The role of extrinsic mechanism which improves fatigue crack resistance of the cast alloy has been examined. A constant load amplitude fatigue crack growth test and a crack closure measurement have been performed using AC4CH cast aluminum alloys. The da/dN-ΔK relation and the crack closure levels were compared between the cast alloys and squeeze cast or powder-metallurgy processed 6061 aluminum alloys. Improved fatigue crack growth resistance at the low ΔK range was shown for the cast alloy in comparison with the powder-metallurgy alloy with finer microstructure, when evaluated by the da/dN-ΔK relation. While, the difference was diminished by evaluaion based on the da/dN-ΔKeff relation. This indicates that the roughness-induced crack closure effect is large in the cast alloys to improve the crack growth resistance in the low ΔK region. This extrinsic mechanism is considered to be originated from the deflected fatigue crack growth path and the resultant rough fatigue fracture surface, which are attributed to the dendritic solidification structure and the eutectic Si particle distribution in the cast alloy.
