Fatigue Crack Initiation Mechanism for Cast 319-T7 Aluminum Alloy

In this study, the fatigue crack initiation behavior of cast 319-T7 aluminum alloy at 25 A degrees C and 150 A degrees C was examined based on detailed scanning electron microscopy (SEM) fractographic analysis. The stress-life (S-N) type, uniaxial fatigue tests were conducted for the 319-T7 specimen at a stress ratio (R) of -1 and at 25 A degrees C and 150 A degrees C, and the fractographs of all the fatigued specimens were documented using SEM. At 25 A degrees C, all the fatigue cracks for the specimens initiated from porosity located at or near the specimen surface. At 150 A degrees C, the fatigue crack initiation occurred at porosities near surface for 11 out of 29 specimens. For those specimens with relatively small or no porosities near surface and fatigued at 150 A degrees C, the fatigue crack initiation by slip band mechanism tended to occur and coarse transgranular facets were observed. The oxide film was often observed on the coarse transgranular facet for the those specimens using the slip band mechanism. The fatigue crack initiation mechanism for cast 319-T7 alloy at 25 A degrees C and 150 A degrees C was proposed based on extensive fractographic analysis results.