Mechanical strength and failure characteristics of cast Mg-9%Al-1%Zn alloys produced by a heated-mold continuous casting process: Fatigue properties

The fatigue properties and failure characteristics of a cast Mg alloy (AZ91: Mg-Al_8.9-Zn_0.6-Mn_0.2) produced by heated-mold continuous casting (HMC) and conventional gravity casting (GC) are investigated. Excellent fatigue properties are obtained for the HMC alloy compared with the GC alloy. The high fatigue strength of the HMC alloy is a reflection of its improved microstructural characteristics, namely, tiny α-Mg grains and fine spherical eutectic structures (β-Mg₁₇Al₁₂). Fatigue cracks propagate mainly in the α-Mg grains and along high-hardness β-phases in both alloys. The direction of fatigue crack growth is altered as the crack reaches the eutectic phases. Because the tiny eutectic phases are distributed randomly in the HMC alloy, a meandering crack path is formed, which results in high crack growth resistance, leading to the high fatigue strength. For the HMC alloy, a striation-like failure mode can be seen in the crack growth stage, and dimple fracture is the dominant feature in the final failure stage. On the other hand, cleavage-like brittle failure with many microcracks can occur in the GC alloy. Severe lattice strain occurs during the final failure stage, especially for the HMC alloy, resulting in strong work hardening, although this is weak in the crack growth site. Severe strain is distributed uniformly around the entire cracks in the HMC alloy when the sample is fractured by monotonic loading. The lattice strains in the HMC alloy under monotonic and cyclic loading are overall higher than those in the GC alloy, owing to the high material ductility of the HMC alloy.