Thermal stability of an Al-6061 alloy processed by high-pressure torsion.

Abstract

The occurrence of static recrystallization (SRX) and grain growth (GG) were examined in an Al-6061 alloy processed by high-pressure torsion for 10 turns at room temperature during isochronal heat treatment in the temperature range of 473 to 723 K for 30 min. The results demonstrated a normal grain growth behaviour in which the mean grain size continuously increased with increasing annealing temperature from 0.44 ± 0.1 µm (deformation state) to 8.7 ± 0.8 µm at 723 K. Coincidentally, the Vickers microhardness decreased from 155 ± 3 (deformation state) to 43.5 ± 4.1 Hv0.1 after annealing at 573 K due to SRX and then reached a steady-state up to 723 K due to GG. A texture analysis revealed the gradual development of a C shear component in the SRX stage (523-623 K) and its dominance in the GG stage (673-723 K). The activation energy for SRX obtained through differential scanning calorimetry analyses was equal to 85.4 ± 4.2 kJ/mol indicating a dominance by grain boundary self-diffusion. The activation energy for GG was estimated by the Arrhenius equation as 68.4 ± 2.4 kJ/mol indicating the poor thermal stability of the HPT-processed Al-6061 alloy.