Investigations On The Portevin-Le Chatelier Effect In Al Alloys: Experiments And Simulations

Many engineering materials exhibit irregular plastic flow, which is referred to as Portevin-Le Chatelier(PLC) effect, in limited regimes of strain rate and temperature. The PLC effect manifests itself as temporal continuous stress serrations on stress-time curves or strain staircase behavior on strain-time curves respectively. Furthermore, the irregular plastic flow results in inhomogeneous deformation with various localization bands. These bands can be static, hopping and sometimes propagating along the specimen. The PLC effect is a kind of typical multi-scale effect and has been increasingly attractive to researchers since its discovery. The unstable plastic flow is generally understood as the consequences of solute-dislocation interaction at the microscopic level, namely, dynamic strain ageing (DSA). Aluminum alloy materials are very suitable for application in automobile industry due to the high strength-weight ratio. However, the PLC effect usually occurs at room temperature for aluminum alloy, which leads to the increasingly geometric perturbation in specimen and decreasingly necking strain. Thus, the material formability decreases, restricting its use in the car parts. Thus, the research on the PLC effect in aluminum alloy is of important theoretical and industrial application significance.In the alloy with solute content higher than the limiting solubility, the solute atoms failed to dissolve in matrix will precipitate from the solid solution and form precipitations. At first, the PLC effects in annealed 5456 and 5052 Al-Mg alloys with different precipitation contents are investigated under different applied strain rates respectively. The results indicate that precipitations strengthen the impediment to the motion of dislocations. The greater the contents of precipitations become, the more pronounced the influences on the PLC effect become.Secondly, 5456 and 5052 Al-Mg alloys are aged at the same temperature for different time, making the content of solute atoms and precipitations changing together. Then, tensile experiments under the same applied strain rate are carried out for the two alloys to investigate the effect of the simultaneous changes in the content of solute atoms and precipitations on the PLC effect. Also, this investigation is carried out for the 2024 Al-Cu alloy in the same way and the experimental results are compared with those in the Al-Mg alloys. The findings indicate that the increasing contents of precipitations make up the influences from the decreasing solute atoms. The effects of solute atoms and precipitations on the PLC effect are equivalent in Al-Mg alloy. However, in Al-Cu alloy, the effect of solute atoms is dominant in the PLC effect. The precipitations fail to make up the influences from the decreasing Cu solute atoms.In accordance to the DSA mechanism, the dislocation densities in 5456 Al-Mg alloy are changed by different prestrain magnitudes, and the effect of prestrain on the PLC effect is investigated. The experimental results indicate that the prestrain affects the PLC effect distinctly. The critical strain shows an abnormal evolution as well.Based upon the experiment of prestrain, tensile experiments are carried out along the different lattice directions in 5456 Al-Mg alloy. The findings show that the PLC effect does not present obvious difference with the lattice directions except the critical strain.A macroscopically phenomenological model is presented for the PLC effect based upon the DSA mechanism. The model takes into account the competition between the mobile dislocations and solute atoms, the interaction between dislocations, and the effect of temperature changes. Combined with a heat conduction equation, the model is finally used in the numerical simulations of the stress-strain curves and the temperature evolutions associated with the PLC effect in 5456 Al-Mg alloy. The numerical results reproduce the experimental findings in an infrared pyrometry experiment. The elastic shrinkage outside of deformation band is simulated as well by the consideration of elastic deformation in the model. It shows that the relevant parameters used in the simulation are rate-dependent and the influence on the flow stress from the temperature increment in specimen is minor.