Effect of Cleanliness on Fatigue Crack Growth Mechanism of IN718.

Abstract

It is widely understood that the fatigue life of nickel-base superalloys is greatly increased as a consequence of the inclusion elimination in the melting process. Electron beam cold hearth remelting is a recently developed technique that offers the most dramatic improvements in the cleanliness of superalloys. However, the relation between fatigue characteristics and cleanliness has not been well investigated. In this work, fatigue crack growth rate tests were conducted at ambient temperature on commercial grade IN 718 and electron beam melted clean IN 718. The fatigue crack growth rate test shows that small surface cracks in the clean materials propagate considerably slower than those in the conventionally processed materials. In contrast to the behavior of small cracks, long through cracks propagate faster in the clean materials than in the conventionally processed materials. Since the small crack initiation and propagation period is the major portion of the fatigue life, the behavior of small cracks has a more important effect on the fatigue life than the behavior of long cracks.