Elongation increase in ultra-fine grained Al-Fe-Si alloy sheets

Microstructures and mechanical properties of ultra-fine grained aluminum alloy sheets (AA1100 and AA8011) manufactured by accumulative roll bonding (ARB) at ambient temperature were investigated. In the AA1100 sheet, the grain size was reduced down to 200 nm by the ARB process and the strength increased up to 360 MPa, accompanied by a remarkable reduction in tensile elongation. Meanwhile, in the AA8011 sheet, the grain size was reduced down to 300 nm until the third ARB cycle, but after the fourth cycle, the grain size did not decrease. Similarly, the tensile strength increased up to 200 MPa at the equivalent strain of 2.4 (3 cycles), but after the fourth cycle, it showed a constant level. It was noteworthy that the total elongation of the AA8011 sheet firstly decreased below 10% by the one cycle ARB but significantly increased with the increasing number of ARB cycles thereafter and eventually reached to 18% by the 12th ARB cycle. The AA8011 included a large number of second-phase particles like Si precipitates and α-AlFeSi intermetallic compounds in the initial state, which brought about scattered orientation of the matrix by inhomogeneous deformation during the ARB process. As a result, the AA8011 sheet had a larger amount of high angle grain boundaries and more equiaxed grains than the AA1100 sheet. The tensile stress-strain curves of the ARB processed AA8011 alloys revealed dynamic recovery type curves at room temperature, which corresponded well to the fact that recovery was enhanced to occur in the ARB processed AA8011 alloy due to the larger fraction of high angle boundaries and purer Al matrix than those in the AA1100. Additionally, dissolution and re-precipitation of Si particles during the ARB were observed in the AA8011 alloy.