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Solute and precipitate effects on magnesium recrystallization and grain growth
Storey, Gillian K.
Storey, Gillian K.
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2021
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Recrystallization and grain growth kinetics of magnesium alloys can vary significantly based upon precipitate and solute content. Rare earth alloying elements, such as Ce, have been shown to stabilize grainsize at elevated temperatures and thereby improve elevated-temperature properties. Quantifying the effects of precipitate fraction and solute content can result in fundamental understanding that will enable optimized industrial processing pathways. The baseline composition for this study is ZK60. Several variations on ZK60 compositions with different Zn and Ce levels were evaluated, which result in variations in second phase insoluble particle type, solute distribution and volume fraction that have strong effects on both grain size and recrystallization. After cold rolling and static annealing heat treatments from 6-60h at 350-400°C, microstructural characterization and microhardness tests were conducted. These results provide insights into mechanical and microstructural characteristics, including hardness, grain size distribution and texture, which are affected by enhanced or retarded recrystallization and grain growth kinetics. Increases in precipitate and solute content increase Zener pinning and solute drag and decreases static recrystallization(SRX) and grain growth kinetics. Texture weakening occurs during SRX and dynamic recrystallization(DRX), while near-basal (0001) orientated grains grow preferentially during grain growth. Modeling of DRX processes is determined by calculations of the critical stress required for DRX initiation. Hot compression tests were carried out on these alloys from 350-400°C via the Gleeble 3500 thermomechanical simulator at various strain rates. Microstructural characterization provides further insights into grain size distribution and texture, which are enhanced or retarded by DRX kinetics. A constitutive model shows that increased strain and lower temperatures require greater stress input for DRX to occur. The flow curves exhibit work hardening, softening and steady states. Ce additions impede DRX kinetics to a greater extent than Zn additions. Texture weakening readily occurs and is observed in the post-DRX microstructures
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