Development of Ultrafine Grain Structure in an Al–Mg–Mn–Sc–Zr Alloy During High-Temperature Multidirectional Isothermal Forging

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Mechanisms of grain refinement under multidirectional isothermal forging (MIF) at 325 °C (~ 0.65 Tm) and the strain rate 10− 4 s− 1 of the Al–Mg-based alloy with complex additions of transition metals were investigated. The starting alloy had an equiaxed grain structure with grain size 25 µm and a uniform distribution of coherent Al3(Sc,Zr) dispersoids of 20–50 nm. A distinguished structural feature in the early MIF stage was the formation of high strain- and misorientation gradients, followed by deformation banding. Due to the sequential changes of the loading axis, such bands were developed in various directions and fragmented the original grains. The number of bands and misorientation of their boundaries gradually rose with strain, resulting in formation of (ultra)fine grain structure with the grain size 2 µm. New grain formation was concluded to occur via continuous dynamic recrystallization and controlled by the nanosized precipitates, which preferably remained stable and coherent with the surrounding matrix.