The effect of high temperature, high strain rate deformation on the evolution of second phase particles in commercial aluminium alloys has been investigated. Three model alloys provided by Novelis have been examined, and the evolution of particles during deformation has been examined for the alloy that most closely resembles the composition of alloys used in commercial applications. The effect of deformation mechanisms was expected to be an enhancement of diffusion controlled processes; therefore the first part of the work was to develop a heat treatment that would produce a fine distribution of dispersoid particles. This heat treatment was then used to prepare material for torsion testing, at strain rates similar to those found during the hot rolling stage of commercial production. Testing was performed at both the end of heat treatment temperature, to remove thermal effects, and at a lower temperature which more closely represents the temperature during commercial rolling. Material was examined by optical microscopy, FEGSEM and TEM and the particle populations were characterised by backscattered FEGSEM imaging and image analysis. This demonstrated that the disperoid particle population develops in multiple ways. Along with the enhancement of coarsening there is a significant shape change to the dispersoid particles, suggesting a change in the character of their interface. It has also been demonstrated that there is nucleation of new particles, despite a long prior hold time, in material deformed at the same temperature as the heat treatment. Material deformed at lower temperatures also demonstrated a larger increase in the volume fraction of dispersoid than material with the same thermal history. A constitutive model for diffusion enhancement and a model for particle evolution have been combined to simulate the effects of thermomechanical processing on the particle population.

Aluminium is increasingly being used in the automotive industry to reduce the weight of vehicles. It is the additions of transition elements such as Mn and Cr that can be picked up during recycling, that can form dispersoid particles during homogenisation. Dispersoids play a significant role in the recrystallization and texture development for wrought Al-Mg-Si alloys by inhibiting grain boundary motion. It is therefore important to understand the precipitation kinetics of such particles. The Mn+Cr dispersoid phases are currently thought to nucleate on β'-Mg1.8Si particles via an intermediate semi-coherent precipitate denoted the u-phase. In this study, Al-Mg-Si alloys with additions of Fe and varying levels of Cr were cast to study the effect of different homogenisation regimes on the dispersoid precipitation mechanisms and final characteristics. Electron Probe Micro Analysis (EPMA) was conducted to study the inhomogeneity of elements in the cast structure and through heating to the homogenisation temperature. It was found that Mg, Si and Fe segregate towards the dendrite edges during solidification while Cr segregates towards the dendrite centre. During heating, the matrix composition of both Mg and Si decrease and increase due to precipitation of Mg+Si phases. Cr and Fe stay segregated during the heating process due to the slower diffusion rates in the face centred cubic Al matrix. Dispersoid free regions have also been observed in the microstructure correlating to the elemental segregation in the as-cast condition. Optical, scanning and transmission electron microscopy was utilised in order to study the change in dispersoid characteristics with varying homogenisation regimes and as a function of distance through a grain. With an increase in homogenisation temperature, the mean size of dispersoids increased but number density decreased. For a longer dwell time, the dispersoids remained approximately the same size but increased in volume fraction and density. Increasing the heating rate did not significantly change the dispersoid size, volume fraction or density. The dispersoids size and number density was also studied as a function of distance through a number of grains with the interplay of nucleation, growth and coarsening discussed. Both α-Al(FeCr)Si and α'-AlCrSi dispersoids were found to exist with a variety of morphologies while the α'-AlCrSi dispersoids were found to have a larger effective diameter.

L’alliage d’aluminium 6061-T6 a été choisi comme matériau de structure du casier et du caisson du cœur de réacteur Jules Horowitz (RJH). Transparent aux neutrons, il doit ses bonnes propriétés mécaniques à la précipitation de fines aiguilles nanométriques appelées béta'' contenant Mg et Si et à la présence de dispersoïdes Al(Cr,Fe,Mn)Si jouant un rôle important dans la résistance à la recristallisation. Le caisson et le casier seront soumis à de forts flux neutroniques à une température avoisinant les 50°C. L’objectif de cette thèse est d’étudier les évolutions microstructurales de l’alliage sous irradiation et plus particulièrement la stabilité des précipités. Pour cela, des études analytiques par irradiations in-situ et ex-situ aux électrons et aux ions à température ambiante et forte dose ont été réalisées ainsi qu’une étude du comportement des précipités sous irradiations aux neutrons à faible dose. La caractérisation fine des précipités par Microscopie Electronique en Transmission a montré que les dispersoïdes sont stables sous irradiation, cependant ils présentent une structure cœur/coquille avec un cœur riche en (Fe, Mn) et une coquille riche en Cr qui s’accentue sous irradiation par accélération de la diffusion. En revanche, les nano-phases type béta’’ sont déstabilisées par l’irradiation. Elles sont dissoutes par irradiation aux ions au profit de l’apparition d’amas riches en Mg, Si, Al, Cu et Cr participant à l’augmentation du durcissement de l’alliage, tandis qu’elles tendent à se transformer en précipités cubiques sous irradiation aux neutrons. / The 6061-T6 Aluminium alloy, whose microstructure contains Al(Fe,Mn,Cr)Si dispersoids and hardening needle-shaped beta” precipitates (Mg, Si), has been chosen as the structural material for the core vessel of the Material Testing Jules Horowitz Nuclear Reactor. Because it will be submitted to high neutron fluxes at a temperature around 50°C, it is necessary to study microstructural evolutions induced by irradiation and especially the stability of the second phase particles. In this work, analytical studies by in-situ and ex-situ electron and ion irradiations have been performed, as well as a study under neutron irradiation. The precipitates characterization by Transmission Electron Microscopy demonstrates that Al(Fe,Mn,Cr)Si dispersoids are driven under irradiation towards their equilibrium configuration, consisting of a core/shell structure, enhanced by irradiation, with a (Fe, Mn) enriched core surrounded by a Cr-enriched shell. In contrast, the (Mg,Si) beta” precipitates are destabilized by irradiation. They dissolve under ion irradiation in favor of a new precipitation of (Mg,Si,Cu,Cr,Al) rich clusters resulting in an increase of the alloy’s hardness. beta’’ precipitates tend towards a transformation to cubic precipitates under neutron irradiation.

The effect of individual and combined zirconium and manganese additions have been compared for an AA2198 6 mm thick sheet in T351 temper regarding their influence primarily on recrystallisation resistance and secondly on fracture toughness and overageing resistance. A complete characterisation of the dispersoid distributions was carried out for a deeper understanding of the effects of the Al3Zr and Al20Cu2Mn3 particles, involving studying their formation from the as-cast and homogenised stage.The most important finding in this work was the lower recrystallisation resistance in the alloy containing 0.1 wt%Zr + 0.3 wt%Mn compared to that containing only 0.1 wt%Zr. This result was rather unexpected, if one considers the opposite microsegregation patterns of Zr and Mn during casting, which leads to dispersoids occupying the majority of the grains’ volume and minimising dispersoid-free zones that could be potential sites for nucleation of recrystallisation. The other two alloys with dispersoid additions 0.05 wt%Zr + 0.3 wt%Mn and 0.4 wt%Mn, were partially and fully recrystallised respectively in the rolled T351 condition.Equally important in this work, was the observation that the opposite microsegregation trend of Zr and Mn sufficed to restrict grain growth in unrecrystallised areas. The 0.1Zr-0.3Mn alloy exhibited the lowest grain size of all alloys, both in the T351 temper and after annealing at 535oC for up to 144 hours. The reason for this was the combined action of Al20Cu2Mn3 dispersoids and Mn solute in the regions where the Zr concentration was low (i.e. near the grain boundaries), which offered additional pinning pressure to those areas compared to the 0.1Zr alloy.The lower recrystallisation resistance of the 0.1Zr-0.3Mn alloy was explained on the grounds of two main factors. The first was the lower subgrain size and hence stored energy within bands of Al20Cu2Mn3 dispersoids, which increased the driving force for recrystallisation in these regions. The second was the interaction between Zr and Mn that led to a decrease in the Al3Zr number density and pinning pressure. Since Zr was the dominant dispersoid family in terms of inhibiting recrystallisation, inevitably this alloy became more prone to recrystallisation. The Al3Zr pinning pressure was found to be much lower especially within bands of Al20Cu2Mn3 dispersoids. The detrimental effect of the Mn addition on the Al3Zr distribution was proven not to result from the dissolution of Zr within Mn-containing phases, and several other phases, at the grain interior and also in grain boundaries. The observed effect could not be precisely explained at this stage.Concerning mechanical properties, the 0.1Zr alloy exhibited the best combination of properties in the Kahn tear tests for fracture toughness. Further, it had a higher overageing resistance compared to the 0.1Zr-0.3Mn alloy.As an overall conclusion from this work, considering all the studied properties here that are essential for damage tolerant applications, the addition of 0.1 wt%Zr to the AA2198 6 mm thick sheet was found to be superior to that of the combined addition of 0.1 wt%Zr + 0.3 wt%Mn.