Experimental Study Of Portevin-Le Chatelier Effect In Alloys By Using Digital Image Correlation

The plastic instability phenomenon,i.e.Luders effect and Portevin-Le Chatelier(PLC)effect,frequently appears in the plastic deformation stage of many alloys under specific range of temperature and strain rate.The Luders effect can be eliminated by applying pre-strain,whereas the PLC effect is characterized by repeated serration in the work hardening stage of stress-strain curves,accompanied by local deformation of high strain gradient on the specimen surface,which makes the material surface rough and decreases the mechanical properties.This research explored the influence of internal and external conditions of materials,such as solute atoms,precipitate,current,stiffness of testing machine,on the PLC effect via digital image correlation(DIC)method.This paper focused on the spatial deformation behavior of the PLC effect combined with microstructure characterization and temporal analysis,revealing physical mechanism.It was important that this work provided the theoretical basis for inhibiting the PLC effect of materials.Main research contents of this paper are as follows:(1)The inhomogeneous local-ized deformation behavior of Al alloys was studied by two-dimensional digital image correlation(2D-DIC)technique and three-dimensional digital image correlation(3D-DIC)technique simultaneously.The theoretical local point strain error correspond-ing to out-of-plane displacement and in-plane displacement under the oblique optical axis was compared with the actual strain difference measured by 3D-DIC and 2D-DIC,which confirmed the validity of 2D-DIC in measuring the macroscopic localized defor-mation behavior of PLC effect.(2)The macro-scale localized deformation in Ni-Co-based superalloys with different content of γ’ precipitate was investigated.The spatial characteristics of PLC band,including band width,band inclination,spatial distribution of in-band strain,were quantitatively characterized by using DIC.It was demonstrated that the localized deformation was more severe in the high γ’ content alloy.Moreover,a special propagation feature,namely±60° zigzag bands characterized by head-to-tail connections,was observed in the high γ’ content alloy,whose relationship with the frac-ture mechanism was qualitatively analyzed.(3)The influence of Zn addition on the PLC effect of Al-Mg Alloy was experimentally studied by 3D-DIC technique.Specifically,the relationships between Zn addition with serration amplitude and critical strain were discussed,as well as the inhibitory effect of Zn addition on the PLC effect.It was pro-posed that the degree of in-band deformation could be determined by comparing the serration amplitude of the two alloys.Moreover,the band width and angle of the PLC band were not affected by the addition of Zn.The special propagation phenomenon of type B band combining continuous and hopping propagation was observed in Zn-added alloy.(4)The PLC effect of Al-Mg Alloy at various strain rates and continuous current was investigated via DIC method,from which we obtained the spatial characteristics and location evolution of the localized deformation band.A water circulation cooling device was designed to eliminate interferences from the Joule heat effect.It was re-vealed that current could effectively inhibit PLC effect and the inhibition degree was related to strain rate,the mechanism of which was further clarified.Additionally,the Luders band corresponding to Luders plateau including serrations and the PLC band were compared based on the spatio-temporal characteristics of the local deformation band.(5)The strain rate sensitivity and specimen thickness dependence of PLC effect were investigated by experiments in testing machine with various stiffness based on DIC technique.The testing machine stiffness was reported to affected serration characteris-tics and the spatial propagation behavior of PLC band.The stiffness of testing machine,specimen thickness and strain rate had a coupling effect on PLC effect.Moreover,the essential characteristics of PLC effect were clarified.It was proposed that the testing machine stiffness had a straightforward impact on the dynamic strain aging(DSA).