Multi-scale Experimental And Theoretical Investigations On The Portevin-Le Chatelier Effect In Al Alloys

The Portevin-Le Chatelier (PLC) effect refers to special plastic instability in many alloys in a certain range of strain rate, temperature and appropriate pre-deformation, which shows serrated flow in stress-strain curves and strain localization in specimen. It is widely accepted that the physical origin of the PLC effect is dynamic strain aging (DSA), i.e. dynamic interaction between mobile dislocations and diffusing solute atoms. As a multi-scale problem ranging from macro deformation bands, meso grains deformation to micro dislocations motion, the PLC effect has been increasingly attractive to researchers after its discovery. Aluminum alloys have been widely used due to a series of advantages such as their high strength-weight ratio and good corrosion resistance, but their formability at room temperature has been severely harmed by the PLC effect. Therefore, it has important theoretical and practical significance to carry out multi-scale experimental and theoretical research on the Portevin-Le Chatelier effect in Al alloys.At first, by the digital speckle pattern interferometry (DSPI) technique, the PLC effect in Al-Mg alloys and Al-Cu alloys is systematic investigated in the present work. The influences of strain rate, test temperature, heat treatment, alloy composition on the PLC effect are studied. A qualitative explanation for above phenomena is made according to the mechanism of DSA. This work is focused on stress field. Then, based on infrared pyrometry, the spatial domain behavior of the PLC effect in Al-Mg alloys is investigated. The modes of temperature rise and the obliquity change of PLC band are analyzed and discussed. The strain rate and deformation inside the PLC band during its formation are also calculated. This work is focused on temperature field. After that, the morphology, composition and distribution under different circumstances of the precipitates are observed by transmission electron microscope (TEM). The multiplication and evolution of dislocations with the increase of strain are studied, as well as the interaction type between dislocations and precipitates. Finally, from the comprehensive analysis of macro and micro experimental results, the extended DSA mechanism based on pipe diffusion is proposed, in which the joint effects of precipitates and solute atoms are considered.On the time domain behavior of the PLC effect, the evolution of stress drop amplitude, reloading time and critical strain with the parameters such as strain rate, test temperature, annealing temperature, quenching temperature, aging, and alloy composition are investigated. The statistical analysis of stress drop amplitude shows that, the PLC effect changes from the self organized criticality to chaos, with the decrease of strain rate, the increase of test temperature or the increase of Mg content in Al-Mg alloys. It displays that precipitates play an important role in the PLC effect from the analysis of quenching and alloy composition experiments. By low temperature experiments, we know that, without the participation of solute atoms, only precipitates can't create the PLC effect.On the spatial domain behavior of the PLC effect, according to temperature field data, the formation and propagation of the three types of PLC bands (type A, B, C) are discussed and some characteristic parameters of the bands such as bandwidth, orientation, and apparent velocity are also obtained. Two modes of temperature rise in the PLC effect are found, and the location of the obliquity change of PLC band is forecasted. Both the experimental and computational results show that elastic shrinkage deformation exists outside the band only during the formation of the type B, C band. Based on this, whether or not shrinkage deformation exists outside the band is proposed as a new standard to define type A, B band.In the microcosmic experiments, the morphology and composition of precipitates are recorded by TEM and energy dispersive spectrometer (EDS). These inferences about content of precipitates in quenching and alloy composition experiments are verified. With the increase of plastic deformation, the dislocation density increases and dislocation configuration evolves from spread, tangled to cell structure. This verifies the proliferation mechanism of dislocation. The phenomenon of interaction between mobile dislocations and precipitates when the PLC effect appears is observed, which shows that the precipitates are also involved in the process of DSA.In the research of mechanism, based on experimental results of macro and micro, according to comprehensive consideration of solute atoms and precipitates, the original DSA mechanism based on pipe diffusion is improved and the general DSA mechanism based on pipe diffusion is proposed. Further more, precipitates only play a role of mobile dislocations blocking similar to forest dislocations, and the participation of solute atoms is a necessary condition to cause the dynamic strain aging.