Investigation of Microstructural Factors Affecting the Plane-Strain Fracture Toughness of Mg–Zn–Y–Al Alloys Processed by Consolidation of Rapidly Solidified Ribbons

High strength nanocrystalline bulk Mg96.75Zn0.85Y2.05Al0.35 (at%) alloys featuring increased fracture toughness were fabricated by the consolidation of rapidly solidified (RS) ribbons. The RS ribbons prepared by single-roller melt-spinning at a cooling rate of ~1.4 × 105 Ks−1 were pressed into a copper billet and then consolidated by hot-extrusion. The microstructure of the alloys consists of α-Mg and long-period stacking ordered (LPSO) phases. The two-phase microstructure evolution in the alloys is influenced by conditions of pre-consolidation/extrusion heat treatment. In the alloys without pre-extrusion heat treatment, plate-shaped LPSO phase precipitates in grain interior. On the other hand, in the alloys with pre-extrusion heat treatment, block-shaped LPSO phase forms around grain boundaries. Formation of the block-shaped LPSO phase induces crack deflection and meandering, resulting in improvement of fracture toughness. The alloy subjected to pre-extrusion heat treatment at ~738 K for 24 h exhibited a good balance of plane-strain fracture toughness (KIc =  ~15 MPam1/2) and tensile yield strength (σ0.2 =  ~400 MPa).