Mechanical Properties And Plastic Deformation Mechanism Of Nano-Twinned 316L Austenitic Stainless Steel

Nanostructured materials have attracted intensive investigation due to their ultra-high strength,but most nanostructured materials have only high strength and unsatisfactory plasticity.In recent years,material researchers found that nano-twin structures not only have super high strength,but also have good plasticity and work hardening capacity.At present,the methods of preparing nano-twin structures are generally divided into two categories:one is deposition methods,such as electrodeposition,magnetron sputtering,and the other is plastic deformation methods.Compared with all kinds of deposition methods,the plastic deformation method is closely related to engineering owing to their high effciency,and has the most industrial application prospects.The low yield strength of austenitic steel seriously limits its industrial application potential and can not meet the harsh industrial environment.How to improve the yield strength of austenitic steel without reducing its plasticity is a challenge for material researchers.In recent years,materials researchers have used plastic deformation methods,such as dynamic plastic deformation(DPD)to prepare high-density nano-twined structures in AISI 316L stainless steel.However,uniaxial tension showed that they have very high yield strength,but almost no plastic deformation ability.The yield strength of nano-twin prepared by DPD can not represent the intrinsic yield strength of nano-twins because they are mixed with structure of nanotwins and nanograins.It is a focal point of this paper to prepare high volume fraction nanotwins and improve the plasticity of deformed nanotwins.In our work,gradient nanostructures were successfully prepared on AISI 316L stainless steel surface by surface mechanical gringing treatment(SMGT)technology,and high volume fraction gradient nanotwins were prepared on sub-surface layer.The micro structural evolution of gradient nanostructures was systematically studied,with emphasis on the mechanical properties of sub-surface nanotwins.The plasticity of nano-twinned structure is improved after annealing and the plastic deformation mechanism of nanotwins was emphatically studied.The main results are as follows:1.The gradient nanostructure of 316L stainless steel was successfully prepared by SMGT technology.From the treated surface to the center,the gradient nanostructure can be devided into(1)nanograin region:0-30?m;(2)nanograin and nanotwin region:30-50?m;(3)nanotwin region:50-170?m;(4)nanotwin and dislocation region:170-250?m.2.Uniaxial tensile test showed that the intrinsic yield strength of nano-twinned 316L stainless steel was about 1 GPa,but its uniform elongation was about 3%.After annealing at 730?,the yield strength of nanotwins decreased to 800MPa,but the uniform elongation increased to 8%.3.The direction of twin planes with respect to the loading direction is 27°,which activates the motion of partial dislocations along twin biundaries,resulting in low strength of nanotwins.After annealing,the dislocation density of nanotwins decreases sharply by an order of magnitude,and the thickness of twin/matrix lamellae widens from 14±1nm to 39±6nm,which may be the reasons for the decrease of strength and the increase of plasticity of nanotwins.4.The dislocation density of annealed samples is increased by about one order of magnitude after tensile test with a strain of 8%.TEM observations showed secondary twinning and high density dislocation in the nanotwins.Dislocation multiplication and secondary twinning may dominate the plastic deformation of annealed nanotwins.