Effects of dendrite cell size and eutectic si particle morphology on fatigue crack growth in cast and HIPed AC4CH alloys

Fatigue crack growth tests were performed for structurally controlled AC4CH cast alloys. Elimination of casting defects from the casts was achieved by means of post-casting HIP treatment. Compliance change measurement was made during the fatigue crack growth test to investigate crack closure. The level of crack closure was comparable among the alloys with different microstructure. Sheroidizing and refining of the eutectic Si particles are essential in terms of reduced propensity of particle fracture, which results in prolonged stable crack growth region of the da/dN-Delta K relationship and improved fatigue crack growth resistance at the high Delta K. At low Delta K, however, the fatigue crack growth rate is controlled by general matrix strength. The higher proof stress due to the irregular shaped Si particles and fine dendrite cell size results in the reduced crack opening displacement of each fatigue cycle or the reduced crack growth rare.