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UBC Theses and Dissertations

On the origin of cluster strengthening in aluminum alloys de Vaucorbeil, Alban


Understanding the influence of atomic clusters formed during natural aging in aluminum alloys is a key problem to control more effectively the strength of alloys both during their processing and life. The yield strength of these alloys is controlled by the interaction between dislocations and solute atoms, and clusters. An empirical scaling law relating the dislocation-obstacle interaction force with the size of clusters has been developed and successfully used to predict the yield stress of a cluster strengthened AA6111 industrial aluminum alloy.It is proposed that the strengthening effect captured by the scaling law could come from the geometrical rearrangement of solute atoms from a random distribution to a clustered distribution, and/or from the change in strength of individual obstacles.A modified areal glide model was employed to investigate the statistical problem of a dislocation moving through a set of clustered point obstacles in the glide plane. The results of these simulations suggest that the degree of clustering of solute atoms does not influence the critical resolved shear stress. Then, molecular statics simulations were used to investigate the origin of the change in strength of individual clusters, in the simple case of Al-Mg alloys. A model based on elastic interaction between the solute atoms/clusters and an edge dislocation was developed and demonstrated to give good predictions for the maximum pinning force of single solutes, dimers and trimers. Using a detailed analysis of the model and the molecular statics simulations, it was shown that the strength of clusters principally comes from the elastic interaction between dislocations and solute atoms forming the clusters. Further, the change of topology of clusters was found to not significantly affect their strength at least in the case of Mg clusters in aluminum. Finally, this model was employed to determine the strengthening contribution of distributions of single solutes, dimers and trimers in binary Al-Mg alloys. The strength was found to roughly depend linearly on the size of clusters, however, its slope is lower than in the case of the AA6111 alloy which predominately contains a combination of Mg-Mg and Mg-Si clusters. The possible reasons for this discrepancy are discussed.

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