## 1 Introduction  
The fine-grained Al-based alloys attract regular interest due to their combination of properties, among which are increased plasticity, formability, and strength [1]. Grain refinement in Al-based alloys is currently the main focus of multiple research studies that reflect relevant industrial needs [2]. The popular Al-based heat-treatable 6 xxx-type Al-Mg-Si-based alloys exhibit good corrosion resistance and outstanding strength-to-weight ratio [3, 4]. Moreover, the Al-Mg-Si-based alloys demonstrate good formability at low temperatures [5]. However, at elevated temperatures, owing to the low alloyed solid solution, the 6 xxx-type alloys are prone to a grain growth that complicates the formation of a fine-grained structure [6]. Thus, the intense dynamic grain growth of these alloys leads to poor superplasticity [7, 8]. The grain growth leads to structure inhomogeneity that provides inhomogeneous flow and results in a decrease in uniform elongation. A fine-grained structure is commonly achieved through a severe plastic deformation (SPD) method that has been proven to be universal and effective for various metals and alloys [9]. The SPD is realized by various techniques that are united by the ultra-high plastic strain accumulated during material processing. Popular SPD techniques, such as the equal channel angular pressing (ECAP) [10], friction stir processing (FSP) [11] and high pression torsion (HPT) [12], have demonstrated high efficiency in grain refinement and mechanical properties improvement. However, the high energy consumption of the previously mentioned SPD approaches limits the sample size and industrial application. A promising SPD technique that allows for processing bulk samples while maintaining their geometry is isothermal multidirectional forging (MDF) [13; 14; 15; 16; 17]. MDF has been effectively used for achieving outstanding grain-refinement in the Al-based alloys [18; 19], by which a grain size of 0.5-2 \(\upmu\)m was achieved in 5 xxx type alloys [20; 21]. A similar grain size was observed in 6 xxx-type alloys after multidirectional forging at cryogen temperatures [22; 23]. Although the MDF treatment effectively refines the grains, the thermal stability of the refined grain structure requires special approach. To suppress grain growth, Al alloys are usually doped with small additions of transition metals (TM) and rare-earth elements (RE) [22; 24; 25], including Sc and Zr which have proven their significant effectiveness [26; 27]. These metals exhibit low equilibrium solubility in aluminum. During rapid solidification, the solubility of Sc and Zr can be increased to allow the formation of a supersaturated solid solution. Subsequent heat treatment provides the decomposition of supersaturated solid solution with a precipitation of fine nanoscale dispersoids of Al\({}_{3}\)Sc or Al\({}_{3}\)Zr phases with an L1\({}_{2}\) crystal structure type [28; 29; 30]. The co-adding of Sc and Zr results in a formation of a complex Al\({}_{3}\)(Sc,Zr) phase with core-shell structure [31; 32] that is distinguished by an increased potential resistance to coarsening at elevated temperatures [33]. The nanoscale dispersoids provide for the Zener pinning effect that suppress static and dynamic grain growth [34]. The positive effect of Sc and Zr co-addition on grain stability at elevated temperatures has been multiply reviewed for thermomechanical-treated Al alloys [35; 36]. As a result, the improved superplastic behavior due to the Zener pinning effect has been previously demonstrated in 2 xxx- [37], 5 xxx- [38; 39; 40; 41], 6 xxx- [42; 43], and 7 xxx-type [44; 45] Al alloys. The effectiveness of microalloying directly depends on the particle parameters. Following the Smith-Zener Equation (1), the pinning pressure increases with increasing particle volume fraction and decreasing particle radius [46]. \[P_{SZ}=\frac{3/\gamma}{2r}, \tag{1}\]  
where \(f\) is the volume fraction, \(r\) is the radius of particles, and \(\gamma\) is the grain boundary energy.