Propagation of microcracks in single crystalline nickel-based superalloys: size effects on the crack opening

The lifetime of single crystals (SX) as established high temperature materials for turbine blades is mainly determined by the propagation of microcracks. So far mainly long crack data are available for SX turbine blade materials measured on the basis of the established K-concept. This data pool is used for the calculation of lifetime although short cracks normally lead to failure. However, microstructurally and physically short cracks behave different from long cracks and cannot be described by using the K-concept. A comprehensive study of short crack data was carried out for the SX nickel-based superalloy CMSX-4 as a model alloy at temperatures from room temperature up to 1223 K. Additionally the crack propagation rate and the crade-tip-opening displacement was investigated close to the crack tip in situ by scanning electron microscopy. Thereby the physical model of alternating slip was investigated locally and it was shown that the local crack opening displacement of short cracks cannot be predicted by elastic–plastic fracture mechanics.