Investigation On Microstructure Evolution And Properties Of Pure Copper And Ingot Iron Under Cryogenic Impact Deformation

Compared with coarse-grained (CG) material, bulk ultra-fine grained (UFG) material possesses more superior physical and mechanical properties. And thus UFG materials become a hot issue in the field of materials research. Severe plastic deformation (SPD) was proved to be an efficiency method in grain refinement and it is one of the most frequently method used in producing bulk UFG materials. Cryogenic impact deformation is a novel SPD method that imposes deformation at cryogenic temperature. Compared with traditional SPD techniques at ambient or elevated temperatures, the use of cryogenic temperatures is justified by suppression of dynamic recovery and stimulation of mechanical twinning. This may enhance the grain refinement efficiency and decrease the strain level necessary to achieve a UFG structure. It is time-saving and cost-efficiency and can be applied to large-scale productions by using a conventional forging machine without any special equipment.This article studied the microstructure evoltution and corrosion properties of pure copper and ingot iron through metallographic examination, TEM, XRD, microhardness tester and potentiodynamic polarization et al. testing methods. The main conclusions of the present study are below:(1) Cryogenic impact deformation of pure copper was carried out and an UFG microstructure was successfully induced in the sample. The grain on the transverse plane is equiaxed with a size range of 150～450 nm, and on the longitudinal plane is lamellar with a width range of 30～220 nm. The microstructure analysis shows that, significant grain refinement occurs in pure copper during the cryogenic impact deformation. The grain refinement process in pure copper was related to evolution of dislocation cells, mechanical twinning and dynamic recrystallization.(2) After annealing at 190 ℃ for 60 min, high density dislocation disappeared. The transverse section was still equiaxed with a size range of 30～220 nm. The longitudinal section was still elongated with a width range from 70 to 100 nm. One thing should be note, large amount of annealed nanotwins with a width of 10～100 nm were also observed on the longitudinal section.(3) The corrsoin tests shows that both of cryogenic impact deformation and post annealing could decrease the corrosion rate of copper in 0.5 M NaCl solution.(4) Cryogenic impact deformation of ingot iron was carried out and the microstructure was refined significantly after deformed to a strain of 2.21. The grains on the transverse section are equiaxed with a size range of 100～300 nm. And the grains on the longitudinal section are elongated with a width range of 100～350 nm. The microstructure analysis shows that, significant grain refinement was induced by mechanically broken of grains and evolution of dislocation cells. Besides, the existence of cementite also contributes to the grain refinement process.(5) After annealing at 190 ℃ for 60 min, dense dislocation disappeared in ingot iron. The grains on the transverse section was still equiaxed with a size of 120-310 nm. The longitudinal section was still elongated with a width of 80-350 nm. No annealed twins were observed.(6) Corrosion tests showed that compared with as-received ingot iron, the cryo-deformed sample exhibited a superior corrosion resistance in 0.5 M NaCl solution. Subsequently annealing could result in a notable improvement in the passive ability and could obtain a more stable passive film.