Tensile compression yield asymmetry of Mg-Zn-Y-Gd-Zr alloy in extruded and annealed states

This study investigates the microstructural changes and mechanical response of GWZ magnesium alloy at room temperature, aiming to understand the factors contributing to the asymmetry observed in tensile and compressive yield behavior. After a long-time annealing treatment, various microstructural features such as texture, grain size, long period stacking ordered (LPSO) phase, and crystal orientation undergo changes, which allows for the investigation of their combined effects on the mechanical properties and yield-asymmetry. The yield-asymmetry of the alloy in the extruded condition was found to be 1.08, increasing to about 1.22 after heat treatment. Annealing creates new texture components that are favorable for extension/contraction twinning. Additionally, the increase in average grain size after annealing increases the probability of extension twin formation. Increasing the twin probability and decreasing the slip-to-twin transfer critical stress enhances the difference between tensile and compressive yield strengths. Additionally, the mentioned phenomenon is the reason for the lower strength level, higher ductility, and higher yield asymmetry of the annealed specimens. Fraction of blocky-LPSO also diminished through annealing, giving way to the formation of lamellar-LPSO within α-grains. Under tensile loading, lamellar LPSO exhibits structural integrity, while under compression, yielding occurs through the formation of kink bands. The high susceptibility of lamellar LPSO for strain induced structural evolution rather than blocky ones, could justify the observed low yield-asymmetry of extruded microstructure.