Fatigue Crack Growth Threshold in an Al-Si-Cu Die Cast Alloy and the Effect of Strontium Modification

This thesis presents an evaluation of the effects of strontium modification on the near threshold fatigue crack growth behavior of a die cast aluminum-10% silicon-3% copper alloy. Strontium is known to refine the eutectic microstructure in aluminum-silicon casting alloys. In the present work, results of near threshold fatigue crack growth testing are presented. Micrographic examination of the crack path and SEM examination of fracture surfaces provide the basis for characterizing the fracture modes. Test results establish distinct fatigue fracture modes for the modified and unmodified eutectic structures. The change in fracture mode correlates to the change in microstructure. The change in fracture mode is also used to explain the observed difference in fatigue properties. The modified alloys exhibit a 10 to 20 percent higher fatigue crack growth threshold compared to an unmodified alloy, under high load ratio testing