Formation Mechanism Of The Portevin-Le Ch(?)telier Effect On 5182 Aluminum Alloy By Surface Mechanical Attrition Treatment

The Portevin-Le Chatelier(PLC) effect, also known as serrated yield effect. Which refers to special plastic instability phenomenon in many alloys under certain temperature and strain rate, which shows on the stress-strain curves of serrated flow and the specimen of strain localization. Dynamic strain aging(DSA) is the concept of PLC effect, which is widely accepted by researchers, i.e. dynamic pinning and unpinning between mobile dislocations and the surrounding solute atoms. Aluminum(Al) alloy have been widely used in automobile industry due to a series of advantages. However, the PLC effect usually occurs at room temperature for Al alloy, which makes the material surface rough and reduce the mechanical properties of the material. Because of the PLC effect, the application of aluminum alloys will be limited. Therefore, the research on the PLC effect is of important significance and industrial applications.Grain refinement caused by severe plastic deformation(SPD) allows obtainingultra-fine grained(UFG) materials, it is reported that SPD may effectively suppress the macroscopic stress serrations characteristic of the PLC effect. Surface mechanical attrition treatment(SMAT) make the sample surface result in severe plastic deformation and grain refinement progressively down to the nanometer regime in the entire sample surface. It can improve the overall performance of metallic materials. Therefore, in this paper, SMAT has been carried out on the 5182 Al alloys prepared by conventional rolling(croase grain) and continuous casting and rolling(fine grain), respectively. By means of electron backscattered diffraction(EBSD), optical microscopy(OM), transmission electron microscopy(TEM), microhardness test and mniaxial tensile test, the microstructure, mechanical properties and the PLC effect of the 5182 Al alloys with SMAT are investigated. The effect of SMAT on the microstructure, mechanical properties and the PLC effect of the two types of 5182 Al alloys has been discussed and analyzed. Finally, the PLC effect of 5182 aluminum alloy was discussed by using the multifractal analysis.After SMAT, a gradient nano structure is formed in the surface of the 5182 Al alloy along the thickness direction. The thickness of the nanocrytalline surface layer is about 60~100?m. After SMAT, the strength of Al alloy increases, but its fracture elongation decreases. The coarse grain aluminum alloy sample exhibits 47.3% higher YS and1.5% higher UTS, but 34.8% lower FE, and the fine grain aluminum alloy sample exhibits 51.8% higher YS and 2.4 % higher UTS, but 39.1% lower FE. This is due to the grain refinement after SMAT, the ability to resist deformation increased, the strength increased, but more grain boundaries inhibit the movement of dislocations, so the plastic reduced. After SMAT, the microhardness of the alloy is improved significantly, and it shows a gradient distribution along the thickness direction. The microhardness of nanocrystalline layer on the alloy surface is the highest, then the microhardness decreases with increasing the depth. The microhardness of the coarse grain aluminum alloy sample is about 93~102 Hv; The microhardness of the fine grain aluminum alloy sample is about 94~103 Hv; This is due to the grain refinement after SMAT.The SMAT technology has a certain effect on the critical strain and stress drop of the PLC effect of 5182 aluminum alloy. The critical strain of the PLC effect increases after SMAT; That is, the PLC effect of Al alloy could be postponed; With the duration of SMAT, the critical strain increased gradually, the critical strain of the coarse grain aluminum alloy sample increased from 0.73% to 1.4%(SMAT 5min), and the critical strain of fine grain aluminum alloy sample increased from 1.12% to 1.4%(SMAT 5min); the serrated strain of the coarse grain decreased from 2.4% to 0.7%, and the serrated strain of the fine grain decreased from 2.3% to 0.9% after SMAT, With increase of duration of SMAT, the serrated strain decreases gradually. After SMAT, the stress amplitude did not obvious increase or decrease, but its distribution is more concentrated. FWHM of the coarse fine aluminum alloy decreased from 18.8 to 2.6; FWHM of the fine grain aluminum alloy decreased from 9.5 to 1.3. But the average stress amplitude increases in a strain range.A set of parameters q and D(q) is introduced to describe the multifractal of the PLC effect of 5182 aluminum alloy. The PLC effect is confirmed by the analysis of the multifractal behavior, both the coarse and fine grain, with the increase of q value, log10Sq(?t)/(q-1) and log10?t gradually showed a linear relationship. The different time interval shows log10Sq(?t)/(q-1) scale. When q is over a certain range, there is no linear relationship between log10Sq(?t)/(q-1) and log10?t, which indicates that there is a certain scale of multifractal behavior. The generalized fractal dimension D(q) is obtained by the method of multifractal analysis, D(q) decreases with the decrease of q, and it reveals the multi fractal structure of the stress curve of PLC effect on a certain scale.