Corrosion fatigue behavior of die-cast and shot-blasted AM60 magnesium alloy

Magnesium (Mg) and its alloys have a long tradition of use as a lightweight material in the field of commercial and especially automotive construction. Unfortunately its poor corrosion resistance restricts its use under corrosive atmosphere. So far, most of the research works on corrosion fatigue properties of Mg alloys have dealt with effect of pre-existing pit or effect of pitting corrosion. In our previous work, it has been found that fatigue cracks initiate and propagate from corrosion-induced surface defects rather than pits. In the present study, fatigue tests of as-cast and shot-blasted AM60 Mg alloy specimens were carried out in three environments: low humidity (relative humidity 55%RH), high humidity (relative humidity 80%RH) and 5% NaCl. Fatigue strengths were degraded in the high humidity and NaCl environments for both the as-cast and the shot-blasted specimens. Cracks were nucleated from corrosion-induced surface defects which then propagated until final failure. Considering the corrosion-induced surface defect as a stress raiser, the stress intensity factor value was estimated. The estimated value was almost coincided with the threshold stress intensity factor range. The shot-blasted specimens showed improved fatigue strength under all the three environments compared to the as-cast specimens. It was speculated that improvement of fatigue strength due to shot-blasting resulted from the hardening and compressive residual stress near surface region induced by shot-blasting.