Microstructure characteristics, yield asymmetry and fracture mechanism of the fine grained thin-wall Mg-6.03Zn-0.55Zr tubes fabricated by hot spinning

Abstract

In this study, thin-wall Mg-6.03Zn-0.55Zr alloy tubes consisted of fine dynamic recrystallized (DRXed) grains and ultrafine grains were successfully obtained by hot spinning process. The results showed that the microstructures of the initial extruded tubes were greatly refined with average grain sizes of 4.46–10.32 µm (300–450 ℃) due to the DRX process, especially at low spinning temperatures. Moreover, the basal texture was transformed from extruded fiber texture to spun tilted plane texture with an angle of ∼10° deviated from the axial direction (AD). More specifically, the basal texture was weakened along AD but was strengthened along the transverse direction (TD) compared with the extruded texture due to the reduced difficulty in pyramidal <c+a> slip activation and the shear deformation induced by the rollers. Consequently, the spun Mg-6.03Zn-0.55Zr alloys showed significant yield asymmetry along AD and TD at room temperature and the increase in spinning temperature had a negative influence on yield strength (YS) that the YS along AD decreased from 290 ± 5 MPa (300 ℃) to 232 ± 2 MPa (450 ℃) and the YS along TD decreased from 236 ± 6 MPa (300 ℃) to 198 ± 4 MPa (450 ℃). Both texture and grain refinement were the important factors in the mechanical properties according to the Hall-Petch relationship due to the varied activation fractions of deformation modes along AD and TD, especially the activation of prismatic <a> slip. Moreover, the fracture mechanism of the spun alloys consisting of fine DRXed grains and ultrafine grains was unusual. The aggregated ultrafine grains played a negative role in the fracture mechanism, because the ultrafine grains were transformed from the dislocation cells and the micro-crack easily occurred in the ultrafine grains.