Search

The influence of hot deformation on the evolution of size, shape, and fraction of dispersoids has been studied in a simple 3xxx aluminium alloy by means of hot torsion testing. It has been shown that at high strain rates, deformation leads to spheroidization of the dispersoids, an increase in number density, and an increase in volume fraction. The increase in number density and volume fraction are associated with precipitation of new particles. The enhancement of manganese diffusion is a key factor in promoting rapid dispersoid evolution during deformation. A model has been developed to estimate the effect of deformation induced vacancies and dislocations on diffusion. This predicts that an order of magnitude increase in diffusion coefficient between may occur under typical hot deformation conditions, consistent with the rapid microstructural changes measured experimentally.

The development of both dispersoid and constituent particle types during high temperature deformation has been investigated. Using torsion testing, which enables good temperature and strain rate control, the development of particles in terms of individual properties and the overall population has been examined during extended high strain rate deformation. Torsion tests also allow material that has the same thermal history but different levels of strain within a single sample to be compared. Quantitative comparison of particles has been performed using high resolution SEM imaging. Strain has been shown to have an important influence on particle evolution, beyond changing the kinetics of particle evolution alone. It has been demonstrated that the shape of the dispersoids is altered when they are evolving under the action of strain compared to that obtained from a thermal effect.

Second phase particles in wrought aluminium alloys are crucial in controlling recrystallization and texture. In Al-Mn-Fe-Si (3xxx) alloys, the size, spacing, and distribution of both large constituent particles and small dispersoids are manipulated by heat treatment to obtain the required final microstructure and texture for operations such as can-making. Understanding how these particles evolve as a function of process conditions is thus critical to optimize alloy performance. In this study, a novel 3-dimensional technique involving serial sectioning in the scanning electron microscope (SEM) has been used to analyse the intermetallic particles found in an as-cast and homogenized Al-Mn-Fe-Si alloy. This has allowed an accurate determination of the size and shape of the constituent particles and dispersoids derived from a 3-dimensional dataset. It is demonstrated that a proper consideration of the 3-dimensional microstructure reveals important features that are not obvious from 2-dimensional sections alone.

A numerical model based on the Kampmann and Wagner method was developed to predict the evolution of precipitate distribution in 7xxx aluminium alloy during non-isothermal heat treatments. The model considers the nucleation, growth and coarsening/dissolution of the metastable and equilibrium precipitate phases, η' and η with their stochiometric composition, MgZn2. Constitutive model equations for nucleation were based on the classical theory of nucleation whilst growth and coarsening were treated using classical phase transformation theory. The transition between η' and η, where η' acts as a precursor for η was also accounted for in the model. Differential scanning calorimetry was used to calibrate the homogeneous precipitation kinetics. The model also predicts the evolution of grain boundary precipitates and their effect on precipitate free zone size. Jominy end quench tests were performed to calibrate grain boundary precipitation kinetics. Precipitation on dislocations and dispersoids was considered. The dislocation and dispersoid densities were varied to represent different regions of a grain and therefore account for the spatial distribution of preferential heterogeneous precipitation sites. Comparison between the model prediction and experimental characterisation of the microstructure evolution of a friction stir welded 7449 aluminium alloy was found to be reasonably consistent.

The effect of friction stir welding (FSW) and post weld heat treatment (PWHT) on the second phase particle distribution and cross weld hardness profile in AA7449 plate has been investigated. The alloy was received in an underaged condition, welded, then PWHT to give an overaged condition (in the parent material) . The effect of this complex treatment on the precipitate distribution in the weld and parent plate has been investigated over a range of length scales using small angle X-ray scattering (SAXS), TEM and FEGSEM. It is shown that the PWHT does not improve the hardness in the heat affected zone (HAZ), which is the location of the strength minimum after welding, but it does reduce the difference between the hardness in the HAZ and the nugget and parent hardness. The reduction in nugget strength after PWHT is particularly marked and is due to replacement of fine GP zones formed on post weld natural ageing by coarse overaged precipitates.

Two models to predict the microstructural evolution and post-weld properties of friction stir welds in aerospace aluminium alloys are presented. The first model is a develop- ment of an existing semi-empirical method for the prediction of hardness profiles after welding, calibrated using isothermal hardness data. Post-weld natural ageing is accounted for, and a new method that predicts natural ageing kinetics is introduced. Once calibrated, the model is shown to accurately predict weld hardness profiles. However, this model does not explicitly predict the microstructure and therefore cannot readily be extended to model other properties. It can also only be applied to alloys welded in peak or overaged conditions. The second model aims to explicitly predict the heterogeneous precipitate distributions obtained after welding for any initial condition. It is based on classical kinetic theory and the numerical framework of Kampmann and Wagner. Multiple nucleation sites and multiple phases are accounted for. This model provides detailed microstructural information required for prediction of complex properties.

The relationship between plane strain fracture toughness, KIc, tensile properties and the microstructure of two high strength 7xxx alloys has been examined. Two formulations of the 7449 composition were examined, specifically, one containing Zr as the dispersoid forming element, whilst the other contained Mn. Fractography revealed coarse voiding at intermetallics, fine tensile voiding at dispersoids and a combined intergranular/transgranular shear fracture mode. In the Zr containing alloy in particular, the observed increase in toughness with overageing is accompanied by a change in fracture mode from predominantly intergranular/transgranular shear failure to coarse voiding. A new fracture toughness model has been derived based on the microstructurally dependent work hardening factor, KA, introduced in Ashby’s theory of work hardening. This model predicts a linear relationship between KIc and KA^0.85/?ys^0.35 which is shown to be consistent with the experimental data.