Tensile flow behavior of AZ31 magnesium alloy processed by severe plastic deformation and post-annealing at moderately high temperatures

Abstract

The uniaxial tensile flow behavior of an AZ31 magnesium alloy was examined after processing by severe plastic deformation (SPD) and short time post-annealing. The SPD process of constrained groove pressing (CGP) was performed to three cycles at temperatures of 503–453K. After CGP, the alloy was further annealed at 473K for a short duration of 3min. By using this procedure, a bimodal fine-grained microstructure with an average grain size of 3.5μm was produced. In the room temperature tensile tests, the yield strength and elongation to failure were improved by 34% and 11%, respectively as compared with the initial material before SPD. The tensile deformation behavior and deformation mechanism of the alloy were further examined at moderately high temperatures from 373 to 523K under initial strain rates from 1×10−3 to 1.0s−1. Examination of the microstructures near the fractured tips indicate that dynamic recrystallization could occur in the fine-grained alloy at a relatively low temperature of 423K and the maximum strain rate sensitivity of 0.14 was measured at 523K. The apparent activation energy tested at 423–473K with initial strain rates of 1×10−3-1.0s−1 was estimated to be 123.6kJmol−1, which is close to the value for lattice self-diffusion of magnesium. The results suggest that climb-controlled dislocation creep is the dominant deformation process associated with lattice diffusion. Despite the relatively fine-grained microstructure, superplastic-like ductility was not observed because of the low thermal stability of the microstructure at elevated temperatures, and is attributed to the high stored strain energy in the microstructure imparted by the CGP. Nevertheless, enhanced ductility was measured at 523K and 1×10−3s−1 where a maximum elongation to failure was 100±2.5%, and dynamic recovery was observed as the main restoration process.