Description

What looks like the Misty Mountains from Tolkien’s The Hobbit is actually a colorized electron micrograph of a fracture surface. Cobalt-base superalloys are promising for turbine engine applications where cyclic loading and unloading at high temperatures can lead to catastrophic fatigue failures.

This backscattered electron micrograph was taken using a FEI Nova Nano 650 SEM equipped with a Schottky field emission gun. The Co-base superalloy (CoAlWTiNb) was cast into single crystal bars using the Bridgman Method, heat treated, and then machined into dogbone test specimens. Heat treatment produced a γ-γ' microstructure with a small amount of extra phases. Fatigue testing was performed in the UCSB Mechanical Test Lab. Stress cycles were applied with R = 0.0 at 750 °C to simulate the start/stop cycle of a gas turbine engine. The gauge length of the specimen was cross-sectioned, mounted in an epoxy thermoset resin, and then polished for imaging using standard metallography techniques. Image processing was performed using GIMP. The unmagnified dimensions of this image are 173 x 115 μm.