| Microdefect is an important control medium for mechanical and physical properties of metal materials.By introducing high density micro-defects(such as interface,dislocation,vacancy,etc.)into severe plastic deformation,the mechanical strength of metal structural materials can be greatly improved.According to the above strategies,ultra-finegrained/nanocrystalline metal materials have several times higher strength and hardness than traditional materials.But at the same time,numerous studies have shown that high storage energy is also introduced into the high density micro-defects,which makes the thermal stability of ultrafine/nano-grained metal materials worse,and the strength of ultrafine/nano-grained metal materials is easy to deteriorate during service,thus limiting the practical industrial application.Therefore,it is of great scientific and engineering significance to design and fabricate ultrafine/nano-grained metal materials with high strength and thermal stability.As a metal material widely used in national defense,electric power and transportation industry,copper and its alloys have very broad research and application prospects.Based on micro-defect regulation and microalloying of Zr element,combined with severe plastic deformation,low temperature rolling and aging process,a Cu-Zr alloy with high strength,high thermal stability and high electrical conductivity was prepared.The EBSD and TEM were used to characterize the microstructure evolution of the alloy during the preparation process.The effects of Zr and its segregation behavior on the thermal stability of the Cu-Zr alloy and the mechanical properties of the Cu-Zr alloy were investigated by isochronous and isothermal aging processes.The main conclusions of the study are as follows:(1)Ultrafine grains with an average size of 310 nm and lamellar structure with an average thickness of 82.5 nm were obtained by ECAP and ECAP+Cryo-Rolling processes in 0.07 wt.% Zr bulk Cu alloys.Contrastive experiments show that microalloyed Zr and its segregation behavior can coordinate the geometric deformation mode to a certain extent,and achieve the effect of fining lamellar spacing.At the same time,the difference between the actual grain size and the theoretical lamellar thickness indicates that the geometric refinement model can not be perfectly matched during the low temperature rolling process.There are also mechanisms for the co-coordination of plastic deformation such as shear band,deformation twinning and grain coarsening in the alloy,which ultimately achieve steady grain size under multiple mechanisms.(2)The ultrafine grained Cu-Zr alloy exhibits much higher thermal stability than the pure copper material during the aging treatment.Related experimental results and reports indicate that Zr atoms produce grain boundary segregation during aging treatment.Thermodynamics can reduce the driving force of crystal growth by reducing grain boundary energy.On the kinetic principle,Zr grain boundary segregation pinning effect is used to suppress large angle crystal.The long-range migration of the boundary achieves the purpose of improving the thermal stability of the alloy.At the same time,the grain boundary strengthening effect of Zr makes the softening temperature of ultrafine grained Cu-Zr alloy increase from 150 °C to 450 °C of pure copper.The discontinuous static recrystallization of block pure copper by ECAP and rolling due to uneven deformation has been transformed into continuous static recrystallization.(3)The aging process can significantly improve the mechanical and physical properties of the ultrafine grained/nano-lamellar Cu-Zr alloy.The EACA sample obtained a good combination of high strength(760 MPa)and high conductivity(86% IACS)after one-hour of secondary aging at 300°C.Theoretical calculations and experimental analysis show that the high strength is the coupling of fine grian strengthening,deformation twin strengthing,dislocation strengthening,segregation strengthening and other mechanisms.The high electrical conductivity is derived from low lattice distortion inside the grain and low dislocation defects at the grain boundary. |
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