Researches On Grain Boundary Characteristics And Recrystallization Behavior Of Ultrafine-Grained Pure Aluminum

As compared with coarse-grained(CG)microstructures,the ultrafine-grained(UGF)microstructures(grain size～100 nm-1μm)usually have special physical and chemical properties as well as high mechanical properties,as a result of which,there have being receiving extensive attentions on the researches of bulk UFG materials processing.However,the UFG microstructures prepared by plastic deformation usually have a mount of deformation defects,large deformation storage energy,as well as high grain boundary energy state deviation from the equilibrium state,all those leading to a poor thermal stability of such microstructures.The UFG microstructures may recrystallize and begin to grow with a sharp decrease of mechanical properties even at a low temperature.Therefore,the thermal stability of the UFG microstructure has become an important factor restricting the application of bulk UFG materials.In the present work,CP aluminum was used as it have a high level SFE energy and a relatively simplegrain boundary configurations under plastic deformation.The specimens were subjected to a severe plastic deformed with ECAP and carried out post-annealing experiments at different temperatures with a series of holding times,and at the same time,the investigations on recrystallization behaviors&grain boundary characteristics were systematically executed.The main conclusions of the study are as follows:(1)The CP aluminum was subjected to 1-10 passes of ECAP extrusion deformation at room temperature,and the microhardness of the deformed material was about 1.5 times high than that of the annealed materials.The microstructure observation showed that with the increase of extrusion passes,the grain size of pure aluminum was continuously refined,and the smallest grain size was obtained after 8p,～0.15μm.The proportion of high-angle grain boundaries in the structure gradually increases,and the average grain orientation difference was close to 20.6~o.(2)The 8p sample was carried out isothermal annealing at 100°C and 150°C.The grain size got a slightly increased but still remained at a submicron level of～0.55μm,after annealing at 5min,10min,15min,20min,respectively.After annealing at150°C with 5 minutes,the microstructure was mainly composed of high-angle grain boundaries(fraction～66.5%),while it had changed into 58.3%of low-angle grain boundaries when keeping with 15min,and this was varied consist of half to half in low&high grain boundaries when keeping with 20min,remaining a grain size of～0.58μm.(3)After ECAP deformation,sub-micron structures with low defect densities were formed inside the pure aluminum.With the further increase of deformation,the grain size refinement is not obvious,but the grain boundary orientation difference further increases,forming a recrystallized sub-micron structure dominated by large-angle grain boundary.The submicron grains of CP Al undergoes continuous recrystallization under isothermal annealing,apparent with low defect density and little grain size growth.After annealing at 150°C for 15 min,the recrystallized structure contains a large number of small-angle grain boundaries,which is related to the rotation and merging of grains during the recrystallization process.The microstructure keeps stable after a subsequent annealing.