui contiennent des polymères hydrosolubles confinés dans les lamelles d’eau de l’empilement. L’effet du polymère confiné sur la stabilité de la structure lamellaire est en particulier étudié grâce à une mesure directe des interactions entre les membranes de tensioactif grâce la technique du Surface Force Apparatus (SFA). Les systèmes étudiés sont des fluides complexes et très visqueux ce qui a donné lieu au développement d’une nouvelle méthode pour analyser les données collectées. En l’absence de polymère dissous dans l’empilement lamellaire, la valeur du module de compressibilité élastique mesurée est remarquablement interprétée avec la théorie électrostatique corrigée des corrélations des contre-ions. En présence de polymère, au fur et à mesure que les macromolécules remplacent le contenu en eau, le module élastique de compressibilité de l’empilement lamellaire chute,signature d’une interaction attractive due à la présence des macromolécules. Néanmoins, les développements théoriques proposés ne parviennent pas à interpréter quantitativement cette décroissance. De plus, des comportements très intéressants et inattendus ont été mis en évidence lors de l’approche d’un point critique : la présence de dislocations de très grands vecteurs de Burgers à grande séparation ainsi que des phénomènes d’avalanches. En particulier, des énergies de nucléation de dislocation ont pu être extraites. / This work deals with lyotropic lamellar mesophases which contain confined hydrosoluble polymers in the water layers of the stack. In particular the effect of confined polymers on the stability of the lamellar structure is investigated through a direct measurement of the interactions between the surfactant membranes thanks to the Surface Force Apparatus (SFA) technique. The investigated systems are complex fluids and samples are extremely viscous, this is why a new procedure for analyzing the collected data has been developed. In the absence of dissolved polymer in the lamellar stack, the measured value of the elastic compressibility modulus is remarkably interpreted with the electrostatic interaction corrected from the counterions correlations. In the presence of polymer, as long as the macromolecules replace the water content the elastic compressibilitymodulus of the lamellar stack decreases, evidencing an attractive interaction due to the macromolecules. Nevertheless, the proposed theoretical developments fail to quantitatively interpret this drop. Moreover, very interesting and unexpected phenomena have been evidenced on the approach of one critical point: dislocations of large Burgers vectors at large separations and avalanche phenomena. In particular dislocation nucleation energies have been extracted.

Die vorliegende Arbeit beschaeftigt sich mit dem strukturellen Aufbau von (1-100) M-plane GaN, das mit plasmaunterstuetzter Molekularstrahlepitaxie auf gamma-LiAlO2(100) Substraten gewachsen wurde. Die heteroepitaktische Ausrichtung einerseits, sowie die Mikrostruktur und die Erzeugungsmechanismen der Defekte andererseits, wurde mit der Transmissionselektronenemikroskopie (TEM) systematisch untersucht. Das gamma-LiAlO2 Substrat reagiert heftig im Mikroskop unter Bestrahlung mit hochenergetischen Elektronen. Waehrend dieser Strahlenschaedigung verliert das Material seine urspruengliche kristalline Struktur und vollzieht eine Phasentransformation, die anhand einer Serie von Feinbereichsbeugungsdiagrammen nachgewiesen werden konnte. Die atomare Grenzflaechenstruktur zwischen epitaktisch gewachsenem alpha-GaN(1-100) und tetragonalem gamma-LiAlO2 Substrat ist mittels HRTEM untersucht worden. Die neuartige Epitaxiebeziehung ist mit Elektronenbeugung bestaetigt worden und lautet folgendermassen: (1-100)GaN liegt parallel zu (100)gamma-LiAlO2 und [11-20]GaN ist parallel zu [001]gamma-LiAlO2. Die Realstruktur der M-plane GaN Schichten, die auf (100)gamma-LiAlO2 gewachsen werden, unterscheidet sich erheblich von der in C-plane Orientierung hergestellten Epischichten. Ausfuehrliche TEM Untersuchungen zeigen, dass die M-plane Schichten vor allem intrinsische (I1 und I2) und extrinsische (E) Stapelfehler in der Basalebene enthalten. Der vorherrschende I2 Stapelfehler besitzt keine Komponente des Verschiebungsvektors senkrecht zur Ebene und ist damit nicht geeignet, epitaktische Dehnung entlang der [11-20] Richtung abzubauen. Darueberhinaus ist eine komplexe Grenze in der (10-10) Prismen- flaeche entdeckt worden, die zur Grenzflaeche geneigt verlaeuft. Die Defekte in den M-plane GaN Epischichten werden waehrend der anfaenglichen Keimbildungsphase erzeugt. Atomare Stufen entlang der [001] Richtung auf dem LiAlO2 Substrat fuehren zur Bildung von Stapelfehlern vom Typ I2. / In this work the structure of (1-100)M-plane GaN epitaxially grown on gamma-LiAlO2(100) by using plasmaassisted molecular beam epitaxy (PAMBE) is studied. The heteroepitaxial alignment and the microstructure of M-plane GaN as well as the defect formation in the layer are systematically investigated by using transmission electron microscopy (TEM). The gamma-LiALO2 substrate reacts under irradiation of high-energy electrons in the TEM (200-300 keV).The material looses its original crystalline structure during this process undergoing irradiation damage followed by a phase transformation as it is verified by a series of selected area diffraction patterns taken under constant electron dose. The result is a structural phase transformation from the tetragonal gamma to the trigonal alpha phase. The atomic interface structure of epitaxially grown hexagonal alpha-GaN(1-100) layers on tetragonal gamma-LiAlO2 (100) substrates is investigated by means of HRTEM. The novel epitaxial orientation relationship verified by electron diffraction is given by (1-100)GaN parallel to (100)gamma-LiAlO2 and [11-20]GaN parallel to [001]gamma-LiAlO2. The defect structure of M-plane GaN epilayers grown on gamma-LiAlO2(100) substrates is different to that of C-plane GaN. Our detailed TEM studies reveal that the M-plane layers mainly contain intrinsic I1 and I2 and extrinsic E basal plane stacking faults. The dominant I2 stacking fault has no out-of-plane displacement vector component and is thus not qualified for epitaxial strain relief along the [11-20] axis. Beyond this, a complex type of planar defect is detected in the (10-10) prism plane which is inclined with respect to the interface. The study of nucleation samples shows that the surface morphology is directly correlated to the generation of the dominant planar defects. Atomic steps along the [001] direction in the gamma-LiAlO2 substrate result in the formation of basal plane stacking faults I2.