Crack growth mechanisms in an aged superalloy at high temperature

• Analysis of service cracks in turbine blades.
• Transgranular service cracks exhibit recrystallized layer along crack faces.
• Recrystallization can be reproduced with adapted experiments.
• Transcrystalline crack extension can be observed even at elevated temperature.
• Recrystallization is a two-step process which takes place on a short time scale.

Turbine blades made of Ni-base superalloy IN738 in stationary gas turbine had been removed during routine maintenance procedures after 10,000–20,000 h of operation. Crack growth in this material was studied by performing lab tests with specimens directly cut from the hot area of these blades with testing temperatures of 20 °C and 700 °C. The crack paths in these tests were analyzed and compared to those of service cracks. It was found that even though intergranular crack growth was dominant at elevated temperatures there was still a certain amount of transgranular crack propagation in all cases. Transgranular crack growth under operating conditions led to local recrystallization in the plastic zone ahead of the crack tip. The implications of the recrystallization process on the crack growth rate are discussed in the paper.