Effects of large amounts of Mg (5–13 wt%) on hot compressive deformation behavior and processing maps of Al-Mg alloys

Abstract

The effects of the addition of large amounts of Mg (5–13 wt%) on the hot compressive behavior and processing maps of Al-Mg alloys were systematically examined in a wide strain rate range between 5 × 10−4 s−1 and 10 s−1 in a temperature range between 598 and 698 K. As the Mg content in the Al-Mg alloy increased from 5 to 7 wt%, the fraction of dynamically recrystallized (DRXed) grains decreased and then increased, followed by the further increase of Mg content up to 13 wt%. The Al-13Mg alloy exhibited superior post-deformation microstructures compared to the Al-Mg alloys with the lower Mg Al-Mg alloys. For example, after compressive deformation at high strain rates of 1 and 10 s−1 at 698 K, the Al-13Mg alloy had considerably higher fractions of DRXed grains (67–98%) and much smaller average grain sizes (13–19 μm) than the fractions of DRXed grains (5–43%) and the average grain sizes (63–485 μm) of the Al-Mg alloys with the Mg contents ≤ 10 wt%. The decreased fraction of DRXed grains with an increase in Mg content from 5 to 7 wt% was attributed to the suppression of dynamic recovery, resulting in a decrease in the kinetics of continuous dynamic recrystallization (CDRX). The increased fraction of DRX grains with the increase in Mg from 7 to 13 wt% could be attributed to the transition from CDRX to discontinuous dynamic recrystallization by the significant reduction in stacking fault energy with the addition of large amounts of Mg. The Al-Mg alloys exhibited the solute drag creep at low strain rates and power law breakdown (PLB) at high strain rates. With the addition of large amounts of Mg, however, the onset of PLB was delayed to a higher strain rate and a lower temperature. As a result, the Al-13Mg alloy could exhibit higher power dissipation efficiency and smaller instability regions compared to that of the lower Mg content Al-Mg alloys in the processing maps. This outcome agreed with the observations on the microstructures after compressive deformation.