The Thermal Stability Of UFG Cu Processed By ECAP And HPT

Severe plastic deformation techniques(SPD)enjoy great popularity owing to their ability to produce extensive grain refinement in manufacturing ultrafine-grained/nano-grained materials.However,as a result of deformation to giant plastic strains involved in SPD processing,large energies are stored in the materials,attributed to a high concentration of high angle grain boundaries and crystal lattice defects(such as dislocations,vacancies,etc).Thus,ultrafine-grained/nano-grained materials are metastable in nature,which affects the thermal stability.From the viewpoint of industrial application,the investigation of the thermal stability of the severely deformed materials is on the incline in evaluating their suitability.Lots of previous studies focused on ultrafine-grained(UFG)materials processed via a kind of SPD technique,with many significant results reported on the thermal stability and recrystallization behavior.However,the strain paths of different severe plastic deformation methods are different and the microstructure and thermal stability of ultrafine grained grains produced by those SPD methods are also distinct.It is necessary to make a systematic comparative study on the thermal stability of UFG structures obtained by a variety of severe plastic deformation methods.On the other hand,the dynamic process of the evolution of the material structure can be divided into two modes:the discontinuous transformation involving two obvious stages of the nucleation and growth and the continuous(uniform)transformation with the nucleation and growth stages failed to be resolved.As far as the thermal stability of the single-phase deformation structure is concerned,the discontinuous recrystallization and the discontinuous recrystallization are presented.The effect of the two different modes of microstructure transformation on thermal stability is also needed to be studied in depth.In view of the above problems,this dissertation uses typical methods of equal-channel angular pressing(ECAP)and and high pressure torsion(HPT)to prepare UFG copper with hardness into a steady-state with further straining,to study the thermal stability.Adopt parameters of the change rate of hardness with the annealing temperature and the recrystallization temperature(the peak temperature of the change rate)to characterise the thermal stability and analyse the dynamic process of the evolution of microstructure under conventional ex situ annealing and quasi-in situ annealing in order to study the thermal stability and its internal mechanism of UFG structures.The main innovative conclusions of the study are as follows:(1)Compared with ECAP without back pressure,the peak value of the softening rate of hardness with annealing temperature of UFG copper manufactured by ECAP with backpressure(BP-ECAP)increases from～2.6 to～2.9 HV/℃.the temperature of recrystallization decreases from～153 to 147℃.the activation energy of recrystallization decreases from～99 to～91kJ/mol,and the thermal stability decreases;the stored energy accordingly increases from～8.61 to～9.11 MJ/m3;the recrystallization temperature and thermal stability decreases with the increase of storage energy,which conforms to the regular rule of the relation between the recrystallization temperature and thermal stability as well as the storage energy of deformation.(2)Compared with ECAP,the peak value of the softening rate of hardness with annealing temperature of UFG copper produced by HPT decreases from～2.6 to～1.1 HV/℃,with the recrystallization temperature increasing from～153 up to 183℃,showing higher thermal stability;the corresponding stored energy rises from～8.61 to～9.39 MJ/m3;with the increase of stored energy,the recrystallization temperature and thermal stability rise up.The relationship between recrystallization temperature as well as thermal stability and stored energy show abnormal regularity.(3)The microstructure analysis shows that the grain size of UFG copper manufactured by ECAP with and without the back pressure exhibits biomodal distribution stage.The grain size and the fraction of high angle grain boundaries(HAGBs)increase sharply with the increase of annealing temperature,which shows obvious discontinuous recrystallization.leading to the the conventional law that the recrystallization temperature and thermal stability decreases with the increase of storage energy.In the case of UFG copper processed via HPT,the grain size has the same single peak distribution.The fraction of HAGBs increases continuously,uniformly and gradually with the increase of annealing temperature,which shows typical continuous recrystallization characteristics,as result of the phenomena that although copper has high stored capacity,it has relatively high thermal stability.(4)The uniform microstructure distribution,low strain gradient,equilibrium grain boundary migration rate and driving force(dominated by grain boundary energy)in UFG copper processed via HPT resulting in a good stability of grain boundary network,which is formed by high proportion of HAGBs(>70%).Thus,the migration of grain boundary in the near range of several grains turns out to be restricted,the grain growth by swallowing other peripheral gains via the rapid long-range migration of boundaries in discontinuous recrystallization has been inhibited,which leads to continuous recrystallization.The interface energy is the dominant factor in the migration of grain boundary.Although UFG copper manufactured by ECAP has up to 60%high angle grain boundary,but there is still a considerable amount of the local microstructures characterized by elongated grains with a high strain gradient.In the annealing process,dozens of grains can be swallowed by HAGBs driven by the strain gradient(the stored energy gradient)in the form of long-range migration,with the formation of large grains posessing the grain growth advantage than the surrounding grains,leading to discontinuous recrystallization.Strain gradient(storage energy)is the dominant factor in grain boundary migration.