Effects of Zr addition on the multi-scale second-phase particles and fracture behavior for Mg-3Gd-1Zn alloy

Here we report the effects of Zr addition (0.1-0.3 at.%) on the multi-scale second-phase particles and fracture behavior for Mg-3Gd-1Zn (at.%) alloys. Microstructural features of the alloys in the as-cast, solutionized, and aged conditions have been investigated in detail. The results have revealed that the cast alloys are mainly composed of α-Mg matrix, eutectic phase (Mg, Zn)3Gd, 14H long period stacking ordered (LPSO) structure and cuboid shaped GdH2 phase. After solution heat treatment, the eutectic phase (Mg, Zn)3Gd has transformed to X phase with 14H-LPSO structure, which has different distributions and morphologies due to different Zr additions. When the Zr addition is 0.3 at.%, a large amount of microscale Mg(Gd, Zn, Zr) particles appear along the grain boundaries. After aging heat treatment, Mg-3Gd-1Zn alloy with basal precipitates γ'' exhibits poor age-strengthening response. In contrast, Zr addition to the Mg-3Gd-1Zn alloy gave rise to a strong age-strengthening response. The strength improved significantly due to dense distribution of nanoscale prismatic plate β' and β1 precipitates, which are not observed in the Zr-free alloys. One key finding of this study is that Zr contributes to formation of the prismatic precipitates, and their amount increase with increase of Zr addition. The mechanical damage for these alloys is a sequence of microscale GdH2 or Mg (Gd, Zn, Zr) particles cracking, followed by fracture of the surrounding particles, and finally the growth/coalescence of micron-voids in the α-Mg matrix. The fracture of microscale particles was quantitatively analyzed by a Weibull model.