Modeling of Precipitation Sequence and Ageing Kinetics in Al-Mg-Si Alloys

Abstract

Al-Mg-Si alloys are heat treatable alloys in which strength is obtained by precipitation hardening. Precipitates, formed from a supersaturated solid solution during ageing heat treatment, are GP-zones, B", B´ and B-Mg2Si. Precipitation kinetics and strength vary with alloy composition and process parameters. There is still a need for property and process optimization and therefore to investigate precipitation in this system. Therefore, the general objective is to develop process models to study precipitation kinetics and precipitation sequence in Al-Mg-Si alloys. Precipitates in Al-Mg-Si alloys usually nucleate with a spherical morphology but grow with an elongated shape. An age-hardening model is used to study the effects of morphology of precipitate on the strengthening of Al-Mg-Si alloys (Chapter 2). The results show that age-hardening models, assuming elongated precipitates of constant aspect ratio, do not give an overall better prediction of precipitation and strength evolution during ageing and the assumption of spherical precipitates remains, as a first approximation, an acceptable assumption. The Kampmann-Wagner numerical (KWN) framework, as a base ageing model, is introduced in chapter 3. The KWN model is a method for modelling coupled nucleation, growth, and coarsening. In this method, the precipitate size distribution is simulated using a finite difference method. Current precipitation models, applied to aluminium alloys, usually assume that thermodynamic equilibrium is always fulfilled at the precipitate-matrix interface. This implies the assumption of an infinite interface mobility, which means very fast transformation of matrix to precipitate as soon as the local equilibrium is disturbed by diffusion. The validity of this assumption has been investigated in chapter 4. A modified version of KWN model is introduced in which a mixed-mode growth model has been implemented instead of the only diffusion-controlled growth equation. Using this model a comprehensive systematic study has been done on the effects of diffusivity, and type of precipitate, i.e. interface energy and mobility, on the kinetics and character of precipitation in Al-Mg-Si alloys. The results show that changes in the interfacial energy have almost no effect on the precipitation character. However, changes in diffusivity and interface mobility have significant influence on the character of precipitation. For example, it is shown that there is a certain radius below which precipitation character is always interface controlled. In chapter 5, the complexity of the precipitation sequence and its effect on the precipitation kinetics during ageing has been investigated with a multi-component multi-precipitate model, based on the assumption of maximum Gibbs free energy dissipation. In this modelling framework it is possible to consider simultaneous formation of GP-zones, B'', B', B, and free-Si. The model predicts that a large fraction of nuclei of different precipitate species form during quenching from solutionizing temperature and during heating to ageing temperature. Nucleation is first followed by the growth of the less stable species, which dissolve at some point in favor of more stable precipitates. In the end, only thermodynamically stable precipitates like b and free-Si remain in the alloy. The model also confirms that maximum strength is reached when B'' is the dominant precipitate. The effects of secondary precipitates, induced by interrupted ageing, on the age hardening of Al-Mg-Si alloys have been presented in chapter 6. In the interrupted ageing the alloy is first aged at an elevated temperature (e.g. 170 °C), quenched and then exposed to a lower temperature (e.g. 25-100 °C), and aged again at elevated temperatures. From the results it appears that the influence of secondary precipitates is highly dependent on the interruption temperature. Secondary precipitation stimulated by interruption at temperatures below 50 °C has almost no influence on the alloy strength, while when the interruption temperature is above 50 °C, it increases the hardness significantly. The proposed scenario to explain this behavior is based on the temperature-dependent competitive growth of GP-I and GP-II precipitates. According to this model, interruption temperature below 50 °C stimulates the formation of GP-I zones, which have a very slow kinetics of transformation and therefore they have almost no influence on the mechanical properties. On the other hand, when the alloy is interrupted at temperatures above 50 °C, the formation of GP-II zones is more likely to take place, consequently resulting in the higher density of ?" precipitates during re-ageing and better mechanical properties. Previous precipitation models were linked to a strength model to predict the yield strength evolution during ageing. However, the whole work-hardening behavior is also important for many applications of Al-Mg-Si alloys. In the last chapter, the influence of ageing on work-hardening is investigated from tensile tests. A modified version of Kocks-Mecking-Estrin (KME) model is then employed to simulate the work-hardening response as a function of the precipitation state. Results reveal that underaged material shows a linear decrease of the work-hardening rate with flow stress, while overaged material shows an initial constant work-hardening rate before decreasing linearly. This distinct behavior has been related to the stability of the Orowan loops. In conclusion, the thesis addresses several important issues concerning precipitation and work-hardening behavior of Al-Mg-Si alloys, including precipitates morphology, precipitation sequence, precipitation character, and interrupted ageing. This allows for a better understanding of precipitation sequence, precipitation and hardening kinetics in these alloys. The results of this study can be used for optimization of both chemical composition and ageing parameters in order to achieve desirable microstructure and mechanical properties.