Materials Science Forum Vols. 794-796

Abstract: On the basis of the balance equations of chemical and phase composition of the Al-Mg-Li and Al-Cu-Li alloys developed a method of determining the amount of intermetallic phases from the experimentally measured value of the lattice parameter of α-solid solution. In alloys of Al-Mg (Cu)-Li the relative phase ratio of the δ'(Al₃Li) and S₁ (T₁) are determined by the atomic percentage ratio of the Li and Mg (Cu).

Abstract: A conventional way to determine precipitation kinetics in heat treatable aluminium alloys is to monitor the associated solute loss by in-situ resistivity. A Gleeble machine is used to perform so called isothermal quenching (IQ) resistivity measurements. IQ consists in quenching the alloy down to a given temperature and holding it at this temperature. The results are validated against measurements performed with a classical four-points method using continuous current on the same alloy.

Abstract: Two examples of precipitation studies (in Al-Li-Cu and Al-Li-Mg alloys) are shown to demonstrate the complementarity of atom probe tomography, small-angle-scattering and differential scanning calorimetry for precipitation studies. It will be used to unravel an unexpected two-step precipitation behavior of T₁ in Al-Li-Cu and to ascertain precipitates size in Al-Li-Mg. through a model free comparison between atom probe and SAXS.

Abstract: Al-Mg-Si-(Cu), i.e. AA6xxx, alloys are widely used light alloys which can be effectively strengthened through precipitation hardening. The final microstructure, and thus properties, of these alloys after common artificial aging treatments are largely determined by the composition-dependent nano-scale clustering and precipitation that occur during the earliest stage of aging. Therefore, multi-length scale analysis of the earliest-stage of precipitation can provide critical knowledge in understanding the basis for the microstructural evolution during aging and attaining the desired microstructures and properties. Here, we investigate the effect of alloy composition on the evolution of early-stage clusters and precipitates during aging at 180°C using high resolution transmission electron microscopy. The results map a sequential evolution of clusters with an FCC structure but different morphology/orientation characteristics. GP-zones with structures other than FCC, also form in the early stages of aging. The composition-dependent kinetics of β” phase precipitation and hardening behavior are discussed in light of the results from differential scanning calorimetry experiments, microhardness measurements, and conventional transmission electron microscopy.

Abstract: Isothermal time-temperature-precipitation (TTP) diagrams deliver important material data like temperature and time ranges critical for precipitation. During the last years an in-situ calorimetric method to record continuous cooling precipitation diagrams has been developed to application level by our group. However, isothermal TTP-diagrams were still determined by ex-situ analyses only. In this work in-situ measurements of precipitation reactions were carried out during isothermal soaking. Therefore the whole heat treatment cycle was performed in a differential scanning calorimeter (DSC). Al-Mg-Si-alloys 6063 and 6005A were analysed. Solution annealing and overcritical quenching to several temperatures between 450 °C and 250 °C was followed by isothermal soaking. Based on the heat flow curves during isothermal soaking TTP-diagrams were determined. Further microstructure investigations by scanning electron microscopy and hardness tests after artificial ageing were performed. Both alloys show similar results. In the TTP-diagramms three so-called ”C-curves” could be observed. Every C-curve is expected to represent precipitation of a different phase. Hardness and microstructure investigations correspond with the data of the TTP-diagramms.

Abstract: Al-Cu-Li alloys are extensively used for aerospace applications. The main hardening phase is the T₁ phase that precipitates as thin platelets on {111}_Al planes. To facilitate its nucleation, different minor alloying elements are added and dislocations are introduced by cold deformation before the ageing treatment. The impact of these additions in combination with the presence of dislocations on precipitate nucleation and growth needs a deeper understanding. In this work, we investigated the precipitation kinetics of the T₁ phase in alloys containing a common content of Cu and Li and different contents of minor solutes (Mg, Ag) where these elements are present either together or independently. A general overview on the precipitation kinetics was achieved by in-situ small-angle X-ray scattering and hardness measurements. The evaluation of precipitation kinetics reveals that magnesium plays an important role during precipitation by enhancing nucleation kinetics. Additionally, a smaller yet measureable effect of Ag, both in the presence and absence of Mg has been evidenced.

Abstract: The grain boundaries of a fibrous Al-Mg-Si-Cu alloy have been investigated with Transmission Electron Microscopy. The compositions have been mapped by Energy Dispersive X-ray Spectroscopy. The alloy has been aged for 12 hours at 155°C after solution heat treatment and is in a slightly underaged condition. The precipitates nucleated on the high angle grain boundaries are coarse, while the precipitates on the low angle grain boundaries are smaller and more numerous. The precipitates on both types of grain boundaries has been identified as Q'-type. Copper is segregated to both the low and high angle grain boundaries. The effect of this segregation will be discussed with regards to the corrosion properties of the alloy.

Abstract: Two types of nanoclusters are formed during low temperature aging and play important roles in age-hardening of Al-Mg-Si alloys. The formation behavior of these nanoclusters depends on the alloy composition and heat-treatment process. In this work, the various alloys with different Mg and Si concentration were used in order to clarify the influence of alloy composition on the nanocluster formation using differential scanning calorimetry (DSC), hardness and electrical resistivity measurements. Based on the DSC results, two overlapped exothermic peaks were clearly detected, showing the formation of Cluster (1) and Cluster (2) in all examined alloys with different alloy composition. These two overlapped peaks are separated by the Gaussian function method to analyze the volume fraction of nanoclusters quantitatively. It is found that the Si and Mg concentration of Al-Mg-Si alloys has a marked effect on the nanocluster formation. The formation of Cluster (1) is more related with the Si concentration, whereas Cluster (2) is correlated with both of the Mg and Si concentration. Furthermore, the important point is that the formation behavior of nanoclusters strongly depends on the Mg/Si ratio of the alloys. The formation of nanoclusters is most enhanced when the Mg/Si ratio is approximately 1.0.

Abstract: Vacancies are the simplest type of lattice defect. However, they play a major role in the kinetics of diffusional processes, such as solid-state precipitation, where mass transport is directly proportional to the concentration of vacancies. We present a physical modelling framework, where we simulate the evolution of excess vacancies on the example of Al-alloys during simplified time-temperature treatments. Interaction energies between solute atoms and vacancies are evaluated by first-principle analysis. Assuming that the escape of vacancies from existing traps is dependent on temperature and binding energies, we explore the life-time of non-equilibrium vacancies and the natural and artificial aging response of Al alloys. The predictions of the model are finally compared to experimental data.

Abstract: In order to elucidate some of the differences between Al-Mg-Si and Al-Mg-Ge alloys and the role of Cu, a series of Al-Mg-Ge, Al-Mg-Si and Al-Mg-Ge-Si alloys, some of them containing Cu, are investigated by positron annihilation lifetime spectroscopy during natural ageing. Al-Mg-Ge alloys show qualitatively the same evolution of positron lifetime τ_1C with time as Al-Mg-Si alloys, namely an initial decrease, followed by a re-increase, after which τ_1C drops to an equilibrium value. However, for alloys with equal Mg contents, Ge gives rise to a notably slower ageing kinetics than Si, pointing at effects of atomic size or solute-vacancy binding energies. Adding Cu to both Al-Mg-Ge and Al-Mg-Si alloys slows down the initial formation of clusters but promotes their further growth.