An Evaluation of the Microstructure and Microhardness in an Al–Zn–Mg Alloy Processed by ECAP and Post-ECAP Heat Treatments

Herein the microstructure evolution and microhardness variations in a peak-aged Al–Zn–Mg alloy after equal-channel angular pressing (ECAP) for 1–8 passes followed by heat treatments at temperatures of 393–473 K for 5–20 h are examined. The results show that ECAP processing hardens the alloy, and the hardness increases with each pass due to grain refinement, the formation of large numbers of fine precipitates, and the introduction of a high dislocation density. Post-ECAP annealing at 393 K up to 20 h after 1 and 4 passes of ECAP leads to a further increase in microhardness and this increase is substantial for an annealing time of 20 h after 1 pass due primarily to the extensive formation of a nano-sized η′ phase. After ECAP for 8 passes, post-ECAP annealing at 393 K for 5 h or more leads to softening due to the occurrence of recrystallization and the transformation of fine η′ to coarse η. Post-ECAP anneals at the higher temperatures of 423 and 473 K are not capable of producing high hardness values.