**Sheng Li, Zhongguo Huang and Shunyao Jin \***

School of Mechanical Engineering, University of Science and Technology Beijing, Beijing 100083, China; lishengsir@163.com (S.L.); jsy\_white@126.com (Z.H.)

**\*** Correspondence: jinshunyao@ustb.edu.cn; Tel.: +86-010-6233-2467

Received: 18 December 2018; Accepted: 8 January 2019; Published: 10 January 2019

**Abstract:** A fine-grained 5A70 alloy sheet was obtained through a combination of rolling and heat treatment, with a total deformation reduction of 90% and an average grain size of 8.48 μm. The alloy was studied at 400, 450, 500, and 550 ◦C and exhibited excellent elongation-to-failures of 205, 321, 398, and 437% with coefficients for the strain rate sensitivity of 0.42, 0.40, 0.47 and 0.46, respectively. Electron backscatter diffraction (EBSD) results revealed that the massive grain boundaries were high angle boundaries, suggesting that boundary sliding and grain rotation occurred during superplastic deformation. The X-ray diffraction (XRD) and energy dispersive spectrometer (EDS) results indicated that the compositions were the Al6(MnFe) and Mg-rich phase particles of the deformed 5A70 alloy. In addition, the weakening of the pinning effect led to abnormal grain growth at 500 and 550 ◦C, resulting in strain hardening. Transmission electron microscopy (TEM) examinations demonstrated that the applied stress at the head of the precipitated particles and/or grain boundaries exceeded the matrix-structure-promoted cavity nucleation. Cavities grew, interlinked, and coalesced, which resulted in crack formation that eventually led to superplastic fractures. Filaments formed at the fracture surfaces because of second phase precipitation at grain boundaries and the formation of Mg-rich oxides.

**Keywords:** 5A70 aluminum alloy; fine-grained structure; superplastic tension; grain boundary sliding; precipitated phase; strain hardening
