The strength contribution via heterostructure in a Mg-Gd-Zn-Mn alloy

Abstract

Deformation by extrusion is an effective means to enhance the performance of magnesium alloys. However, the resulting microstructure heterogeneity always lead to poor plasticity. Therefore, it is crucial to explicit the impact of heterogeneous structure on the deformation mechanism of the alloys. In this work, the microstructures evolution, mechanical properties, and fracture mechanism of Mg-xGd-1Zn-0.5Mn (x = 0, 2, 4, and 6) alloys were investigated. The elongation reached its maximum (40%) when the Gd content was 2%, which was attributed to the formation of the RE-texture and grain refinement. As the Gd content was further increased, the alloy developed a heterostructure consisting of both undynamic recrystallization (UDRX) grains and dynamic recrystallization grains (DRX). The exist of the heterostructures resulted in an increased strength which was attributed to the influences of texture strengthening derived from UDRX grains, the lamellar LPSO fiber strengthening and fine grain strengthening of DRX grains. At the initial stage of deformation in the heterostructure, the basal slip of DRX grains play a dominant role, while the UDRX grains strengthens the alloy effectively. The lamellar LPSO precipitated in the UDRX grains suppressed twinning effectively. As the concentrated stresses were enough to break through the barrier for twining of LPSO, numerous twins nucleated in the deformed grain thus resulting in the nucleation of cracks. During the process of tensile deformation, twins experienced greater basal slip resolved shear stress than that of the parent grains, resulting in significant basal slip activated due to twin. Consequently, there was a high stress concentration at the twin boundaries, leading to the nucleation of cracks. The varied deformation amount also resulted in the crack initiating at the interface of DRX grain and UDRX grain. This investigation provides valuable insights into the influence of a heterostructure on the deformation mechanism of Mg-Gd-Zn-Mn alloys.