Mechanical properties and failure characteristics of cast and extruded Mg₉₇Y₂Zn₁ alloys with LPSO phase

Mechanical properties and failure characteristics of Mg₉₇Y₂Zn₁ (at%) alloys, prepared by gravity casting and extrusion processes, are examined experimentally. High tensile and fatigue strengths are obtained for extruded Mg₉₇Y₂Zn₁ alloys, with the values being approximately twice as high as those for cast Mg₉₇Y₂Zn₁ alloys. Such high strengths are attributed to the high internal stress arising from the complicated microstructure and severe lattice strain in the α-Mg and long-period stacking order phases. On the other hand, a high fracture strain is obtained for cast Mg₉₇Y₂Zn₁ alloy, which is affected not only by the lower internal stress, but also severe kink deformation. The failure characteristics, e.g., crack growth characteristics are examined via direct observation during tensile and fatigue tests at micro- and nano-scales. Meandering fatigue crack growth is obtained for the cast Mg₉₇Y₂Zn₁ alloys. The mean roughness of the fracture surface for cast Mg₉₇Y₂Zn₁ alloys is more than twice as high as that for commercially available AM60 and AZ91 cast alloys. A coarse fracture surface that was observed in the cast Mg₉₇Y₂Zn₁ alloys would make severe roughness-induced crack closure, leading to high fatigue strength.