Influence of severe plastic deformation on dynamic strain aging of ultrafine grained Al-Mg alloys
Final Report Abstract
The investigations carried out during the first project period revealed a strong influence of SPD processing on the DSA effect of Al-Mg alloys at room temperature (RT), which can be attributed to larger mobile and forest dislocation densities as well as an obviously refined grain size caused by SPD. This decreased the obstacle spacing, increased the obstacle density and thus, intensified the DSA effect by increasing pinning stresses and prolonging the aging and waiting time of arrested mobile dislocations at obstacles. Consequently, serrated flow is amplifieed and the onset of serrated flow is retarded. Increasing Mg content strengthened the influence of SPD on DSA of Al-Mg alloys. Furthermore, the asymmetric instantaneous stress response at the moment of strain rate jump indicates that the influence of SPD on the instantaneous properties was different for the rate-down and rate-up tests. Specifically, the nature of the asymmetry was attributed to the different magnitude of effective pinning stress, exerted by aging solute atoms on arrested mobile dislocation, at different strain rates. On the other hand, the steady state strain rate sensitivity was found to be not so sensitive to the strain rates. Due to DSA the steady state strain rate sensitivity were negative and near zero, and yield a saddle shape curve with respect to progressive SPD strain. Contrary to coarse grains, the critical strain increased with increasing solute concentration in UFG Al-Mg alloys. Overall, the room temperature behaviour of ultrafine grained Al-Mg alloys was interpreted in terms of activated deformation mechanisms and the interaction of dislocations with solute atoms. In the second period of the project, the mechanical response and dynamic strain aging of solid solution Al-Mg alloys subjected to SPD were studied at elevated temperatures. The study revealed that the thermal stability is sensitively dependant on the solute concentration where the pinning of grain and cell boundaries by Mg atoms is assumed to cause the observed higher thermal stability in alloys with higher Mg contents. Recovery and recrystallization were observed in lower alloyed Al-1Mg samples at 200°C causing the formation of a bimodal microstructure, although the fraction of recrystallized grains was small. The strain rate sensitivity increased with temperature and CCDP pass indicating a decrease of the activation volume. This observation is due to a change in the activation of dislocation glide and dislocation climb with temperature and strain rate. A model which predicts the relative activity of these mechanisms and the corresponding mechanical response was developed. Finally, the activation energies for serrated flow in Al-Mg alloys were evaluated based on the experimentally observed critical strains at different temperatures and strain rates.
Publications
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Influence of severe plastic deformation on dynamic strain aging of ultrafine grained Al-Mg alloys, Acta Mater 76 (2014) 54-67.
S. Zhao, C. Meng, F. Mao, W. Hu, and G. Gottstein
