Effect of strontium modification on near-threshold fatigue crack growth in an Al-Si-Cu die cast alloy

The effects of strontium modification on microstructure and fatigue properties in a die cast commercial aluminum-silicon alloy are demonstrated. Strontium additions of 0.010 and 0.018 wt.% drastically change the morphology of the eutectic silicon. The influence of these microstructural changes on fatigue properties is evaluated through fatigue crack growth testing. Examination of the fracture surfaces and the crack path establish distinct fatigue fracture modes for the modified and unmodified eutectic structures. Changes in fracture mode and crack path are correlated to the microstructure changes. A higher energy fracture mode and increased crack path tortuosity explain the observed improvement in fatigue properties for the modified alloys. Strontium modified alloys exhibit a 10-20% higher fatigue crack growth threshold compared to an unmodified alloy for testing at a load ratio of 0.5. No difference was observed for testing at a load ratio of 0.1.