Role of second phases and grain boundaries on dynamic recrystallization behavior in ZK60 magnesium alloy

Highlights: • The ingot conditions before extrusion significantly influence the microstructure and mechanical properties of as-extruded ZK60 alloys. • The fine initial grains have a large potential to form a considerable amount of DRX nuclei sites and refine the final grains. • The primary second-phase particles have significant PSN effect during the extrusion process. • Fine grain structure is achieved when the PSN driving force is greater than the pinning pressure of fine precipitates. -- Abstract: Dynamic recrystallization (DRX) and the related phenomena take place during thermomechanical processing that furnishes the final microstructure of a product. The microstructure of most Mg alloys composed of Mg matrices and second-phase particles, which significantly affect the DRX behavior and the formation of final microstructure. In the present work, four specific and inter-related topics were investigated according to four typical ingot conditions (as-homogenized, as-cast, cast-aged, and homogenization-aged) before hot extrusion on Mg-6.0Zn-0.5Zr (ZK60) alloy: the effect of initial grain boundaries on the DRX nucleation, the role of primary particles on the particle-stimulated nucleation (PSN) effect, the effect of precipitates on DRX nucleation and grain boundary migration, and the influence of two types second-phase particles on DRX behavior. After extrusion, the average grain size of cast-aged ZK60 is the finest, which is refined from 20.5 to 2.8 μm. With the grains significantly refined, the mechanical properties of the cast-aged ZK60 are improved. The tensile yield stress, ultimate tensile stress, fracture elongation, compressive yield stress, and tension-compression yield asymmetry of the cast-aged ZK60 are 261 MPa, 338 MPa, 26.9%, 252 MPa, and 0.97, respectively. The results show that initial grain boundaries have huge potential to promote DRX nucleation. Coarse particles distributed at grain boundaries accelerate grain boundary nucleation. And the ones distributed in the grains have significant PSN effect to promote DRX nucleation. The fine precipitates distributed at new grain boundaries produce a pinning effect on the migration of grain boundaries, which prevent the growth of DRX grains. The combination of fine initial grains, coarse particles, and fine precipitates generate the best grain refinement during the DRX process. Therefore, after hot extrusion, the ZK60 alloy with cast-aged state before extrusion exhibits fine microstructure and good mechanical properties. The influence mechanisms of second phase and grain boundary on the dynamic recrystallization behavior were discussed in detail.