Warm Tensile Deformation and Fracture Behavior of AZ31 Magnesium Alloy Sheets Processed by Constrained Groove Pressing

Constrained groove pressing (CGP), as a promising severe plastic deformation method suitable for processing sheet metals, was applied to AZ31 magnesium alloy sheets, and ultra-fine grained structure was achieved after two passes of CGP at 473 K. A bimodal structure was also developed due to partial dynamic recrystallization (DRX) and deformation homogeneity. Tensile tests at temperatures from room temperature to 523 K and strain rates from 1 × 10⁻⁴ to 1 × 10⁻¹ s⁻¹ were conducted to the processed alloy sheets, and their tensile deformation and fracture behavior were investigated in this work. The maximum elongation to failure of 38.7% is achieved at 1 × 10⁻⁴ s⁻¹ and 473 K, and the yield strength and tensile strength are 37.0  and 43.0 MPa, respectively. The strain hardening ability increases gradually with increasing strain rate at elevated temperatures, and its dependence on strain rate is more significant at lower temperatures. The strain rate sensitivity coefficient gradually increases with increasing temperature, and the relatively high values of ~ 0.17 and ~ 0.14 are obtained at 473 and 523 K, respectively, indicating the absence of superplastic behavior. The fracture morphology shows that with increasing temperature, the fracture mode changes from brittle fracture to ductile fracture, which is closely related to the microstructural evolution during tensile deformation. The grain coarsening at 523 K may result in the slight decreases of elongation to failure and strain rate sensitivity coefficient as well as the recovery of strain hardening exponent. The apparent activation energy at 423-523 K is estimated to be 68.8-105.5 kJ/mol. Conclusively, DRX and grain growth should be the dominant mechanism accounting for the warm tensile deformation of AZ31 magnesium alloy sheets processed by CGP, while grain boundary sliding mechanism may contribute little, due to the developed bimodal structure during CGP and the relatively low temperatures for the tensile tests.