Study On Tensile Fracture Properties Of Industrial Pure Zirconium With Surface Nanocrystallization

Zirconium alloys have excellent corrosion resistance,mechanical properties and machinability,and are often used in nuclear and chemical industries.The mechanical properties and corrosion resistance of materials are very sensitive to the structure and properties of material surface,and the nanocrystallization of material is an effective way to improve the material properties.This paper through the surface nanocrystallization of Zr-3 by ultrasonic shot peening studies the structure and mechanical properties of surface gradient after surface nanocrystallization.The deformation of slip system and twin system in the process of plastic deformation provides a reference for the application and development of surface nanocrystallization zirconium and its alloy.The slip system and twin system,microstructure evolution and texture evolution of industrial pure zirconium were determined by tensile deformation of coarse grain samples.The results show that the cylinder(10(?)0)[11(?)0] slide system is the main one,and the other slide systems are in coordination with each other.Stretching perpendicular to the direction of rolling,the main deformation is the base plane(0001)< 11(?)0 > slip system,{10(?)2} < 10(?)1 >twin and other slip system.Parallel to the rolling direction and perpendicular to the rolling direction,with the increase of deformation degree,the low angle grain boundary increases,the strain increases,the dislocation density first increases and then decreases.The single peak texture is formed by stretching parallel to the rolling direction.The {0001} plane inclined to TD direction by 30°.The texture enhancement of< 10(?)0 > is parallel to RD direction.Typical plate texture {0001} < 10(?)0 > is formed by stretching perpendicular to the direction of rolling.The surface gradient nanostructure layer with thickness of 200 μm was prepared on the surface of pure zirconium by ultrasonic shot peening and the microstructure characteristics of surface gradient structures were analyzed.The results show that there are structure size gradient,strain gradient,low angle grain boundary gradient and dislocation density gradient.During the deformation process,{10(?)2} < 10(?)1 > and{11(?)1} < (?)(?)26 > tensile twins are produced.The twins are shuttle shaped,and the twins nucleate and grow at the grain boundary or inside the grain.In the low strain layer near the substrate,twin deformation is the main plastic deformation mechanism.In the high strain layer near the surface,dislocation slip becomes the main plastic deformation mechanism.With the increase of deformation,coarse grains are transformed into nanocrystals.Based on the tensile experiment of surface nanocrystallization samples,The mechanical properties and the evolution process of tensile structure of the gradient nanostructure samples were studied.The results show that compared with the coarsegrained samples,the tensile strength of the surface nanocrystalline samples is improved and the plasticity is reduced.During the in-situ tensile process of the surface nanomaterials,the gradient nanostructure layers crack in many places.With the development of the tensile process,the crack grows gradually.The crack only expands in the gradient nanostructure layer,but not to the interior of the matrix.The twin crystals disappeared after the gradient nanostructure was stretched and deformed.The macroscopic fracture mode of coarse grain tensile samples belongs to positive fracture,while the macroscopic fracture mode of surface nanometer samples belongs to cutting.