Roles of LPSO phases on dynamic recrystallization of high strain rate multi-directional free forged Mg-Gd-Er-Zn-Zr alloy and its strengthening mechanisms

Abstract

High strain rate multi-directional free forging (HSMDFF) is an effective method to expand the industrial application of Mg alloys. However, dynamic recrystallization (DRX) and strengthening mechanisms of the HSMDFFed Mg alloys have not been understood. In this work, the evolution of the LPSO phase and its roles on DRX of Mg-8Gd-1Er–1Zn-0.6Zr (wt.%) (GEZ811) alloy at various cumulative strains during the HSMDFF process are studied, and the corresponding strengthening mechanisms are analyzed. As the cumulative strain increases, the lamellar LPSO phase undergoes four stages: kinking, tearing, local breaking, and breaking into submicron particles, while the block LPSO phase is kinked, torn and broken with a long-plate morphology. The LPSO-induced DRX (LDRX) is dominant while the continuous dynamic recrystallization (CDRX) is complementary for the grain refinement of the HSMDFFed alloy. At ∑Δ ε = 2.64, the comprehensive mechanical performance of the HSMDFFed GEZ811 alloy is optimal, and its tensile yield strength (TYS), ultimate tensile strength (UTS) and elongation (EL) are 302.4 MPa, 370.3 MPa and 11.8%, respectively. The combined strengthening effects of grain boundary, LPSO phase and dislocation are responsible for the enhancement of mechanical properties, in which the grain boundary strengthening is dominant. The present work is beneficial to provide a low-cost approach for designing and fabricating forged Mg-RE alloys with advanced mechanical performance.