Tensile and High-Cycle Fatigue Properties of Die-Cast SEN6 Magnesium Alloy

This study investigates the microstructural characteristics, tensile properties, and high-cycle fatigue properties of a die-cast Mg–6Al–0.1Mn–0.1Zn–0.3Ca–0.2Y (SEN6) alloy with high corrosion and ignition resistances. The average grain size of an hourglass-shaped die-cast specimen of the SEN6 alloy is smaller at the surface region (~ 7 μm) compared to the center region (12–15 μm) due to the higher cooling rate at the surface region. Four types of pores with different sizes and roundness features, namely, gas pores, island-shrinkage pores, gas-shrinkage pores, and net-shrinkage pores, are distributed throughout the specimen. The presence of small amounts of Ca and Y in the SEN6 alloy results in the formation of Al₂Ca, Al₈Mn₄Y, and Al₂Y particles. Coarse Mg₁₇Al₁₂ and Al₂Ca particles are present along the grain boundaries, and relatively fine Al₈Mn₄Y and Al₂Y particles are distributed inside the grains. The die-cast SEN6 alloy exhibits a tensile yield strength of 121.4 MPa, ultimate tensile strength (UTS) of 172.5 MPa, elongation of 5.1%, and fatigue limit (FL) of 50 MPa. The island-shrinkage and net-shrinkage pores act as crack initiation sites during tension owing to their irregular shape and sharp tips. In contrast, fatigue cracks initiate at the gas-shrinkage pores and Al₂Ca particles present at the sub-surface. Larger gas-shrinkage pores located closer to the surface induce higher stress concentration around them during cyclic loading. The high hardness of Al₂Ca particles causes a significant difference in micro-plasticity between the α-Mg matrix and the particles, leading to the formation of fatigue cracks. The FL and UTS of die-cast Mg alloys are linearly related: FL = 0.74 × UTS − 73.5; this relationship is observed in the die-cast SEN6 alloy as well. Graphic Abstract: [Figure not available: see fulltext.]