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Long-term thermal stability of Equal Channel Angular Pressed 2024 aluminum alloy

The strength of bulk metallic materials can be improved by creating ultra-fine grained structure via severe plastic deformation (SPD). However, the thermal stability of severely deformed materials has been a major issue that restricts their practical use within the industry. Although there are studies on the thermal stability of SPD metals, the long-term annealing response of particularly complex alloys, such as the age hardenable ones, remains undetermined. In the present study, annealing behavior of the single pass Equal Channel Angular Pressed age hardenable 2024 Al alloy was investigated in the 038-0.52 homologous temperature range for up to 1000 h. Microstructures and the corresponding mechanical properties of the samples were determined by transmission electron microscopy, electron back-scatter diffraction analyses, and micro-hardness measurements. After long annealing durations at 80 degrees C and 120 degrees C, a secondary hardening was observed whereas a fast softening occurred at 200 degrees C. At 150 degrees C, however, a softening followed by a slight secondary hardening was also detected. The increased coarsening rate of S precipitates accompanied with dislocation annihilation was found to be the major cause of the hardness loss. Furthermore, dislocation-rich structure and Mg clusters remaining from the S precipitate dissolution eased the nucleation of Omega precipitates. which are responsible for the secondary hardening. It was concluded that below 120 degrees C the single pass ECAPed Al 2024 components preserve their improved hardness for a prolonged period of time.