A comprehensive investigation of the strengthening effects of dislocations, texture and low and high angle grain boundaries in ultrafine grained AA6063 aluminum alloy

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The effect of equal channel angular pressing (ECAP) on the microstructure and mechanical properties of AA6063 aluminum alloy was investigated. For this purpose, samples of AA6063 aluminum alloy were deformed up to 10 passes using ECAP and the evolution of microstructure, texture and dislocation density was investigated. It was found that the dislocation density increased and cells mostly surrounded by low angle boundaries (LABs) formed after 2 passes ECAP. Increasing the dislocation density continued with further processing and reached a maximum at the 4th ECAP pass. With further deformation to 6 passes, the dislocation density reduced, the fraction of high angle boundaries (HABs) increased and the averages of cell size and grain size reduced, significantly, which indicated the occurrence of grain refinement. However, a slight increase in dislocation density, average cell size (ACS) and average grain size (AGS), indicating coarsening, occurred when 10 passes of ECAP are imposed. Hardness, yield (YS) and ultimate tensile strength (UTS) increased continuously although the most significant enhancement occurred after 2 passes of ECAP. Investigating the correlation between the microstructure and mechanical properties indicated that Hall-Petch relationship was valid for ultrafine grained AA6063 before the coarsening started provided that the property of interest was correlated to average cell size but not the average grain size.