Fabrication Of Cu/Nb Composite Sheets By Accumulative Roll Bonding And Cryorolling And The Study Of Microstructure And Mechanical Properties

Cu/Nb nanolayered composite is an ideal nuclear reactor material for its high strength,good thermal stability and excellent radiation resistance.Improving the strength and thermal stability of Cu/Nb layered composites and the preparation process of large size Cu/Nb layered composites are the focus of current research on Cu/Nb composites.In order to further improve the mechanical properties and thermal stability of Cu/Nb composite strips,the traditional Cu/Nb rolling process is improved by adding Nb during the accumulative roll bonding process.The main research contents and innovations are as follows:Firstly,the effect of adding new Nb layers during accumulative roll bonding on the microstructure and mechanical properties of Cu/Nb composite strip was studied.It was found that the introduction of new Nb layers could change the distribution of shear stress and inhibit the occurrence of Nb layers’ compression and fracture.In addition,the addition of the new Nb layer also significantly increases the interfacial bonding strength and the number of interfaces,thereby improving the mechanical properties of the Cu/Nb composite ribbons.For example,the strength of Cu/Nb composite strips prepared by Nb additions 5 times and Nb additions 9 times during the accumulative rolling process was increased by 16.7 % and 23.1 %,respectively,compared with those without Nb addition in the accumulative rolling process.Secondly,the experimental study found that the reduction of the interfacial distance of the Cu/Nb composite ribbon delayed the occurrence of recrystallization,and the thermal stability performance was also significantly improved.A special interface transition zone(ITZ)interface was found in the Cu/Nb composite ribbon prepared by Nb addition process for 9 times in the process of cumulative lamination.The high density of dislocations in the ITZ interface can bring strength further improve.The width of the ITZ interface increases with the increase of temperature during the heat treatment process,so that the Cu/Nb composite strip prepared by adding Nb nine times during the accumulative rolling process has better thermal stability.Its strength decreased by only 9.2 % after heat treatment at 500 ℃,while the strength of Cu/Nb composite thin strip prepared without Nb during the cumulative rolling process decreased by 16.7 %.Thirdly,the Cu/Nb composite thin strip prepared by accumulative roll bounding was further cryorolled.The dynamic recovery and recrystallization are inhibited by the ultra-low temperature environment of cryorolling,and the dislocation density in Cu and Nb layers is increased.At the same time,the ultra-low deformation temperature changes the deformation mechanism and promotes the occurrence of twinning in the Cu layer with lower fault energy.Therefore,the strength of the Cu/Nb composite thin strip after cryorolling is increased by 3%～6% compared with that of room temperature rolling.It was also found that the twinning caused the change of Cu/Nb interface structure,and the interface with{112}<110>Cu||{112}<111>Nb orientation relationship formed during the accumulative roll bonding process is changed to a lower energy one with{110}<111>Cu||{001}<110 >Nb-oriented interface.Finally,the effect of cryorolling on the thermal stability of Cu/Nb composite strips was investigated.It was found that the hardness and strength of the cryogenically rolled Cu/Nb composite strips decreased to a lesser extent after heat treatment.Among them,the hardness of the cryorolled Cu/Nb composite strip decreased by only 2.4% after heat treatment at 450 ℃,while the hardness of the cold rolled strip decreased by 8.4%,indicating that cryorolling can improve the thermal stability of the Cu/Nb composite strip.In addition,after heat treatment at 500 °C,a large number of annealing twins were found in the Cu layer of the cryogenically rolled Cu/Nb composite thin strip,and there were highdensity dislocation accumulations at the annealing twins;The occurrence of premature recrystallization was found in the Nb layer.Therefore,the strength and plasticity of the cryorolled Cu/Nb composite strip after heat treatment at 500 ℃ are better than those of room temperature rolled.