SEM online investigation of fatigue crack initiation and propagation in cast magnesium alloy

Magnesium (Mg) alloys have a unique combination of properties including low density and easy formability that are attractive for numerous technological applications. The requirement to reduce the weight of engineering components has recently triggered substantial research effort on the structural properties of this kind of alloy [1–3]. Compared to other lightweight wrought and cast materials such as aluminum and titanium, little work has been performed on the fatigue behavior of cast magnesium. Initial studies examined the low and high cycle fatigue of cast Mg [4, 5] but they concluded that traditional fatigue life prediction tools are often inaccurate for the cast Mg. Therefore, it is of great interest to search for a relationship between the fatigue failure mechanisms and microstructure in the cast Mg [6–9]. To gain a better understanding on the fatigue behaviors of this alloy, we carried out the fatigue crack initiation and propagation tests and investigated the fatigue crack growth path with notched specimens of the cast AM50 alloy with online SEM observation. The nominal composition of AM50 alloy contain about 5 wt% aluminum. The testing specimens with a 14 mm gage length and a 2.5 mm by 2.3 mm gage cross section were machined from plates supplied by Hydro’s Inc. The testing specimen had a notch radius of about ρ = 80 μm to elevate the local stress state in controlling the site of fatigue crack initiation. The surfaces of the samples were carefully polished prior to fatigue tests. All fatigue tests were performed in the chamber of the SEM. Fatigue tests were carried out at a stress ratio R = 0.1 under a sinusoidal variation. The maximum stress of 120 MPa was chosen as it is below the yield stress σ0.2 = 140 MPa. The whole process of fatigue crack initiation and propagation was monitored and recorded in situ at a frequency of 0.01 Hz at room temperature in order to clearly observe the open and closed crack status. The microstructure of cast AM50 alloy is present in the brittle eutectic phase Mg17Al12 (Its micro-hardness is about 88.34 MPa) with the α-Mg grains (Its micro-hardness is about 66.86 MPa). The microstructure has a significant influence on the me-