**1. Introduction**

Due to the excellent combination of strength, corrosion resistance, weldability, formability, and low cost, Al-Mg alloys are widely used in the aircraft, shipbuilding, and automotive industries [1]. However, Al-Mg alloys are not heat treatable, so high strengths are achieved through solution hardening using the Mg atoms that are retained in the solid solution, through precipitation hardening from the second phase particles, or through strain hardening effects [2–5]. Accordingly, additional contents of alloying elements, Mn, Ti, and Zr, refine the recrystallized grain size, while the Cu and Mg effectively improve the strength of Al-Mg alloys. The subcrystal and grain sizes of Al-Mg alloys are refined with the increasing Mg concentration. Furthermore, recrystallization can promote the evolution of the grain orientation towards high-angle grain boundaries [6,7]. It is known that the increase in magnesium content (3–8.5%) provides finer grains and improves the elongation-to-failure, *δ*, in Al-Mg alloys [8–10].

Numerous domestic and literature studies have been conducted concerning different methods used to prepare Al-Mg alloys and investigations on their superplasticity in the preceding decades. The equal channel angular pressing (ECAP) [11–16] and friction stir welding/processing (FSW/FSP) [17–22] technologies were used to prepare the fine/ultrafine grain structure of Al-Mg alloys to investigate their superplasticity. For example, the 5083 alloy was prepared using the FSW method and its superplastic behavior was investigated in the temperature range from 250 to 450 ◦C [23]. The results revealed that reasonable superplasticity with an elongation-to-failure of 570% was achieved at 300 ◦C and 8.3 × <sup>10</sup>−<sup>3</sup> <sup>s</sup>−1. In addition, the research regarding the superplasticity of Al-Mg alloys with an Mg content lower than 5.5 wt.% and refined structure showed excellent elongation. With the development of the lightweight aviation industry and the requirements of sonic and supersonic aircraft, the superplastic form of aluminum alloy sheet metal can survive severe plastic deformations, reduce the weight, and increase the strength for complex structures. Therefore, high-strength Al-Mg alloys have significant advantages in stamping parts, including the fuselage, stringer, vertical tail skin, etc. [24]. It is well-known that a high Mg content enhances the strength of Al-Mg alloys; however, the precipitation of a large amount of Mg-rich phase particles can lead to strain hardening in superplastic tension. The requirements for military, aluminum-alloy, superplastic sheet products are continually increasing, so the demand for rolling and heat treatment (RHT) methods to prepare superplastic sheets is urgent. In this paper, the RHT method will be used to obtain a fine-grained 5A70 alloy, mainly for large sheet metal parts in industrial applications.

Structural superplasticity is the ability of polycrystalline materials to exhibit high tensile elongations without the formation of a neck prior to fracture because of a high valued strain rate sensitivity, *m* [25]. Strain rates during superplastic deformation with the fine-grained 5A70 alloy obey the following relationship [26–30]:

$$\dot{\varepsilon} = A \frac{GD\_0 b}{kT} \left(\frac{b}{d}\right)^p \left(\frac{\sigma - \sigma\_0}{G}\right)^n \exp\left(-\frac{Q}{RT}\right),\tag{1}$$

where, *A* is a dimensionless material constant; *b* is the Burgers vector; *d* is the grain size; *σ* is the applied stress; *σ*<sup>0</sup> is the threshold stress; *G* is the shear modulus; *k* is Boltzmann's constant; *T* is the absolute temperature; *p* is the exponent of the inverse grain size, which ranges from 2 to 3; *n* is the stress exponent, which is defined as 1/*m*; *D*<sup>0</sup> is a frequency factor; *Q* is the activation energy; and *R* is the gas constant. For the majority of superplastic materials, the rate-controlling process of superplastic deformation is grain boundary sliding (GBS) [31]. An analysis of the superplastic tension testing and surface observation showed that lattice diffusion dominated the GBS mechanism in superplastic tension of the 5A70 alloy. This mechanism, through the diffusion activation energy and the strain rate sensitivity, was defined as a constant rate with a temperature dependence of the fine-grained structure. Therefore, this study focused on revealing the superplastic behavior characteristics of the 5A70 alloy with fine-grained structures that depend on temperatures, strain rates, and precipitated phases during superplastic deformation.
