Accelerated mass transfer in the Al-6 wt.% Cu alloy during high-pressure torsion

The high pressure torsion (HPT) of as-cast alloy Al-6 wt.% Cu at 5 GPa, room temperature, various numbers of anvil revolutions n and a torsion rate of 1 rpm was carried out. HPT under these conditions reduces the grain size in the Al-based solid solution to 100 – 200 nm after n = 5 revolutions. The lattice parameter a decreases from 0.404626 nm in the initial alloy to 0.40420 nm at n = 5, indicating copper enrichment of the aluminum solid solution (Al) during HPT. Notably, the lattice parameter reaches a steady-state value after only 0.1– 0.3 revolutions. Using the simplified formula L = (Dt)0.5 the diffusion coefficient D was estimated for mass transfer during HPT. The effective volume diffusion coefficient DHPT, corresponding to transport over 200 nm in 30 seconds, is DHPT = 3 ×1016 m2 / s, which is about 12 orders of magnitude higher than the conventional volume diffusion coefficient at room temperature. This conventional coefficient reaches the same value at an effective temperature Teff of 380 ± 20°C. Therefore, the effective temperature of HPT for this alloy is approximately 380 ± 20°C, meaning that mass transfer at room temperature during HPT matches the rate of conventional diffusion at this elevated temperature. This suggests a high steady-state concentration of lattice defects in HPT, which promotes accelerated mass transfer.