A TEM Kikuchi pattern study of ECAP AA1200 via routes A, C, B_C

摘要

Shear plastic deformation in ECAP occurs through the formation, movement and storage of dislocations. The differences which exist between shearing characteristics lead to important implications concerning the optimum processing route. It is known that the effectiveness of cell evolution into an array of high-angle boundaries (HABs) is in the order B_C>C>A, or A>B_C>C, depending on the two-channel-angle intersection of the ECAP die. In route A, large portions of HABs are continuously and progressively generated, while routes C and B_C cause fully redundant deformation at each 2n passes, and each 4n passes, respectively. This study is focused on dislocation generation, storage and recombination during ECAP for routes A, C and B_C. Kikuchi bands identified with TEM were used to quantitatively measure the cell and grain boundary misorientation. ECAP was performed on an AA1200 commercially pure aluminium alloy up to 6 = 8.64. A different hierarchy for HAB generation efficiency was found. Thermal stability was studied by annealing the alloy at 0.5,0.6,0.7 T_M (where T_M is the alloy melting point) for 2 h after the severe plastic deformation. It appeared that, even if route B_C involves the fastest microstructure grain refining, route C is likely to be the most stable upon reheating.