Papers by Author: A. Miroux

Abstract: The complete evolution of solute content and second phases during full-scale industrial processing of AA3103 sheets has been measured. During pre-heating, dispersoids, which appear as plates or small polyhedra grow and the Mn solute content decreases. During subsequent breakdown rolling the dispersoid number-density increases significantly. The measured decrease of solute Mn after hot rolling and coil cooling is attributed to constituent particle growth, whereas the solute depletion during the final back-annealing is mainly caused by the growth of the dispersoids. These observations are compared to the predictions obtained by a semi-physical model for precipitation. Although simulations have been performed without any retro-fitting, for hot rolling the results compare quantitatively well with experiment, while for coil cooling and back annealing the modelled Mn solute depletion is underestimated. The precipitation process is found to be very sensitive to the microstructure, which illustrates the importance of coupling precipitation models with work hardening and softening models to obtain reliable predictions.

Abstract: A simple model for recovery of a subgrain structure is used to distinguish and explain the respective influence of the deformed microstructure characteristics, annealing temperature and concurrent precipitation kinetics on the nucleation kinetics of recrystallisation. Simulation results demonstrate how the balance between recovery and precipitation kinetics controls nucleation.

Abstract: To obtain further progress and a more detailed understanding of the mechanisms involved in recrystallisation, new and more accurate techniques such as in-situ observations are necessary. This innovative method has been used to monitor the recrystallisation process in a FEGSEM equipped with hot stage. Observations are done in backscatter mode with particular attention to orientation contrast. EBSD maps of the observed areas can be acquired before and after recrystallisation. Details of the movement of the interfaces between the recrystallised region and the parent structure are recorded and analysed. The results show that the grain boundaries observed do not move smoothly but with a jerky motion. The recrystallising front sweeps through small areas, corresponding to single sub-grains or small groups of them, very rapidly and then stops at other sub-grain boundaries for varying time before progressing to the following area.

Abstract: A diffusion controlled precipitation model based on classical nucleation and growth theory has been implemented to simulate the precipitation kinetics in a hot rolled supersaturated Al- Mn alloy (AA3103). The modelling approach explicitly includes the effect of concurrent recrystallisation on precipitation and considers the simultaneous evolution and interaction of two precipitate populations that vary significantly in size, i.e. constituent particles and dispersoids. Comparison with experimental results shows that this classical modelling approach predicts incorrect nucleus density and too high precipitation rates, which cannot be simply corrected by parameter fitting. Reasons for this discrepancy are discussed in terms of selection of nucleation sites, the effect of diffusion in a multi-component system, various diffusion paths and the possible influence of precipitate shape and size distribution. The model is subsequently altered by introducing two additional parameters that control the Mn solute concentration at the particle-matrix interface. This more phenomenological model is successful in reproducing the experimental precipitation kinetics, both in deformed and undeformed aluminium matrix, and the effect of concurrent recrystallisation for a wide temperature range.

Abstract: The recrystallisation kinetics experimentally measured on a supersaturated AA3103 after hot rolling is analysed using a numerical model. It is shown that temperature and nucleation inhibition due to precipitation on the subgrain structure are the two most important parameters in controlling the recrystallisation kinetics. On the opposite, particle pinning of recrystallised grain boundaries is negligible while pinning of subgrain boundaries during recovery and solute drag are relevant but second order effects.

Abstract: The softening kinetics of a 0.19 wt% C 1.5 wt% Mn steel deformed at two intercritical temperatures have been characterised using the stress relaxation technique. Recrystallisation of intercritical austenite has been modelled using a single grain model (Chen et al., 2002 [1]), whilst recovery of both intercritical austenite and ferrite has been modelled using a model in the literature [Verdier et al., 1999 [2]). The models are combined to predict the overall softening kinetics with a rule of mixtures formulation. Comparison of the model with experiment shows significant deviations. The reasons are discussed with reference to the mixture rule and to the local stress-strain distribution which exists in the deformed samples. A simple modification to the model is proposed which takes into account the effect of a local stress distribution in deformed austenite.