Fatigue Crack Growth in Co and Ni Base Directionally Solidified Eutectic Composites

A study of fatigue crack growth behavior in four directionally solidified eutectic composites has been carried out in air at room temperature and at 870°C (1600 °F). The alloys investigated included two cobalt-base alloys, CoTaC and CoTaC+Cr, and two nickel base alloys, γ′-Mo and γ/γ′-δ. Fatigue crack growth was studied in directions parallel and perpendicular to the solidification direction. It was found that the addition of chromium to the CoTaC alloy strengthened the interface and promoted trans-fiber crack growth. In addition, the effect of interlamellar spacing on the rate of fatigue crack growth was examined for the γ/γ′-δ alloy. A reduction in interlamellar spacing reduced the crack growth rate. The growth rates are related to the corresponding stress intensity factors and are discussed in the light of prior investigations reported in the literature.