Influences of the Al3Sc particle content on the evolution of bimodal grain structure and mechanical properties of Al–Mg–Sc alloys processed by hard-plate rolling

The present study investigated influences of the Al3Sc particle content on the evolution of bimodal grain structure and mechanical properties at both room and elevated temperatures of Al–7Mg–Sc alloys processed by hard-plate rolling (HPR). Bimodal grain structures were obtained in HPRed Al–7Mg–Sc alloys where ultrafine/fine grains decreased in size and increased in amount with increasing volume fraction of Al3Sc precipitates. The formation and evolution of bimodal grain structure are associated with (i) the different stability of various-orientated grains, and (ii) the volume fraction of Al3Sc precipitates. Ultrafine/fine (sub) grains form preferentially along grain boundaries (GBs) of <001>//normal direction (ND)- and <111>//ND-oriented coarse grains during HPR. In contrast, the <101>//ND-oriented coarse grains show relatively high stability nearly without subdivision. Furthermore, increased volume fraction of Al3Sc precipitates in Al–7Mg–0.4Sc promotes dislocation accumulation, favoring recrystallization nucleation, and meanwhile restricts grain growth, which results in more and refined ultrafine/fine grains. Thereby, the refined grain structure and increased volume fraction of Al3Sc precipitates in the Al–7Mg–0.4Sc alloy contribute to improved room-temperature yield strength (YS) and elongation, i.e. ~482 MPa and ~8%, which is much higher than that of ~420 MPa and ~4% in the binary Al–7Mg alloy. Meanwhile, Al–7Mg–0.4Sc exhibited higher hardness when annealed at 150–300 °C and also higher YS when subjected to a tensile test at ≤ 200 °C due to the more stabilized grain structure pinned by dispersed Al3Sc and additional strengthening effect from Al3Sc precipitates. Nevertheless, when subjected to a tensile test at ≥ 250 °C, the Al–7Mg–0.4Sc alloy exhibited much lower YS than binary Al–7Mg. The transition could be explained by more favored grain boundary sliding (GBS) and great dislocation loss, due to the easier occurrence of dynamic recrystallization (DRX) in Al–7Mg–0.4Sc during tensile test at elevated temperatures.