Structural Evolution in the Quenched Al–Zn–Mg–Fe–Ni Alloy during Severe Plastic Deformation and Annealing

The evolution of the structure and phase composition of the Al–Zn–Mg–Fe–Ni alloy (nikalin) during severe plastic deformation by high pressure torsion and annealing has been investigated using high-resolution TEM techniques. The deformation of the quenched alloy has been found to result in a submicron composite comprising an Al matrix and various fine aluminides. The severe structural refinement takes place along with phase transformations, which dissolve the metastable Al₃Zr-phase particles and cause the nanosized strengthening phases Т (Al₂Mg₃Zn₃) and η' (MgZn₂) to precipitate from the Al solid solution which has been supersaturated in result of quenching and deformation. Nikalin has been determined to retain its submicrocrystalline state during low-temperature annealing (200°C, 4 h) due to the barrier effect of fine hardening phase particles, fixing grain boundaries. High-temperature heating to 400°C for four hours transfers nikalin to the overaged state, in which it is characterized to consist of a recrystallized matrix with grains 2–3 µm in size, eutectic micron-sized aluminides, and stable Al₂Mg₃Zn₃ and MgZn₂-phase particles with sizes of no more or equal to 500 nm.