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In this study investigations were made into the effects of Ultrasonic Impact Treatment (UIT) on pre-exfoliated AA 2024 T351. Electrochemical tests were conducted to determine any changes in electrochemical behaviour of the alloys due to UIT condition. Uniaxial monotonic, cyclic and fatigue crack propagation (FCG) tests were conducted on material in As-Received (AR) condition and UIT condition in air and in a corrosive environment by pre-exfoliating the test specimens at pre-defined set of time periods and temperature range (20 °C). It was clear that the fatigue performance was severely reduced by the introduction of the corrosion environment for AR specimens. SEM analysis suggests that UIT conditioned AA 2024-T351 exhibits resistance to exfoliation corrosion at ambient and temperatures ranging between 20°C to 40°C when compared to AR specimens. However, these results are not supported by potentiodynamic polarisation curves which show a decrease in corrosion resistance of UIT specimens. Also it is understood that there is a considerable amount of Cu refinement and enrichment near the surface when the AA 2024-T351 is subjected to exfoliation corrosion tests. Whilst hydrogen is in an atomic state, it can be adsorbed onto the metal surface and consequently diffuses into the matrix and can have serious detrimental effects. A reference line for minimal pre-existing hydrogen in the alloy is identified and the magnitude of hydrogen is found to be 180 Arbitrary Hydrogen Units (AHU). It is also found that in corrosion environment, the hydrogen ingress and further charging is prominent at ambient temperatures for AR samples, and showed damage over the full width of cross-section. The following conclusions were drawn: 1. UIT surface treatments, followed by exfoliation corrosion have shown increased resistance to a reduction in mechanical properties, notably tensile and yield strength. Fractographic analysis further supported this finding by showing smaller average brittle failure depths for UIT specimens when compared to AR specimens. SEM analysis of AR samples (without exfoliation) showed a crystallographic contribution to the mode of failure where high density slip bands are formed and the initial failure exhibits a step format. 2. It is observed that during exfoliation, hydrogen ingress and adsorption is more prominent at ambient temperatures for AR samples. 3. A slight improvement In Low Cycle Fatigue (LCF) life is observed for pre-exfoliated and UIT conditioned sample but not for UIT conditioned samples only. Little effect is observed for either treatment in the High Cycle Fatigue (HCF) region. 4. Fatigue crack initiation occurred from the edges for all samples. 5. Fatigue crack propagation of exfoliated specimens exhibited faster crack propagation than As-Received specimens. UIT caused retardation in crack propagation rate in AR samples but not in corroded samples. Failure of pre-exfoliated, UIT treated samples failed within the treated strip. It is also noted that crack deviation can occur when the crack tip reaches a secondary phase particle. 6. Nanocrystallisation generates uniformity of the surface which refines the secondary phase particles and helps mitigate crack initiation sites.

L’objectif de ce travail de thèse était d’étudier le comportement en fatigue oligocyclique de l’acier Fe-3Si utilisé pour les rotors des nouveaux alterno-démarreurs développés par Valeo. L’étude pris en compte la relation entre la réponse mécanique et la microstructure, les mécanismes de fatigue, l’évaluation de la déformation à une échelle mésoscopique et une optimisation du dimensionnement du rotor. Les essais de fatigue, menés sur des éprouvettes d’épaisseur 350μm, ont montré un durcissement cyclique du matériau et le mode de propagation de la fissure, fragile, souligne sa faible ténacité. La formation de structure de dislocations observées par Microscopie Electronique en Transmission (TEM) et par cECCI (Controlled Electron Channelling Contrast Imaging) est dépendante de la déformation et la variation de déformation totale Δεt=0.7% apparaît comme une valeur de transition. En-dessous de Δεt =0.7%, des arrangements homogènes de dislocations et un amorçage de la fissure transgranulaire sont observés. Au-dessus de Δεt =0.7%, des structures veines-canaux et murs-canaux sont observés et l'amorçage de fissure est intergranulaire. Une étude EBSD (Electron Back-Scattered Diffraction) basée sur les paramètres KAM, GROD et GOS menée sur les éprouvettes fatiguées montre un gradient de déformation dans les grains et une localisation de la déformation aux joints de grain au-dessus de Δεt =0.7%. Enfin, en prenant en compte les propriétés en fatigue de l’acier Fe-3Si dans la base de données matériaux de la modélisation par éléments finis, des légères modifications de la géométrie du rotor ont été effectuées ce qui améliore les performances électromagnétiques de la machine. / The objective of the PhD work was to investigate the low cycle fatigue (LCF) behaviour of Fe-3Si steel employed for rotors of new starter-alternators developed by Valeo. The study took into account the relation between mechanical response and microstructure, the fatigue mechanisms, the assessment of strain at a mesoscopic scale and an optimisation of the rotor design. Low cycle fatigue tests performed on 350μm thick specimen pointed out a cyclic hardening of the material. The brittle crack propagation mode underlined the low toughness of the material. The formation of dislocations structures observed by Transmission Electron Microscopy (TEM) and controlled Electron Channelling Contrast Imaging (cECCI) was strain dependant. The total strain variation Δεt =0.7% appeared as a transition value. Below Δεt =0.7%, homogeneous arrangement of dislocations and transgranular crack initiation are observed. Above Δεt =0.7%, a mixture of veins-channels and wall-channels were observed inside the grains and cracking became intergranular. An Electron Back-Scattered Diffraction (EBSD) study was then performed on the specimen fatigued above Δεt=0.7% and approached with KAM, GROD and GOS parameters. It pointed out the strain gradient within grain and highlighted the strain localisation at grain boundaries. Finally, by taking into account LCF properties of the Fe-3Si steel in the material database of the finite element modelling, slight modifications of the rotor geometry have been performed which improves the electromagnetic performances of the machine.

Orientador: Carlos Mauricio Lepienski / Co-orientador: Carlos Eugenio Foerster / Dissertação(mestrado) - Universidade Federal do Paraná