THE CREEP FRACTURE OF A SINGLE-CRYSTAL SUPERALLOY

The creep fracture of the single-crystal superalloy SRR99 at 750-degre es-C was studied for crystals oriented close to the [001] and [011] di rections. The results indicate that the fracture processes operating w ithin both orientations are highly dependent on the deformation mechan isms which operate during creep deformation. In [001] orientations, ho mogeneous deformation leads to the formation of macroscopically 'squar e'' mode I cracks with sides parallel to [011] directions. These crack s propagate through the gamma' precipitates from microporosity. Final rupture in this orientation occurs by the interlinkage of these mode I cracks by fracture along four {111} planes when the resolved shear st ress on the remaining material is equal to the critical resolves shear stress of the alloy. Conversely, failure in [011] orientations occurs in a fast and catastrophic manner, with a single crack growing along heterogeneous shear bands which lie along {111} type planes.