Microstructure Evolution and Mechanical Properties of Mg-Gd-Y-Zn-Zr Alloy Prepared via Pre-aging Treatment Prior to Upsetting–Extrusion–Upsetting Passes

The upsetting–extrusion–upsetting (UEU) process adopted in this paper is one of the severe plastic deformation (SPD) methods. This process can significantly refine grains and achieve strength–plastic synergy, which can provide a reference for the industrial application of the production process. In this paper, the UEU experiment of Mg-9.5Gd-4Y-2.2Zn-0.5Zr (wt.%) alloy prepared via pre-aging treatment before each pass was carried out. The microstructure, texture evolution, and mechanical properties of different stages of the process were studied. The results showed that the main mechanism of grain refinement was the dynamic recrystallization (DRX). With the increase in UEU process passes, the alloy began to be dominated by the discontinuous dynamic recrystallization (DDRX), followed by both the continuous dynamic recrystallization (CDRX) and DDRX. The two mechanisms worked together to refine the grains significantly, and the alloy formed a bimodal microstructure. There are a large number of long-period stacking ordered (LPSO) phases in Mg-9.5Gd-4Y-2.2Zn-0.5Zr (wt.%) alloy. During the deformation process, the lamellar LPSO phases were twisted and fractured, the block-shaped LPSO phases were broken, and the second phase particles were formed. The broken block-shaped LPSO phases and the coarse second phase particles promoted recrystallization through particle-stimulated nucleation (PSN) mechanism, while the fine second phase particles played a pinning role in the matrix and hindered the growth of DRX. The fractured lamellar LPSO had higher strain energy at the fracture, which promoted recrystallization and formed a large number of DRXed particles at the fracture. The comprehensive performance of the sample after UEU process is the best, reaching the balance of strength–ductility. The yield strength (YS), ultimate tensile strength (UTS), and elongation (EL) of the sample are 175 MPa, 258 MPa, and 10.1%, respectively.