Materials Science Forum Vols. 519-521

Abstract: Important activities in the aluminum industry are the development of new alloys, and the optimization of thermo-mechanical treatments to obtain desired performance. The strength and formability of aluminum alloys depend on the distribution and scale of precipitating phases, on the grain size and grain orientation distribution, on the distribution and scale of flaws, and on the presence of residual stresses. Thus it is useful to have detailed quantitative data on the crystal structures and volume fractions of phases that form during thermomechanical treatment, on the kinetics of solid state reactions, on the distribution of grain orientations, and on the stresses that develop during mechanical testing and forming. Neutron scattering is a powerful tool that can provide unique data to guide the development of improved materials and processes. Of particular interest are in-situ experiments: such experiments are uniquely suited to neutron diffraction because of the high penetrating power of neutrons, which allows data to be collected from materials subjected to realistic conditions (load, temperature, atmosphere) in specialized sample environments. In this presentation, we discuss several examples of neutron scattering studies, including residual strain mapping, in-situ loading experiments, texture analysis, powder diffraction, and tomography.

Abstract: GP and GPB zone formation in Al-Cu-Mg alloys proceeds rapidly at room temperature immediately subsequent to STQ. This structure evolution is well known for GP zones but not for GPB zones. In many age-hardenable Al-Cu-Mg alloys this vacancy assisted diffusion of Cu from solid solution to form zones is essentially complete within 50 hours with only a small residual quantity of Cu remaining in solid solution. The alloy then remains in this metastable state. This zone formation is observed here using 63Cu NMR for the alloys AA2014 and AA2124 which lie in the α-θ (GP) and α-S (GPB) phase fields respectively. However these zones which form so readily at room temperature are unstable on aging at higher temperatures. Rapid dissolution of the zones, and their reversion back into solid solution at elevated temperatures is explicitly demonstrated by 63Cu nuclear magnetic resonance (NMR). At this stage the Cu is shown to remain stably in solid solution at room temperature. Further aging at the same elevated temperature is then shown to reform the zones with further continuous evolution to either the θ-phases (AA2014) or S-phase (AA2124).

Abstract: This paper examines the challenges which are encountered when using electrical resistivity measurements for characterization of microstructures in aluminum alloys. Experimental examples are provided of electrical resistivity studies conducted on two aluminum alloys, a heattreatable alloy (AA6111) and a non-heat-treatable alloy (AA5754), which demonstrate how the technique can be used to characterize changes in the microstructure. Results on AA6111 show that the dependence of the measurement on solute atoms and fine scale precipitates makes deconvolution of the resistivity signal non-trivial and therefore, utilization of supplementary technique(s) in conjunction with electrical resistivity measurements is essential. In the next example, room temperature electrical resistivity measurements as a function of cold work for AA5754 illustrate a larger resistivity contribution from dislocations in this alloy as compared to that reported for pure aluminum. The interaction of solutes and dislocations is cited as the possible source for the increased dislocation contribution.

Abstract: In the present study, a thermomechanical simulator (Gleeble 3500) was used to represent the solid–state bonding of aluminium in hollow dies during the extrusion of AA7020 alloy. The variations in strain, strain rate, temperature and pressure reflected those typically encountered during extrusion. The results showed that to form a good bond, strain was the most influential parameter, along with the time of deformation (bonding time). Temperature and pressure were found to be less influential within the parameter ranges investigated. On the basis of the results obtained, the ‘stretching’ of the interface has been proposed to be a critical parameter to quantify the solid–state bonding phenomenon.

Abstract: Melt-spun flakes and air atomised powder of a multi-component Al-Si-Fe-X alloy were consolidated by field assisted (FAST) or spark plasma sintering (SPS) in vacuum using steel dies and punches. Experiments were carried out at 350, 400, 450 and 500°C under applied loads ranging from 81 to 283 MPa. The resulting compacts were microstructurally and mechanically characterized. Ultimate strength values up to 1000 MPa and plastic strains up to 20% were observed during compression tests. The effect of the powder shape on the sintering behaviour is compared. The effect of process parameters such as temperature and applied load on the densification and mechanical properties is discussed. It was found that compacts sintered from melt-spun flakes resulted in a higher strength and ductility than compacts produced from air-atomized powder, sintered under identical conditions.

Abstract: Microhardness and hardness maps have been measured in different sections of samples processed by Equal Channel Angular Pressing (ECAP). In order to investigate the homogeneity of the hardening induced by ECAP as a function of deformation, the hardness maps have been based on samples deformed at different strains (after one and four passes). After one pass the specimen’s outer side, both on its cross and longitudinal planes, is less hard than all other zones. Moreover, after 4 passes via route Bc, the hardening induced on the sample cross section is lower at the surface than in the central area of the billet. The reduced hardness regions are compared to the distributions of plastic equivalent strain generated by finite element analysis.

Abstract: The main focus in this work is to investigate the effect of crystallographic texture, grain structure and dispersoids on formability and toughness in some industrial 6xxx and 7xxx series alloys. Materials of these alloys showing strong cube textures or β-fibre deformation textures in as extruded condition have been compared with the same alloys processed by rolling and heat treatment to obtain a random texture. It is found that the formability depends on the temper and the texture and that the effect of the latter is path dependent. Materials with a random texture have a significant higher formability in terms of uniform elongation than materials with cube texture when deformed in the W-temper condition. Forming in other deformation modes shows less difference between the cube and random texture. However, a fibrous grain structure with a sharp β-fibre texture shows an anomalous behaviour when deformed in the biaxial deformation regime. Toughness, in terms of Charpy energy and local strains in the necking area, is significantly higher for materials with a cube texture as compared to materials with random textures. This difference is explained by variations in the dispersoid levels, grain structures (size and grain boundary misorientation) and the texture.

Abstract: 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.