Mechanical Properties Of The Ultrafine Grained Materials Under Complex Stresses

The ultrafine grained (UFG) materials have higher strength than their coarse-grained (CG) counterparts. However, the low tensile plasticity limits their industrial applications, due to the low dislocation accumulation capability. Many researchers tried to increase their tensile plasticity through designing the microstructure. At the same time, other researchers focused on the deformation conditions (strain rate/temperature). Research have found that low temperature and high strain rate will increase the tensile plasticity of UFG materials, but how the deformation mode (stress state) influence the mechanical properties of the ultrafine grained materials is still a open question.Our work focused on the mechanical properties of the UFG materials under complex stress state by exploring the post-necking elongation and the elongation of the notched region of the designed notch tests. The UFG Cu, Al, Ti and their CG counterparts were chosen for the investigation. The 3D optical measuring techniques (ARAMIS) were introduced to the tensile tests to uncover the plastic deformation process. The stress state and stress triaxiality in the necking and notched region were given by finite element modeling (FEM). The information about ductile fracture and plastic deformation mechanisms were reveled by scanning electron microscopy (SEM). The main conclusions as follows:1. In despite of the great difference between the UFG and CG materials in their overall elongation to failure, their post-necking elongation is comparable, which deformed under the complex stress state. Notched tensile tests were designed for further study of the plasticity under the complex stress state based on copper. The results found that the UFG Cu has higher strength and comparable plastic capability with their CG counterparts under the complex stress state.2. The fracture strain (εf) decreases with the stress triaxiality increasing. Besides, the εf of the CG Cu decreases more obviously than the UFG Cu with the increase of the stress triaxiality. Ductile failure is typically considered to occur by the progress of void nucleation, growth and coalescence. The cavity expansion rate is faster at higher stress triaxiality. The cavity expansion is mediated by the dislocation movement, while the grain boundary blocking the dislocation sliding making it difficult for the cavity to expanding from one grain to another in the UFG Cu.3. Fractographic observations in the necking region and uniform deformed parts identified that stress state affects the deformation mechanisms of the UFG materials. Multi-axial stress state (when mean stress increased) will active more dislocation slip system and/or other deformation mechanisms (grain boundary sliding) in the plastic deformation.