In this study, the effects of Mg on the fatigue crack growth (FCG) characteristics of precipitation-hardened Al6061 alloys containing Zr and excess Mg are examined. The growth behavior of microstructurally large cracks is investigated via rotating-bending fatigue tests conducted at room temperature. Analyses of the crack propagation and striation features show that excess Mg promotes the occurrence of Mode I fatigue cracks. These facts suggest that dynamic strain aging due to Mg restricts dislocations motion, resulting in greater work hardening, and generates a large number of active slip systems, leading to more non-localized slip and a large area of striation formation. Consequently, it is concluded that this phenomenon induces highly stable crack growth in Al alloys with excess Mg, which influences the fatigue crack growth rate (FCGR) scatter of microstructurally large cracks. A reasonable mechanism of strain-aging-induced Mode I FCG in Al alloys is proposed on the basis of the morphological aspects of fatigue striation formation.
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