The electroplastic(EP) tensile properties of 5A90 Al–Li alloys compared with thermal tension were investigated.The microstructural variation at different conditions was observed by SEM and TEM.The current density significantly influences the elongation and the flow stress.With increasing current density,wider and deeper dimples on the fracture surfaces and less dislocation density and pile-ups in the EP tension samples were observed compared with roomtemperature and thermal tension,which indicates the plasticity improvement and flow stress reduction.The EP effect(EPE) mainly results from a comprehensive function of Joule heating and pure EPE.Among them,Joule heating effect is perhaps a dominant factor.
The microstructure characteristics and plastic deformation behavior of SUS304 metastable austenitic stainless steel sheets have been investigated during tensile process at diferent strain rates at room temperature.The yield stress continuously increases with strain rates due to low fraction of martensite transformed from austenite at 0.2%plastic stain.While the ultimate tensile stress(UTS)and elongation gradually decreases and then slightly increases with increase in strain rate from 0.0005 s 1to 0.1 s 1,which is attributed to the variation of the martensite fraction that is afected seriously by adiabatic heating.A higher temperature increase in the tensile specimens restricts the martensitic transformation at high strain rate.The strain rate of 0.1 s 1is considered as a transition deformation rate from quasi-static state to plastic forming,where the transformed martensitic content is very small in a higher strain rate range.Anomalous stress peaks in the later half stage of deformation occur at a very low strain rate(i.e.,0.0005 s 1)result from X-shaped strain localization repeatedly sweeping over the specimen.With increasing strain rates,the variation of dimple number density follows similar trend as that of UTS and ductility because martensite fraction mostly influences void nucleation and growth.
Through in situ transmission electron microscopy observation on SUS304 metastable austenitic stainless steel during stretching at room temperature,it is found that e martensite plates were induced preferentially from the sites of dislocation pile-ups.With increasing deformation,some of ε thin martensite platelets disappear and reversibly transform toγ austenite without heating treatment,which is different from the previous result that ε martensite can entirely transform toα'martensite.Then,some of deformation twins appear and grow along the vertical direction of ε martensite due to(111)_γ⊥(1010)_ε.Moreover,it is directly observed that multiple transformation mechanisms via γ→ε→γ,γ→ε,γ→α′,γ→ε→α′,γ→ deformation twins →α′ can co-exist.
