Influence of Zr addition on the microstructure evolution and thermal stability of Al-Mg-Mn alloy processed by ECAP at elevated temperature

Two Al-Mg-Mn alloys having similar compositions but with and without Zr addition were subjected to equal channel angular pressing (ECAP) at 350 degrees C using route Bc and a 90 degrees die, followed by water quenching or air cooling. A series of annealing experiments were conducted at various temperatures from 400 to 460 degrees C on the water-quenched alloys. Fine structures with grain sizes of about 1-2 mu m were obtained in these alloys after six passes. The water-quenched alloy containing Zr exhibited finer structure and higher strength compared to the Zr-free alloy in the same cooling condition, which was due to the existence of Al3Zr dispersoids. And in air cooling condition, the difference of strength between these two alloys increased further. This was because that the static recovery and static recrystallization during the air cooling process decreased the strength of the Zr-free alloy, while the fine Al3Zr dispersoids inhibited the restoration in the alloy containing Zr. In addition, in the annealing experiments, secondary recrystallization took place in the Zr-free alloy annealed at 410 degrees C for 1 h, and it was completed when annealed at 460 degrees C for 1 h. While the alloy containing Zr kept stable up to 460 degrees C for the pinning effect of the Al3Zr dispersoids. Tensile tests showed that the strength of the Zr-free alloy decreased dramatically after annealing at 460 degrees C for 1 h, while the alloy containing Zr exhibited almost no decline at the same condition. (c) 2006 Elsevier B.V. All rights reserved.