**1. Introduction**

The massive magnesium solid solution from the dispersion phase and the magnesium content in aluminum alloys improve the strength properties, fatigue strength, wear resistance, joint properties and the forming properties [1–3]. Aluminum alloys have been extensively researched with regard to superplastic formation due to their non-heat-treatment properties. In recent decades, aluminum alloy superplastic structures have been cited in the locations of Su-27, MIG-26 and J-10 stamping parts such as the fuselage, stringer, vertical tail skin, etc. [4]. It is well known that aluminum alloys exhibit superplastic fractures due to the presence of the cavity caused by the Mg-rich phase particles. However, most early studies related a less than 5 mass% magnesium content of aluminum alloys with excellent elongation.

Numerous researchers have reported the superplasticity of the Al–Mg series aluminum alloys [5–8], super-strength aluminum alloys (Al–Mg–Cu series) [9,10] and ultra-high-strength aluminum alloys (Al–Zn–Mg–Cu series) using equal channel angular pressing (ECAP), friction stir processing (FSP) and high-pressure rotation (HPT) methods [11–14]. It demonstrates that superplastic flow can be achieved in aluminum alloy with small grain sizes, typically less than 10 μm. In order to satisfy the requirement of a 2 mm thickness of superplastic stamping products, the precipitated phases in the ingot casting billet were controlled, combined with heat treatment between the cold rolling of the 5A70 aluminum alloy. The obtained superplastic elongation-to-failure value, δ, was greatly improved when compared to the existing commercial sheets [15].

In this study, attention was focused on the exact mechanism of cavity nucleation and cavity growth during the superplastic deformation to study the cavity behavior in detail. The cavity behavior found at different tensile stages and in different superplastic fractures were precisely identified via supporting microscopy evidence. However, cavitation is a function of external factors such as temperatures and grain sizes, which are both related to the second phase particles. It was found that the cavity nucleation, growth, interlinkage and coalescence during superplastic deformation were caused extensively by the precipitated phase [16–20].

### **2. Materials and Methods**

The chemical composition of the 5A70 alloy is shown in Table 1. The prepared experimental alloy ingot was homogenized and annealed at 450 ◦C for 40 h. Then the ingot alloy was rolled into a billet with a rectangular normal direction plane of 255 mm × 255 mm at 380 ◦C. From the state of the extruding ingot, a billet with a plate shape (200 mm × 200 mm × 25 mm) was processed, and the natural aging treatment was ≥240 h. The size and distribution of the precipitated phases in the smelted and forged processes were controlled to promote the nucleation during dynamic recrystallization, and the billet was subjected to 14 passes of cold rolling on a 350 mm reversing cold mill, and full recrystallization was inserted when the sheet was 10 and 5 mm thick using a GS-2-1200 box-type resistance furnace (Tianjin Zhonghuan Lab Furnace Co., Ltd., Tianjing, China). Ultimately, the 2 mm thickness of fine-grained (FG) 5A70 alloy superplastic sheet was obtained, and the initial grain size of the rolling direction plane was 8.48 μm. Further details of the rolling process and the full recrystallization system were reported in a previous work [15].

**Table 1.** Chemical compositions of the studied 5A70 aluminum alloy (wt.%).


Specimens with 8 mm gage length and 4 mm gage width were machined in the parallel rolling direction. Superplastic tensile tests were performed on an AG-250KINC Instron machine (Shanghai Gold Casting Instrument Analysis Co., Ltd., Shanghai, China) with a microprocessor control pad in the NV63-CV high temperature furnace. The tests were performed at different temperatures ranged from 400 to 550 ◦C, and the initial strain rate was 1 × <sup>10</sup>−<sup>3</sup> <sup>s</sup>−<sup>1</sup> in an air condition. In addition, type-K thermocouples were used to detect the furnace and the temperature was controlled within approximately ±2 ◦C along the entire gage length during the tests. The specimen was insulated at 340 ◦C and 10 min for full recrystallization; moreover, it can control the temperature equilibrium during the heating process. Immediately, the same heating rate, 21 ◦C/min, was utilized to reach the target temperature and maintained for 2 min.

The microstructural characterization and analysis of the 5A70 alloy were carried out using a Jeol-7100 (JEOL Ltd., Tokyo, Japan) transmission electron microscope (TEM) and a 600FEG (FEI Corporation, Hillsboro, OR, USA) scanning electron microscope (SEM) or structural characterization analysis. The average grain size was found using the linear intercept method and the OIM software (6.2.0 x86 version, EDAX Inc., Draper, UT, USA). The boundary orientation was measured by utilizing pixel-to-pixel measurements. To carry out the structural characterization, specimens of the SEM were cut 5 mm from the superplastic fracture surface.
