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

School of Mechanical Engineering, University of Science and Technology Beijing, Beijing 100083, China; lishengsir@163.com (S.L.); jinshunyao@ustb.edu.cn (S.J.)

**\*** Correspondence: jsy\_white@126.com; Tel.: +86-010-6233-2467

Received: 18 November 2018; Accepted: 12 December 2018; Published: 14 December 2018

**Abstract:** The study of the exact physical mechanism of cavity nucleation and growth is significant in terms of predicting the extent of internal damage following superplastic deformation. The 5A70 alloy was processed by cold rolling for 14 passes with a total reduction deformation of 90% (20–2 mm) and the heat treatment was inserted at a thickness of 10 and 5 mm at 340 ◦C for 30 min. The superplastic tensile tests were performed at 400, 450, 500, 550 ◦C and the initial strain rate was 1 × <sup>10</sup>−<sup>3</sup> <sup>s</sup>−1. Cavities were observed at the head of the particle and the interface of the grain boundaries. It is suggested that the cavity was nucleated during the sliding/climbing of the dislocations, due to the precipitate pinning effect and the impeding grain boundary during grain boundary sliding (GBS). In this study, the results demonstrated a clear transition from diffusion growth to superplastic diffusion growth and plastic-controlled growth at a cavity radius larger than 1.52 and 13.90 μm. The cavity nucleation, growth, interlinkage and coalescence under the applied stress during the superplastic deformation, as well as the crack formation and expansion during the deformation, ultimately led to the superplastic fracture.

**Keywords:** 5A70 aluminum alloy; superplastic tensile; cavity growth; fracture
