| The Ultra-Fine-Grained copper(UFG-Copper)with fine structure performs excellent properties,such as ductibility,electrical conductivity and heat conductivity and so on.It has become an important raw material in the following industrial departments as electric apparatus instrument,chemical industry, shipbuilding,aviation and machinery,etc.,and has wide application prospects.At present,that the Ultra-Fine-Grained material is unable to be prepared continuously limits its application in succession.In addition,it is also a key problem need to be solved in application how to obtain uniform and stable UFG material with excellent properties.In this thesis,the UFG-Copper has been prepared by Asymmetrical Accumulative Rolling Bonding(AARB),which can also realize to the industrial production in succession.for UFG-Copper,therefore,it is an extremely potential technology for UFG-Copper industrialization.The copper with big grains was processed by AARB and treated by recrystallization annealing,UFG-Copper was obtained finally in this thesis.On this basis,the structures and properties of UFG-Copper were researched,and the results were as follows:1.Effects of process conditions of AARB and recrystallization annealing on structures and properties of UFG-Copper were studied,including effects of equivalent strain,recrystallization annealing temperate and recrystallization annealing time on deformed structure,grains sizes,micro hardness,tensile strength, yield strength and electrical conductivity of UFG-Copper.The best process conditions and parameters prepared for uniform and stable UFG-Copper with high strength and good electrical conductivity,have been obtained.(1)The Copper with big grains was processed by AARB and the equivalent strain(ε)was equal to 4.8,the UFG obtained is composed of lot of sub-structure.Then these UFG was heat-treated at 220℃for 30～50min and the UFG-Copper with the grain sizes of 200～500nm has been prepared.(2)Tensile strength(σb)of UFG-Copper is 424.5Mpa,yield strength(σ0.2)is 323.1Mpa,the micro hardness on normal plane,transition plane,rolling plane is 125HV,110HV,103HV respectively and the electrical conductivity is 76.3 MS/m.2.The structures and orientation texture evolution of UFG-Copper during AARB were studied and the results were as follows:(1)Whenεis lower or equal to 1.6,the slips happen on grain boundaries and crystal planes owing to the shearing force.There occur many sub-grains possessing small angle boundaries and 5μm grains sizes within the big grains. At this moment,the shearing force by asymmetrical rolling plays a main role in the course of grain refinement.Whenεis between 2.4 and 4.0,the amounts of small grains which have big angle boundaries and the density of dislocation in big grains containing many sub-grains increase with the rise ofε.When e is lower or equal to 4.0, with increasingε,the interface are bonded gradually and some defects produced by AARB have disappeared on interface.At this moment,that the change of neutral plane position plays a main role in the course of interface bonding,the grain refinement degree heightens whenεincreases.When e is higher than 4.8,the copper with big grains is made up of two kinds of structure regions in the course of deforming,namely fine sub-grains region and high density dislocation region.The high density dislocation region reduces and the sub-grains region increases with the rise ofε,the interfaces produced by AARB have bonded completely at this moment.No matter what e is,the shearing force brought by asymmetrical rolling plays a main role in interface bonding,which leads to the position difference of real neutral plane and the geometric neutral plane.(2)The difference in preparing UFG used by AARB and ARB lies in:the shearing force is added in AARB process,which enhances the friction of grains,improves the slips of boundaries and crystal plane,and promotes the bonding of interfaces.The formation of big angle boundaries is caused by geometric deformation during ARB process,while the formation of big angle boundaries is related to plastic slips of dislocations during AARB process.(3)The deformed texture of the samples prepared by AARB shows the characteristic of cold-rolled texture-Copper texture,the number of grains with C orientation is the largest,which is the same to some research on that of the rolling copper early,but the texture position deviates owing to the existing of shearing stress.The strength of rolling texture becomes stronger with increasingε.The deformation goes on by means of the dislocation slip.3.UFG-Copper was treated by recrystallization annealing whenεwas equal to 4.8,and then the reerystallization structure and texture were observed,the results were as follows:(1)Whenεwas equal to 4.8,the recrystallization course of UFG-Copper obtained is that the recovery course happens firstly and its recovery mechanism is mainly sub-structures regularity and polygonization recovery at a small region.The new grain cores form gradually with the longer annealing time,which are two nucleation formations during recrystallization course,one is sub-grains turning and aggregating to nucleate,and the other is transferring of sub-grain boundaries and growing up to nucleate for sub-grain.The grains with the sizes of 200～500nm come into being when the recrystallization annealing time is controlled between 35min and 50min. UFG-Copper is uniform and stable after recrystallization.(2)At the beginning of recrystallization,the texture sustains the characteristics of rolling texture and the strong recovery happens in deformed copper.With the longer annealing time,there appear some component textures,such as cube texture,R texture,S texture and B/G texture,and the discontinuous recrystallization takes place,at this moment,the formation of crystal nucleus is completed by the transferring of big angle boundaries.(3)During the course of recrystallization for copper prepared by common rolling,crystal nucleus with cube orientation come into being preferentially, which will sustain the priority of growth during the growing up,the main texture is cube texture after recrystallization annealing.While during the course of recrystallization for copper prepared by AARB,the directional growth mechanism is limited and there are all kinds of recrystallization textures existing,the crystal nucleus having been formed with all kinds of orientation grows up together,so,the recrystallization texture is diffuse,which makes the anisotropy of the copper by AARB is better than that of the copper by common rolling and recrystallization.4.The micro hardness,tensile properties and electrical conductivity of UFG-Copper under the different conditions were determined and the change of properties was analyzed.(1)During the course of AARB,the strength of copper increases with the rise ofε,but its elongation rate decreases.UFG-Copper with good properties(ε= 4.8)can be obtained when recrystallized annealing at 220℃for 35～50 minutes.When the recrystallization annealing time is longer than 35 minutes, the remaining strain is banished,but the remaining strain of UFG-Copper begins to increase when deformed again.(2)The yield strength and grains sizes of the recrystallization UFG-Copper prepared by AARB withε=4.8,still meet to traditional Hall-Petch relation,the UFG-Copper has good ductibility at room temperature.(3)Electrical resistivity of UFG-Copper increases and electrical conductivity decreases with the rise of equivalent strain.Electrical conductivity of UFG-Copper prepared by AARB withε=4.8 and recrystallization is the same to that of annealing copper of No.1.5.UFG-Copper possessing uniform structure,high strength and ductibility, good electrical conductivity has been prepared by AARB and recrystallization annealing,which is one of the most important research achievements of this thesis. UFG-Copper prepared by AARB was proposed firstly,this can provide a new method for preparing UFG materials.The structure and texture evolution in the course of UFG-Copper prepared by AARB have not been reported yet,but this work was researched here,in the meantime,the mechanism of grain refinement was analyzed and discussed too.6.Electrical resistivity of UFG-Copper prepared by AARB at the optimal process conditions is the same to that of annealing copper of No.1.The electrical resistivity has little change during the process of UFG-Copper,which are different to traditional theory,but above characteristics need to be investigated further. |
