Investigation On Microstructure And Properties Of Nano/ultrafine-grained Copper Processed By Cryogenic Friction Stir Processing

The high proportion of grain boundaries in nano/ultrafine-grained(NG/UFG)metals have an important influence on the mechanicl properties,and it will cuase the inverse relationship between the strength and the microstructure stability of the materials,which affects the application of NG/UFG metals.How to improve the inverse relationship between strength-ductility and strength-microstructure stability has been a hot area in the field of metal strength and ductility.Therefore,this paper focuses on the deformation mechanism,the microstructure evolution behavior and the microstructure stability of NG/UFG mixture structured copper(Cu)prepared by cryognic friction stir processing(CFSP).And then reveals the important role of the nonequilibrium grain boundary in nanocrystalline microstructure and the related mechanism to the microstructure stability of nanograined Cu.The analysis of the deformation mechanism of NG/UFG Cu shows that the main deformation mechanisms are grain boundary sliding,dislocation emission from grain boundary,dislocation absorption in the NG grain boundaries and dislocation pile-up in the UFGs.And the non-uniform microstructure makes the CFSP NG/UFG Cu exhibit good mechanical properties with high strength of 550 MPa and elongation of 25%.The energy states of nonequilibrium grain boundary has an important influence on the mechanical propertites of the NG/UFG Cu.Nonequilibrium grain boundaries with high energy states inhibit the grain boundary sliding,leading to dislocation emission from the grain boundaries,and then absorbed by the opposite grain boundary in small grains.It causes a softening effect.While the emitted dislocations in the large grains will pile up in the interior of grains,leading to the increase of strain hardening rate of the metal.The nonequilibrium grain boundaries with lower energy states provide conditions for grain boundary sliding and the strength is reduced.When the nonequilibrium change to equilibrium grain boundaries,the grain boundary sliding is suppressed again and the strength will increase.It is pointed out that in addition to the relationship between grain size and mechanical properties,the nonequilibrium grain boundary plays an equally important role in determining the mechanical properties of the grain refined metal by SPD.The analysis of microstructure evolution of CFSP Cu shows that the grain refinement is achieved through two ways: When the rotational speed of the stir tool is low,the grains are refined through the dislocation cells transforming into subgrains,and then transforming into final equiaxed grains with high angle grain boundaries.When the rotational speed is high,the grians are refined via the subdivision of lamellar structures formed during deformation by the dense dislocation walls.The dynamic recovery during deformation plays an important role in the increase of misorientation of the grian boundaries.And the “hard shell” restraint effect caused by the base metal to the deformed materials in the low temperature to increase the strain is put forward.The introduction of large strain and no recrystallization process during the evolution of the microstructures creates conditions for the formation of nonequilibrium grain boundaries.The analysis of the microstructure stability of NG/UFG mixture structured Cu shows that the microstructure stability of the NG/UFG metals is related to the dislocation mobility,and the grain coarsening has a softening effect on the mechanilcal properties.Abnormal grain growth occurs by grian boundary migration to release the fluating microstrain in the microstructure of CFSP Cu during long-term strage at room temperature.During tensile deformation,the grains grow uniformly.At low strain rates,the process of dislocation absorption at grain boundaries leads to the reduction of diffusion activation energy,and promotes grain boundary migration to achieve grain growth.At high strain rates,grain boundary migration is inhibited by the dislocations in the grain boundaries.And the grain boundaries maybe widened via the interaction of grian boundaries and dislocations leaing to coalescence of adjacent grains.The nonequilibrium grain boundaries promote the process of grain coarsening by promoting twin formation and dislocation emission.The crystal structure determining the mobility of dislocation is an important factor affecting the stability of microstructure.It is an effective measure to improve the stability of NG/UFG structure by in-situ precipitation of the second phase at the grain boundary to reduce the energy of the system and pin the grain boundary to reduce the mobility of the grain boundary.The nonequilibrium grain boundary with high energy state will increase the strength of fine-grained metal,at the same time the microstructure stability will be reduced.In the process of grain refinement,it is necessary to comprehensively consider the influence of grain boundary energy on the mechanical properties and microstructure of the material.