Effect Of Annealing Treatment On The Microstructure And Mechanical Property Of The Ultrafine-grained Aluminum Processed By SPD

ABSTRACT:By the tensile mechanical testing and fatigue growth test, with use of Transmission Electron Microscope (TEM), Scanning Electron Microscopy (SEM) and EBSD observation, the size and morphology of pure Al and Al-4Cu alloy, as well as the microstructural and property evolution of commercial pure Al with subsequent anneal treatment in different temperature, which were all subjected to Multi-axil compression (MAC) were studied. The main results are as follows:(1) On annealing the grain structures coarsen and transform from lamellar to equiaxed ones. Remarkably, the fraction of high angle grain boundaries drastically increases from29.3%to76.3%after annealing at60℃. Meanwhile, a significant decrease of lattice microstrain is observed after annealing, from0.0839%to0.0731%at130℃which may account for recrystallization.(2) A controlled30min annealing treatment on ultrafine-grained (UFG) Al at60℃can result obviously in a higher strength and a lower elongation, which may be associated with the nucleation and subsequent motion of dislocations in grain boundaries. As the annealing temperature is above60℃the yield strength decreases and elongation increases gradually, which is attributed to the grain coarsening and microstructural enhancement.(3) A slower crack growth rate and a lowest threshold stress intensity factor△Kth characterize the60℃annealed sample when compared with high temperature annealed samples, which is related to the size of RPZ and the fraction of high angle boundary. A comparison of fatigue crack growth curves between130℃and200℃annealed samples indicates that grain size can result in a higher susceptibility to crack initiation. In the contrary, it is inconspicuous in the high AK regime. While such behavior reveals the possibility of a roughness induced crack closure mechanism only near the threshold growth rates.(4) The existence of solute atom of Al-4Cu alloy were associated with the irregular morphology of subgrain and higher dislocation density than pure Al. However, the former exhibited smaller grain size with length about200nm and width about100nn than the latter with600nm. Also, under the room temperature grain refinement were suppressed by the re-dissolution of severely deformed precipitated phase. And under50℃the re-dissolution and re-precipitation of precipitated phase contributed to grain refinement.