High Cycle Fatigue Mechanisms in a Cast AM60B Magnesium Alloy

Horstemeyer, M., Yang, N., Gall, K. A., McDowell, D. L., Fan, J., & Gullett, P. (2002). High Cycle Fatigue Mechanisms in a Cast AM60B Magnesium Alloy. Fatigue and Fracture of Engineering Materials and Structures. Blackwell Science Ltd. 25(11), 1045-1056.

We examine micromechanisms of fatigue crack initiation and growth in a cast AM60B magnesium alloy by relating dendrite cell size and porosity under different strain amplitudes in high cycle fatigue conditions. Fatigue cracks formed at casting pores within the specimen and near the surface, depending on the relative pore sizes. When the pore that initiated the fatigue crack decreased from approximately 110 μm to 80 μm, the fatigue life increased two times. After initiation, the fatigue cracks grew through two distinct stages before final overload specimen failure. At low maximum crack tip driving forces (Kmax less than or equal 2.3 MPa root m), the fatigue crack propagated preferentially through the α-Mg dendrite cells. At high maximum crack tip driving forces (Kmax > 2.3 MPa root m), the fatigue crack propagated primarily through the β-AlMg particle laden interdendritic regions. Based on these observations, any proposed mechanism-based fatigue model for cast Mg alloys must incorporate the change in growth mechanisms for different applied maximum stress intensity factors, in addition to the effect of pore size on the propensity to form a fatigue crack.