Tensile and Fatigue Strength, Fatigue Crack Propagation Rate, and Fracture Behavior of CaO-Added AM30 Alloy

This work investigated the tensile and fatigue strength, fatigue crack propagation rate and corresponding mechanism, and fracture behavior (under the tensile and cyclic loading) of the extruded CaO-AM30 alloy. The microstructure observations shown that the average grain size of AM30 base alloy was 7.8 mu m, which decreased to 3.5 mu m in the CaO-AM30 alloy. In both alloys, Mg17Al12 and Al6(Mn, Fe) phases were present, and C15 ((Mg, Al)(2)Ca) phases were additionally present in the CaO-AM30 alloy. Also, the average size of the Mg17Al12 and Al6(Mn, Fe) phases was much smaller in the CaO-AM30 alloy than those in the AM30 alloy. As a result of the smaller grains and fine evenly distributed second phases, CaO-AM30 alloy shown an improved tensile strength along with a 25% increase in the elongation. Accordingly, the CaO-AM30 alloy showed higher fatigue strength (168 MPa) than the AM30 alloy (130 MPa) after similar to 10(7) number of cycles. Nevertheless, fatigue crack growth test revealed that the CaO-AM30 alloy has a lower threshold triangle K-th value than the AM30 base alloy. Also, the calculated value for m (log slope of da/dN and triangle K) was 13.64 for AM30 alloy, which increased to 14.15 for the CaO-AM30 alloy. The relatively higher crack propagation rate of the CaO-AM30 was most likely related to the presence of larger plastic deformation zone in it than its grain size, causing the suppression of fatigue crack closure mechanism during the unloading half of the cycle. Hence, this study suggested that the fine grains improve the strength and high-cycle fatigue properties of the Mg alloys, but adversely affect the fatigue crack propagation resistance.