The Effect Of Annealing Treatment On Microstructure And Mechanical Properties Of Ultrafine Grained Pure Cu And Fe

A large amount of internal stress is accumulated in the microstructure of ultrafine grained(UFG)materials prepared by severe plastic deformation(SPD),such as accumulative rolling bonding(ARB)and Equal-channel angular pressing(ECAP).The microstructure and properties are unstable and in a metastable state.Annealing heat treatment can not only reduce the internal energy storage and improve the thermal stability of the UFG materials,but also homogenize the microstructure and improve the comprehensive properties.It is important to reveal the effect of annealing heat treatment on the microstructure,mechanical properties and thermal stability of different UFG materials prepared by different processes.This paper selects pure copper with FCC structure and industrial pure iron with BCC structure as experimental materials,and studies the effects of annealing heat treatment on mechanical properties,microstructure,fracture mechanism and damage morphology through the ARB-Cu,ECAP-Cu and ECAP-Fe prepared by six-pass ARB process and one-pass ECAP process.Through experimental study,it is found that annealing can effectively improve the microstructure of high dislocation density and grain boundary confusion within the ARB-Cu.ARB-Cu after annealing for 10 min and 30 min at 100?250?,the yield strength and tensile strength increase first and then decrease with the increase of annealing temperature.The annealing temperature is 100?and 150?,the yield strength and tensile strength rise first and then decrease with the increase of annealing time.After annealing for 30 min,the strength reaches the maximum and the annealing strengthening effect appears.The microhardness decreases with the increase of annealing temperature and annealing time.The tensile fracture characteristics of the ARB-Cu after annealing treatment show that the toughness increases,the plastic deformation ability is improved,and the activation energy of grain growth is 73 k J/mol,showing a certain thermal stability.The relationship between the interfacial welding rate and the rolling pass of ARB-Cu is satisfied:R(28)(1-0.5~n)?100%.The interfacial welding coefficient and the rolling pass are satisfied:k(28)-0.033n~2(10)0.32n(10)0.39.The optimal annealing process parameter of ARB-Cu is annealing at 150?for 30 min.As the annealing temperature increases,the yield strength and flow stress of the ECAP-Fe annealing samples increase first and then decrease.When annealing temperature is 400?,the strength of the material reaches the maximum value.The microhardness gradually decreased with the increase of annealing temperature.After annealing at different temperatures,the intragranular slip and the initiation of grain rotation,the surface deformation damage degree decreases,showing excellent grain boundary coordinated deformation ability.ECAP-Fe after annealing,the microstructure is mainly composed of equiaxed grains with low dislocation density and small size subgrains.The grain boundaries are gradually clear and the grain size differentiation rate is reduced.The optimum annealing temperature of the ECAP-Fe is annealing at 400?for 10 min.ECAP-Cu after annealing for 10 min at different temperatures shows annealing strengthening effect.The compressive yield stress increases first and then decreases with increasing temperature.Compression surface damage morphology is different from ECAP-Fe damage morphology.The main damage features are compression raised steps and fine shear bands.