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

초록

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 (similar to 7 mu m) compared to the center region (12-15 mu m) 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 alpha-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 x UTS - 73.5; this relationship is observed in the die-cast SEN6 alloy as well.

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